winrt - Dave's Blog


Edge browser & JavaScript UWP app security model comparison

Nov 29, 2:21

There are two main differences in terms of security between a JavaScript UWP app and the Edge browser:

Process Model

A JavaScript UWP app has one process (technically not true with background tasks and other edge cases but ignoring that for the moment) that runs in the corresponding appcontainer defined by the app's appx manifest. This one process is where edgehtml is loaded and is rendering HTML, talking to the network, and executing script. Specifically, the UWP main UI thread is the one where your script is running and calling into WinRT.

In the Edge browser there is a browser process running in the same appcontainer defined by its appx manifest, but there are also tab processes. These tab processes are running in restricted app containers that have fewer appx capabilities. The browser process has XAML loaded and coordinates between tabs and handles some (non-WinRT) brokering from the tab processes. The tab processes load edgehtml and that is where they render HTML, talk to the network and execute script.

There is no way to configure the JavaScript UWP app's process model but using WebViews you can approximate it. You can create out of process WebViews and to some extent configure their capabilities, although not to the same extent as the browser. The WebView processes in this case are similar to the browser's tab processes. See the MSWebViewProcess object for configuring out of process WebView creation. I also implemented out of proc WebView tabs in my JSBrowser fork.


The ApplicationContentUriRules (ACUR) section of the appx manifest lets an application define what URIs are considered app code. See a previous post for the list of ACUR effects.

Notably app code is able to access WinRT APIs. Because of this, DOM security restrictions are loosended to match what is possible with WinRT.

Privileged DOM APIs like geolocation, camera, mic etc require a user prompt in the browser before use. App code does not show the same browser prompt. There still may be an OS prompt – the same prompt that applies to any UWP app, but that’s usually per app not per origin.

App code also gets to use XMLHttpRequest or fetch to access cross origin content. Because UWP apps have separate state, cross origin here might not mean much to an attacker unless your app also has the user login to Facebook or some other interesting cross origin target.

PermalinkCommentsedge javascript security uwp web-security wwa

Changing the User Agent string in UWP WebView

Oct 23, 9:32

There's no perfect way to change the user agent string for the UWP WebView (x-ms-webview in HTML, Windows.UI.Xaml.Controls.WebView in XAML, and Windows.Web.UI.Interop.WebViewControl in Win32) but there are two imperfect methods folks end up using.

The first is to call UrlMkSetSessionOption. This is an old public API that allows you to configure various arcane options including one that is the default user agent string for requests running through urlmon. This API is allowed by the Microsoft Store for UWP apps. The change it applies is process wide which has two potential drawbacks. If you want to be able to have different UA strings set for different requests from a WebView that's not really possible with this solution. The other drawback is if you're using out of process WebView, you need to ensure you're calling into UrlMkSetSessionOption in the WebView's process. You'll need to write third party WinRT that calls UrlMkSetSessionOption, create the out of proc WebView, navigate it to some trusted local page, use AddWebAllowedObject or provide that URI WinRT access, and call into your third party WinRT. You'll need to do that for any new WebView process you create.

The second less generally applicable solution is to use NavigateWithHttpRequestMessage and set the User-Agent HTTP header. In this case you get to control the scope of the user agent string changes but has the limitations that not all sub resource downloads will use this user agent string and for navigations you don't initiate you have to manually intercept and re-request being careful to transfer over all POST body state and HTTP headers correctly. That last part is not actually possible for iframes.

PermalinkCommentsuser-agent uwp webview

Win10 PWA Terminology

May 31, 8:26

Folks familiar with JavaScript UWP apps in Win10 have often been confused by what PWAs in Win10 actually are. TLDR: PWAs in Win10 are simply JavaScript UWP apps. The main difference between these JS UWP Apps and our non-PWA JS UWP apps are our target end developer audience, and how we get Win10 PWAs into the Microsoft Store. See this Win10 blog post on PWAs on Win10 for related info.

Web App

On the web a subset of web sites are web apps. These are web sites that have app like behavior - that is a user might call it an app like Outlook, Maps or Gmail. And they may also have a W3C app manifest.

A subset of web apps are progressive web apps. Progressive web apps are web apps that have a W3C app manifest and a service worker. Various OSes are beginning to support PWAs as first class apps on their platform. This is true for Win10 as well in which PWAs are run as a WWA.

Windows Web App

In Win10 a WWA (Windows Web App) is an unofficial term for a JavaScript UWP app. These are UWP apps so they have an AppxManifest.xml, they are packaged in an Appx package, they run in an App Container, they use WinRT APIs, and are installed via the Microsoft Store. Specific to WWAs though, is that the AppxManifest.xml specifies a StartPage attribute identifying some HTML content to be used as the app. When the app is activated the OS will create a WWAHost.exe process that hosts the HTML content using the EdgeHtml rendering engine.

Packaged vs Hosted Web App

Within that we have a notion of a packaged web app and an HWA (hosted web app). There's no real technical distinction for the end developer between these two. The only real difference is whether the StartPage identifies remote HTML content on the web (HWA), or packaged HTML content from the app's appx package (packaged web app). An end developer may create an app that is a mix of these as well, with HTML content in the package and HTML content from the web. These terms are more like ends on a continuum and identifying two different developer scenarios since the underlying technical aspect is pretty much identical.

Win10 PWA

Win10 PWAs are simply HWAs that specify a StartPage of a URI for a PWA on the web. These are still JavaScript UWP apps with all the same behavior and abilities as other UWP apps. We have two ways of getting PWAs into the Microsoft Store as Win10 PWAs. The first is PWA Builder which is a tool that helps PWA end developers create and submit to the Microsoft Store a Win10 PWA appx package. The second is a crawler that runs over the web looking for PWAs which we convert and submit to the Store using an automated PWA Builder-like tool to create a Win10 PWA from PWAs on the web (see Welcoming PWAs to Win10 for more info). In both cases the conversion involves examining the PWAs W3C app manifest and producing a corresponding AppxManifest.xml. Not all features supported by AppxManifest.xml are also available in the W3c app manifest. But the result of PWA Builder can be a working starting point for end developers who can then update the AppxManifest.xml as they like to support features like share targets or others not available in W3C app manifests.

PermalinkCommentsJS pwa uwp web

Multiple Windows in Win10 JavaScript UWP apps

Mar 10, 1:47

Win10 Changes

In Win8.1 JavaScript UWP apps we supported multiple windows using MSApp DOM APIs. In Win10 we use and window and a new MSApp API getViewId and the previous MSApp APIs are gone:

Win10 Win8.1
Create new window MSApp.createNewView
New window object window MSAppView
viewId MSApp.getViewId(window) MSAppView.viewId

WinRT viewId

We use and window for creating new windows, but then to interact with WinRT APIs we add the MSApp.getViewId API. It takes a window object as a parameter and returns a viewId number that can be used with the various Windows.UI.ViewManagement.ApplicationViewSwitcher APIs.

Delaying Visibility

Views in WinRT normally start hidden and the end developer uses something like TryShowAsStandaloneAsync to display the view once it is fully prepared. In the web world, shows a window immediately and the end user can watch as content is loaded and rendered. To have your new windows act like views in WinRT and not display immediately we have added a option. For example
let newWindow ="", null, "msHideView=yes");

Primary Window Differences

The primary window that is initially opened by the OS acts differently than the secondary windows that it opens:

Primary Secondary Allowed Disallowed
window.close Close app Close window
Navigation restrictions ACUR only No restrictions

The restriction on secondary windows such that they cannot open secondary windows could change in the future depending on feedback.

Same Origin Communication Restrictions

Lastly, there is a very difficult technical issue preventing us from properly supporting synchronous, same-origin, cross-window, script calls. That is, when you open a window that's same origin, script in one window is allowed to directly call functions in the other window and some of these calls will fail. postMessage calls work just fine and is the recommended way to do things if that's possible for you. Otherwise we continue to work on improving this.


Win10 UWP WebView AddWebAllowedObject details

2017 Sep 4, 3:09

The x-ms-webview HTML element has the void addWebAllowedObject(string name, any value) method and the webview XAML element has the void AddWebAllowedObject(String name, Object value) method. The object parameter is projected into the webview’s top-level HTML document’s script engine as a new property on the global object with property name set to the name parameter. It is not injected into the current document but rather it is projected during initialization of the next top-level HTML document to which the webview navigates.


If AddWebAllowedObject is called during a NavigationStarting event handler the object will be injected into the document resulting from the navigation corresponding to that event.

If AddWebAllowedObject is called outside of the NavigationStarting event handler it will apply to the navigation corresponding to the next explicit navigate method called on the webview or the navigation corresponding to the next NavigationStarting event handler that fires, whichever comes first.

To avoid this potential race, you should use AddWebAllowedObject in one of two ways: 1. During a NavigationStarting event handler, 2. Before calling a Navigate method and without returning to the main loop.

If called both before calling a navigate method and in the NavigationStarting event handler then the result is the aggregate of all those calls.

If called multiple times for the same document with the same name the last call wins and the previous are silently ignored.

If AddWebAllowedObject is called for a navigation and that navigation fails or redirects to a different URI, the AddWebAllowedObject call is silently ignored.

After successfully adding an object to a document, the object will no longer be projected once a navigation to a new document occurs.

WinRT access

If AddWebAllowedObject is called for a document with All WinRT access then projection will succeed and the object will be added.

If AddWebAllowedObject is called for a document which has a URI which has no declared WinRT access via ApplicationContentUriRules then Allow for web only WinRT access is given to that document.

If the document has Allow for web only WinRT access then projection will succeed only if the object’s runtimeclass has the Windows.Foundation.Metadata.AllowForWeb metadata attribute.

Object requirements

The object must implement the IAgileObject interface. Because the XAML and HTML webview elements run on ASTA view threads and the webview’s content’s JavaScript thread runs on another ASTA thread a developer should not create their non-agile runtimeclass on the view thread. To encourage end developers to do this correctly we require the object implements IAgileObject.

Property name

The name parameter must be a valid JavaScript property name, otherwise the call will fail silently. If the name is already a property name on the global object, that property is overwritten if the property is configurable. Non-configurable properties on the global object are not overwritten and the AddWebAllowedObject call fails silently. On success, the projected property is writable, configurable, and enumerable.


Some errors as described above fail silently. Other issues, such as lack of IAgileObject or lack of the AllowForWeb attribute result in an error in the JavaScript developer console.


Application Content URI Rule effects

2017 Jun 30, 3:01

Previously I described Application Content URI Rules (ACUR) parsing and ACUR ordering. This post describes what you get from putting a URI in ACUR.

URIs in the ACUR gain the following which is otherwise unavailable:

  • Geoloc API usage
  • Audio and video capture API usage
  • Pointer lock API usage
  • Web notifications API usage
  • IndexedDB API usage
  • Clipboard API usage
  • window.external.notify access from within webview
  • window.close the primary window
  • Top level navigation in the primary window
  • Cross origin XHR and fetch to ms-appx(-web) scheme URIs
  • Cross origin dirtied canvas read access if dirtied by ms-appx(-web) scheme URIs
  • Cross origin text track for video element for tracks from ms-appx(-web) scheme URIs

URIs in the ACUR that also have full WinRT access additionally gain the following:

  • Cross origin XHR and fetch
  • Cross origin dirtied canvas read access
  • Cross origin text track for video element
  • Local audio and video WinRT plugins work with media elements
PermalinkCommentsapplication-content-uri-rules coding javascript programming windows-store

Application Content URI Rules rule ordering

2017 Jun 1, 1:30

Application Content URI Rules (ACUR from now on) defines the bounds on the web that make up a Microsoft Store application. The previous blog post discussed the syntax of the Rule's Match attribute and this time I'll write about the interactions between the Rules elements.


A single ApplicationContentUriRules element may have up to 100 Rule child elements. When determining if a navigation URI matches any of the ACUR the last Rule in the list with a matching match wildcard URI is used. If that Rule is an include rule then the navigation URI is determined to be an application content URI and if that Rule is an exclude rule then the navigation rule is not an application content URI. For example:

Rule Type='include' Match=''/
Rule Type='exclude' Match=''/

Given the above two rules in that order, the navigation URI is not an application content URI because the last matching rule is the exclude rule. Reverse the order of the rules and get the opposite result.


In addition to determining if a navigation URI is application content or not, a Rule may also confer varying levels of WinRT access via the optional WindowsRuntimeAccess attribute which may be set to 'none', 'allowForWeb', or 'all'. If a navigation URI matches multiple different include rules only the last rule is applied even as it applies to the WindowsRuntimeAccess attribute. For example:

Rule Type='include' Match='' WindowsRuntimeAccess='none'/
Rule Type='include' Match='' WindowsRuntimeAccess='all'/

Given the above two rules in that order, the navigation URI will have access to all WinRT APIs because the last matching rule wins. Reverse the rule order and the navigation URI will have no access to WinRT. There is no summation or combining of multiple matching rules - only the last matching rule wins.

PermalinkCommentsapplication-content-uri-rules programming uri windows windows-store

Tweet from David Risney

2016 Nov 4, 4:05
WinMDGraph graphs relationships between WinRT APIs . My initial version is up.

Tweet from David Risney

2016 Nov 2, 1:16
@osterman @ericlaw 'CLR cooks' I assume means you get the .NET projection of the WinRT APIs? That's good enough for most of my purposes.

Tweet from David Risney

2016 Nov 2, 9:30
Parsing WinRT metadata (winmd files) is much easier than last time I tried. There's support in .NET reflection APIs 

Parsing WinMD with .NET reflection APIs

2016 Nov 2, 6:13

Parsing WinMD files, the containers of WinRT API metadata, is relatively simple using the appropriate .NET reflection APIs. However, figuring out which reflection APIs to use is not obvious. I've got a completed C sharp class parsing WinMD files that you can check out for reference.

Use System.Reflection.Assembly.ReflectionOnlyLoad to load the WinMD file. Don't use the normal load methods because the WinMD files contain only metadata. This will load up info about APIs defined in that WinMD, but any references to types outside of that WinMD including types found in the normal OS system WinMD files must be resolved by the app code via the System.Reflection.InteropServices.WindowsRuntimeMetadata.ReflectionOnlyNamespaceResolve event.

In this event handler you must resolve the unknown namespace reference by adding an assembly to the NamespaceResolveEventArgs's ResolvedAssemblies property. If you're only interested in OS system WinMD files you can use System.Reflection.InteropServices.WindowsRuntimeMetadata.ResolveNamespace to turn a namespace into the expected OS system WinMD path and turn that path into an assembly with ReflectionOnlyLoad. code programming winmd winrt

WinRT Toast from PowerShell

2016 Jun 15, 3:54

I've made a PowerShell script to show system toast notifications with WinRT and PowerShell. Along the way I learned several interesting things.

First off calling WinRT from PowerShell involves a strange syntax. If you want to use a class you write [-Class-,-Namespace-,ContentType=WindowsRuntime] first to tell PowerShell about the type. For example here I create a ToastNotification object:

$toast = New-Object Windows.UI.Notifications.ToastNotification -ArgumentList $xml;
And here I call the static method CreateToastNotifier on the ToastNotificationManager class:
$notifier = [Windows.UI.Notifications.ToastNotificationManager]::CreateToastNotifier($AppUserModelId);
With this I can call WinRT methods and this is enough to show a toast but to handle the click requires a little more work.

To handle the user clicking on the toast I need to listen to the Activated event on the Toast object. However Register-ObjectEvent doesn't handle WinRT events. To work around this I created a .NET event wrapper class to turn the WinRT event into a .NET event that Register-ObjectEvent can handle. This is based on Keith Hill's blog post on calling WinRT async methods in PowerShell. With the event wrapper class I can run the following to subscribe to the event:

function WrapToastEvent {
param($target, $eventName);

Add-Type -Path (Join-Path $myPath "PoshWinRT.dll")
$wrapper = new-object "PoshWinRT.EventWrapper[Windows.UI.Notifications.ToastNotification,System.Object]";
$wrapper.Register($target, $eventName);

[void](Register-ObjectEvent -InputObject (WrapToastEvent $toast "Activated") -EventName FireEvent -Action {

To handle the Activated event I want to put focus back on the PowerShell window that created the toast. To do this I need to call the Win32 function SetForegroundWindow. Doing so from PowerShell is surprisingly easy. First you must tell PowerShell about the function:

Add-Type @"
using System;
using System.Runtime.InteropServices;
public class PInvoke {
[DllImport("user32.dll")] [return: MarshalAs(UnmanagedType.Bool)]
public static extern bool SetForegroundWindow(IntPtr hwnd);
Then to call:
[PInvoke]::SetForegroundWindow((Get-Process -id $myWindowPid).MainWindowHandle);

But figuring out the HWND to give to SetForegroundWindow isn't totally straight forward. Get-Process exposes a MainWindowHandle property but if you start a cmd.exe prompt and then run PowerShell inside of that, the PowerShell process has 0 for its MainWindowHandle property. We must follow up process parents until we find one with a MainWindowHandle:

$myWindowPid = $pid;
while ($myWindowPid -gt 0 -and (Get-Process -id $myWindowPid).MainWindowHandle -eq 0) {
$myWindowPid = (gwmi Win32_Process -filter "processid = $($myWindowPid)" | select ParentProcessId).ParentProcessId;
} c# powershell toast winrt

Windows Store App WebView Cross Origin XMLHttpRequest Behavior

2016 Jun 2, 6:45

TL;DR: Web content in a JavaScript Windows Store app or WebView in a Windows Store app that has full access to WinRT also gets to use XHR unrestricted by cross origin checks.

By default web content in a WebView control in a Windows Store App has the same sort of limitations as that web content in a web browser. However, if you give the URI of that web content full access to WinRT, then the web content also gains the ability to use XMLHttpRequest unrestricted by cross origin checks. This means no CORS checks and no OPTIONS requests. This only works if the web content's URI matches a Rule in the ApplicationContentUriRules of your app's manifest and that Rule declares WindowsRuntimeAccess="all". If it declares WinRT access as 'None' or 'AllowForWebOnly' then XHR acts as it normally does.

In terms of security, if you've already given a page access to all of WinRT which includes the HttpRequest class and other networking classes that don't perform cross origin checks, then allowing XHR to skip CORS doesn't make things worse.

PermalinkCommentsjavascript uwa uwp web webview windows winrt xhr

WinRT Launcher API in PowerShell

2016 Mar 31, 10:12
You can call WinRT APIs from PowerShell. Here's a short example using the WinRT Launcher API:
$uri = New-Object System.Uri ""
Note that like using WinRT in .NET, you use the System.Uri .NET class instead of the Windows.Foundation.Uri WinRT class which is not projected and under the covers the system will convert the System.Uri to a Windows.Foundation.Uri.

Tweet from David_Risney

2016 Feb 1, 10:44
Chakra conversions between JavaScript objects and WinRT types: … I've just updated to note how IPropertySet works.

JavaScript Types and WinRT Types

2016 Jan 21, 5:35

MSDN covers the topic of JavaScript and WinRT type conversions provided by Chakra (JavaScript Representation of Windows Runtime Types and Considerations when Using the Windows Runtime API), but for the questions I get about it I’ll try to lay out some specifics of that discussion more plainly. I’ve made a TL;DR JavaScript types and WinRT types summary table.

WinRT Conversion JavaScript
Struct ↔️ JavaScript object with matching property names
Class or interface instance JavaScript object with matching property names
Windows.Foundation.Collections.IPropertySet JavaScript object with arbitrary property names
Any DOM object

Chakra, the JavaScript engine powering the Edge browser and JavaScript Windows Store apps, does the work to project WinRT into JavaScript. It is responsible for, among other things, converting back and forth between JavaScript types and WinRT types. Some basics are intuitive, like a JavaScript string is converted back and forth with WinRT’s string representation. For other basic types check out the MSDN links at the top of the page. For structs, interface instances, class instances, and objects things are more complicated.

A struct, class instance, or interface instance in WinRT is projected into JavaScript as a JavaScript object with corresponding property names and values. This JavaScript object representation of a WinRT type can be passed into other WinRT APIs that take the same underlying type as a parameter. This JavaScript object is special in that Chakra keeps a reference to the underlying WinRT object and so it can be reused with other WinRT APIs.

However, if you start with plain JavaScript objects and want to interact with WinRT APIs that take non-basic WinRT types, your options are less plentiful. You can use a plain JavaScript object as a WinRT struct, so long as the property names on the JavaScript object match the WinRT struct’s. Chakra will implicitly create an instance of the WinRT struct for you when you call a WinRT method that takes that WinRT struct as a parameter and fill in the WinRT struct’s values with the values from the corresponding properties on your JavaScript object.

// C# WinRT component
public struct ExampleStruct
public string String;
public int Int;

public sealed class ExampleStructContainer
ExampleStruct value;
public void Set(ExampleStruct value)
this.value = value;

public ExampleStruct Get()
return this.value;

// JS code
var structContainer = new ExampleWinRTComponent.ExampleNamespace.ExampleStructContainer();
structContainer.set({ string: "abc", int: 123 });
console.log("structContainer.get(): " + JSON.stringify(structContainer.get()));
// structContainer.get(): {"string":"abc","int":123}

You cannot have a plain JavaScript object and use it as a WinRT class instance or WinRT interface instance. Chakra does not provide such a conversion even with ES6 classes.

You cannot take a JavaScript object with arbitrary property names that are unknown at compile time and don’t correspond to a specific WinRT struct and pass that into a WinRT method. If you need to do this, you have to write additional JavaScript code to explicitly convert your arbitrary JavaScript object into an array of property name and value pairs or something else that could be represented in WinRT.

However, the other direction you can do. An instance of a Windows.Foundation.Collections.IPropertySet implementation in WinRT is projected into JavaScript as a JavaScript object with property names and values corresponding to the key and value pairs in the IPropertySet. In this way you can project a WinRT object as a JavaScript object with arbitrary property names and types. But again, the reverse is not possible. Chakra will not convert an arbitrary JavaScript object into an IPropertySet.

// C# WinRT component
public sealed class PropertySetContainer
private Windows.Foundation.Collections.IPropertySet otherValue = null;

public Windows.Foundation.Collections.IPropertySet other
return otherValue;
otherValue = value;

public sealed class PropertySet : Windows.Foundation.Collections.IPropertySet
private IDictionary map = new Dictionary();

public PropertySet()
map.Add("abc", "def");
map.Add("ghi", "jkl");
map.Add("mno", "pqr");
// ... rest of PropertySet implementation is simple wrapper around the map member.

// JS code
var propertySet = new ExampleWinRTComponent.ExampleNamespace.PropertySet();
console.log("propertySet: " + JSON.stringify(propertySet));
// propertySet: {"abc":"def","ghi":"jkl","mno":"pqr"}

var propertySetContainer = new ExampleWinRTComponent.ExampleNamespace.PropertySetContainer();
propertySetContainer.other = propertySet;
console.log("propertySetContainer.other: " + JSON.stringify(propertySetContainer.other));
// propertySetContainer.other: {"abc":"def","ghi":"jkl","mno":"pqr"}

try {
propertySetContainer.other = { "123": "456", "789": "012" };
catch (e) {
console.error("Error setting propertySetContainer.other: " + e);
// Error setting propertySetContainer.other: TypeError: Type mismatch

There’s also no way to implicitly convert a DOM object into a WinRT type. If you want to write third party WinRT code that interacts with the DOM, you must do so indirectly and explicitly in JavaScript code that is interacting with your third party WinRT. You’ll have to extract the information you want from your DOM objects to pass into WinRT methods and similarly have to pass messages out from WinRT that say what actions the JavaScript should perform on the DOM.

PermalinkCommentschakra development javascript winrt

Debugging anecdote - the color transparent black breaks accessibility

2014 May 22, 10:36

Some time back while I was working on getting the Javascript Windows Store app platform running on Windows Phone (now available on the last Windows Phone release!) I had an interesting bug that in retrospect is amusing.

I had just finished a work item to get accessibility working for JS WinPhone apps when I got a new bug: With some set of JS apps, accessibility appeared to be totally broken. At that time in development the only mechanism we had to test accessibility was a test tool that runs on the PC, connects to the phone, and dumps out the accessibility tree of whatever app is running on the phone. In this bug, the tool would spin for a while and then timeout with an error and no accessibility information.

My first thought was this was an issue in my new accessibility code. However, debugging with breakpoints on my code I could see none of my code was run nor the code that should call it. The code that called that code was a more generic messaging system that hit my breakpoints constantly.

Rather than trying to work backward from the failure point, I decided to try and narrow down the repro and work forwards from there. One thing all the apps with the bug had in common was their usage of WinJS, but not all WinJS apps demonstrated the issue. Using a binary search approach on one such app I removed unrelated app code until all that was left was the app's usage of the WinJS AppBar and the bug still occurred. I replaced the WinJS AppBar usage with direct usage of the underlying AppBar WinRT APIs and continued.

Only some calls to the AppBar WinRT object produced the issue:

        var appBar = Windows.UI.WebUI.Core.WebUICommandBar.getForCurrentView(); 
// appBar.opacity = 1;
// appBar.closeDisplayMode = Windows.UI.WebUI.Core.WebUICommandBarClosedDisplayMode.default;
appBar.backgroundColor = Windows.UI.Colors.white; // Bug!
Just setting the background color appeared to cause the issue and I didn't even have to display the AppBar. Through additional trial and error I was blown away to discover that some colors I would set caused the issue and other colors did not. Black wouldn't cause the issue but transparent black would. So would aqua but not white.

I eventually realized that predefined WinRT color values like Windows.UI.Colors.aqua would cause the issue while JS literal based colors didn't cause the issue (Windows.UI.Color is a WinRT struct which projects in JS as a JS literal object with the struct members as JS object properties so its easy to write something like {r: 0, g: 0, b: 0, a: 0} to make a color) and I had been mixing both in my tests without realizing there would be a difference. I debugged into the backgroundColor property setter that consumed the WinRT color struct to see what was different between and {a: 1, r: 0, g: 0, b: 0} and found the two structs to be byte wise exactly the same.

On a hunch I tried my test app with only a reference to the color and otherwise no interaction with the AppBar and not doing anything with the actual reference to the color:;. This too caused the issue. I knew that the implementation for these WinRT const values live in a DLL and guessed that something in the code to create these predefined colors was causing the issue. I debugged in and no luck. Now I also have experienced crusty code that would do exciting things in its DllMain, the function that's called when a DLL is loaded into the process so I tried modifying my C++ code to simply LoadLibrary the DLL containing the WinRT color definition, windows.ui.xaml.dll and found the bug still occurred! A short lived moment of relief as the world seemed to make sense again.

Debugging into DllMain nothing interesting happened. There were interesting calls in there to be sure, but all of them behind conditions that were false. I was again stumped. On another hunch I tried renaming the DLL and only LoadLibrary'ing it and the bug went away. I took a different DLL renamed it windows.ui.xaml.dll and tried LoadLibrary'ing that and the bug came back. Just the name of the DLL was causing the issue.

I searched for the DLL name in our source code index and found hits in the accessibility tool. Grinning I opened the source to find that the accessibility tool's phone side service was trying to determine if a process belonged to a XAML app or not because XAML apps had a different accessibility contract. It did this by checking to see if windows.ui.xaml.dll was loaded in the target process.

At this point I got to fix my main issue and open several new bugs for the variety of problems I had just run into. This is a how to on writing software that is difficult to debug.

PermalinkCommentsbug debug javascript JS technical windows winrt

Serializing JavaScript Promise Execution

2013 Aug 10, 3:07
Occasionally I have need to run a set of unrelated promises in series, for instance an object dealing with a WinRT camera API that can only execute one async operation at a time, or an object handling postMessage message events and must resolve associated async operations in the same order it received the requests. The solution is very simply to keep track of the last promise and when adding a new promise in serial add a continuation of the last promise to execute the new promise and point the last promise at the result. I encapsulate the simple solution in a simple constructor:

    function PromiseExecutionSerializer() {
var lastPromise = WinJS.Promise.wrap(); // Start with an empty fulfilled promise.

this.addPromiseForSerializedExecution = function(promiseFunction) {
lastPromise = lastPromise.then(function () {
// Don't call directly so next promise doesn't get previous result parameter.
return promiseFunction();

The only thing to watch out for is to ensure you don't pass the result of a previous promise onto a subsequent promise that is unrelated.
PermalinkCommentsasync javascript promise technical

URI functions in Windows Store Applications

2013 Jul 25, 1:00


The Modern SDK contains some URI related functionality as do libraries available in particular projection languages. Unfortunately, collectively these APIs do not cover all scenarios in all languages. Specifically, JavaScript and C++ have no URI building APIs, and C++ additionally has no percent-encoding/decoding APIs.
WinRT (JS and C++)
JS Only
C++ Only
.NET Only
Relative resolution
Encode data for including in URI property
Decode data extracted from URI property
Build Query
Parse Query
The Windows.Foudnation.Uri type is not projected into .NET modern applications. Instead those applications use System.Uri and the platform ensures that it is correctly converted back and forth between Windows.Foundation.Uri as appropriate. Accordingly the column marked WinRT above is applicable to JS and C++ modern applications but not .NET modern applications. The only entries above applicable to .NET are the .NET Only column and the WwwFormUrlDecoder in the bottom left which is available to .NET.



This functionality is provided by the WinRT API Windows.Foundation.Uri in C++ and JS, and by System.Uri in .NET.
Parsing a URI pulls it apart into its basic components without decoding or otherwise modifying the contents.
var uri = new Windows.Foundation.Uri("");
console.log(uri.path);// /path%20segment1/path%20segment2

WsDecodeUrl (C++)

WsDecodeUrl is not suitable for general purpose URI parsing.  Use Windows.Foundation.Uri instead.

Build (C#)

URI building is only available in C# via System.UriBuilder.
URI building is the inverse of URI parsing: URI building allows the developer to specify the value of basic components of a URI and the API assembles them into a URI. 
To work around the lack of a URI building API developers will likely concatenate strings to form their URIs.  This can lead to injection bugs if they don’t validate or encode their input properly, but if based on trusted or known input is unlikely to have issues.
            Uri originalUri = new Uri("");
            UriBuilder uriBuilder = new UriBuilder(originalUri);
            uriBuilder.Path = "/path2/";
            Uri newUri = uriBuilder.Uri; //

WsEncodeUrl (C++)

WsEncodeUrl, in addition to building a URI from components also does some encoding.  It encodes non-US-ASCII characters as UTF8, the percent, and a subset of gen-delims based on the URI property: all :/?#[]@ are percent-encoded except :/@ in the path and :/?@ in query and fragment.
Accordingly, WsEncodeUrl is not suitable for general purpose URI building.  It is acceptable to use in the following cases:
- You’re building a URI out of non-encoded URI properties and don’t care about the difference between encoded and decoded characters.  For instance you’re the only one consuming the URI and you uniformly decode URI properties when consuming – for instance using WsDecodeUrl to consume the URI.
- You’re building a URI with URI properties that don’t contain any of the characters that WsEncodeUrl encodes.


This functionality is provided by the WinRT API Windows.Foundation.Uri in C++ and JS and by System.Uri in .NET.  Normalization is applied during construction of the Uri object.
URI normalization is the application of URI normalization rules (including DNS normalization, IDN normalization, percent-encoding normalization, etc.) to the input URI.
        var normalizedUri = new Windows.Foundation.Uri("HTTP://EXAMPLE.COM/p%61th foo/");
        console.log(normalizedUri.absoluteUri); //
This is modulo Win8 812823 in which the Windows.Foundation.Uri.AbsoluteUri property returns a normalized IRI not a normalized URI.  This bug does not affect System.Uri.AbsoluteUri which returns a normalized URI.


This functionality is provided by the WinRT API Windows.Foundation.Uri in C++ and JS and by System.Uri in .NET. 
URI equality determines if two URIs are equal or not necessarily equal.
            var uri1 = new Windows.Foundation.Uri("HTTP://EXAMPLE.COM/p%61th foo/"),
                uri2 = new Windows.Foundation.Uri("");
            console.log(uri1.equals(uri2)); // true

Relative resolution

This functionality is provided by the WinRT API Windows.Foundation.Uri in C++ and JS and by System.Uri in .NET 
Relative resolution is a function that given an absolute URI A and a relative URI B, produces a new absolute URI C.  C is the combination of A and B in which the basic components specified in B override or combine with those in A under rules specified in RFC 3986.
        var baseUri = new Windows.Foundation.Uri(""),
            relativeUri = "/path?query#fragment",
            absoluteUri = baseUri.combineUri(relativeUri);
        console.log(baseUri.absoluteUri);       //
        console.log(absoluteUri.absoluteUri);   //

Encode data for including in URI property

This functionality is available in JavaScript via encodeURIComponent and in C# via System.Uri.EscapeDataString. Although the two methods mentioned above will suffice for this purpose, they do not perform exactly the same operation.
Additionally we now have Windows.Foundation.Uri.EscapeComponent in WinRT, which is available in JavaScript and C++ (not C# since it doesn’t have access to Windows.Foundation.Uri).  This is also slightly different from the previously mentioned mechanisms but works best for this purpose.
Encoding data for inclusion in a URI property is necessary when constructing a URI from data.  In all the above cases the developer is dealing with a URI or substrings of a URI and so the strings are all encoded as appropriate. For instance, in the parsing example the path contains “path%20segment1” and not “path segment1”.  To construct a URI one must first construct the basic components of the URI which involves encoding the data.  For example, if one wanted to include “path segment / example” in the path of a URI, one must percent-encode the ‘ ‘ since it is not allowed in a URI, as well as the ‘/’ since although it is allowed, it is a delimiter and won’t be interpreted as data unless encoded.
If a developer does not have this API provided they can write it themselves.  Percent-encoding methods appear simple to write, but the difficult part is getting the set of characters to encode correct, as well as handling non-US-ASCII characters.
        var uri = new Windows.Foundation.Uri("" +
            "/" + Windows.Foundation.Uri.escapeComponent("path segment / example") +
            "?key=" + Windows.Foundation.Uri.escapeComponent("=&?#"));
        console.log(uri.absoluteUri); //

WsEncodeUrl (C++)

In addition to building a URI from components, WsEncodeUrl also percent-encodes some characters.  However the API is not recommend for this scenario given the particular set of characters that are encoded and the convoluted nature in which a developer would have to use this API in order to use it for this purpose.
There are no general purpose scenarios for which the characters WsEncodeUrl encodes make sense: encode the %, encode a subset of gen-delims but not also encode the sub-delims.  For instance this could not replace encodeURIComponent in a C++ version of the following code snippet since if ‘value’ contained ‘&’ or ‘=’ (both sub-delims) they wouldn’t be encoded and would be confused for delimiters in the name value pairs in the query:
"" + Windows.Foundation.Uri.escapeComponent(value)
Since WsEncodeUrl produces a string URI, to obtain the property they want to encode they’d need to parse the resulting URI.  WsDecodeUrl won’t work because it decodes the property but Windows.Foundation.Uri doesn’t decode.  Accordingly the developer could run their string through WsEncodeUrl then Windows.Foundation.Uri to extract the property.

Decode data extracted from URI property

This functionality is available in JavaScript via decodeURIComponent and in C# via System.Uri.UnescapeDataString. Although the two methods mentioned above will suffice for this purpose, they do not perform exactly the same operation.
Additionally we now also have Windows.Foundation.Uri.UnescapeComponent in WinRT, which is available in JavaScript and C++ (not C# since it doesn’t have access to Windows.Foundation.Uri).  This is also slightly different from the previously mentioned mechanisms but works best for this purpose.
Decoding is necessary when extracting data from a parsed URI property.  For example, if a URI query contains a series of name and value pairs delimited by ‘=’ between names and values, and by ‘&’ between pairs, one must first parse the query into name and value entries and then decode the values.  It is necessary to make this an extra step separate from parsing the URI property so that sub-delimiters (in this case ‘&’ and ‘=’) that are encoded will be interpreted as data, and those that are decoded will be interpreted as delimiters.
If a developer does not have this API provided they can write it themselves.  Percent-decoding methods appear simple to write, but have some tricky parts including correctly handling non-US-ASCII, and remembering not to decode .
In the following example, note that if unescapeComponent were called first, the encoded ‘&’ and ‘=’ would be decoded and interfere with the parsing of the name value pairs in the query.
            var uri = new Windows.Foundation.Uri("");
                function (keyValueString) {
                    var keyValue = keyValueString.split("=");
                    console.log(Windows.Foundation.Uri.unescapeComponent(keyValue[0]) + ": " + Windows.Foundation.Uri.unescapeComponent(keyValue[1]));
                    // foo: bar
                    // array: ['','&','=','#']

WsDecodeUrl (C++)

Since WsDecodeUrl decodes all percent-encoded octets it could be used for general purpose percent-decoding but it takes a URI so would require the dev to construct a stub URI around the string they want to decode.  For example they could prefix “http:///#” to their string, run it through WsDecodeUrl and then extract the fragment property.  It is convoluted but will work correctly.

Parse Query

The query of a URI is often encoded as application/x-www-form-urlencoded which is percent-encoded name value pairs delimited by ‘&’ between pairs and ‘=’ between corresponding names and values.
In WinRT we have a class to parse this form of encoding using Windows.Foundation.WwwFormUrlDecoder.  The queryParsed property on the Windows.Foundation.Uri class is of this type and created with the query of its Uri:
    var uri = Windows.Foundation.Uri("");
        function (pair) {
            console.log("name: " + + ", value: " + pair.value);
            // name: foo, value: bar
            // name: array, value: ['','&','=','#']
    console.log(uri.queryParsed.getFirstValueByName("array")); // ['','&','=','#']
The QueryParsed property is only on Windows.Foundation.Uri and not System.Uri and accordingly is not available in .NET.  However the Windows.Foundation.WwwFormUrlDecoder class is available in C# and can be used manually:
            Uri uri = new Uri("");
            WwwFormUrlDecoder decoder = new WwwFormUrlDecoder(uri.Query);
            foreach (IWwwFormUrlDecoderEntry entry in decoder)
                System.Diagnostics.Debug.WriteLine("name: " + entry.Name + ", value: " + entry.Value);
                // name: foo, value: bar
                // name: array, value: ['','&','=','#']

Build Query

To build a query of name value pairs encoded as application/x-www-form-urlencoded there is no WinRT API to do this directly.  Instead a developer must do this manually making use of the code described in “Encode data for including in URI property”.
In terms of public releases, this property is only in the RC and later builds.
For example in JavaScript a developer may write:
            var uri = new Windows.Foundation.Uri(""),
                query = "?" + Windows.Foundation.Uri.escapeComponent("array") + "=" + Windows.Foundation.Uri.escapeComponent("['','&','=','#']");
            console.log(uri.combine(new Windows.Foundation.Uri(query)).absoluteUri); //'%E3%84%93'%2C'%26'%2C'%3D'%2C'%23'%5D
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WinRT PropertySet Changed Event Danger

2013 Jul 8, 1:46

The Windows Runtime API Windows.Foundation.Collections.PropertySet class​ is a nice string name to object value map that has a changed event that fires when the contents of the map is modified. Be careful with this event because it fires synchronously from the thread on which the PropertySet was modified. If modified from the UI thread, the UI thread will then wait as it synchronously dispatches the changed event to all listeners which could lead to performance issues or especially from the UI thread deadlock. For instance, deadlock if you have two threads both trying to tell each other about changed events for different PropertySets.

PermalinkCommentsdeadlock development propertyset windows windows-runtime winrt
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