Optimizations/refactoring/tweaks to DispatcherQueueHelper (#3498)

## Follow up for #3206
<!-- Add the relevant issue number after the "#" mentioned above (for ex: Fixes #1234) which will automatically close the issue once the PR is merged. -->

<!-- Add a brief overview here of the feature/bug & fix. -->

## PR Type
What kind of change does this PR introduce?
<!-- Please uncomment one or more that apply to this PR. -->

- Bugfix
- Code style update (formatting)
- Refactoring (no functional changes, ~~no api changes~~)
- Optimization

## What is the new behavior?
<!-- Describe how was this issue resolved or changed? -->
This PR includes a number of optimizations, tweaks and refactorings to `DispatcherQueueHelper`:

- Removed unnecessary `null` checks for `function`, and enabled nullability annotations. Those `null` checks were unnecessary since there were also other parameters that could also be `null` (ie. the actual `DispatcherQueue`) anyway, plus we'd still get an exception anyway when those instances are accessed. Instead, we now have nullability annotations to make the intent clearer.
- Avoided the slowdown and memory allocations in case the thread already had access to the dispatcher queue, which was caused by the C# compiler (sadly) always preemptively instantiating the display class for the closure of the lambda expression used in the other branch (I made a sharplab repro [here](https://sharplab.io/#v2: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) that illustrates the issue). Instead now the fallback path is moved to a separate method, so that the IL has the correct codegen.
- Renamed the class to `DispatcherQueueExtensions`, since these are all extensions and not a helper class
- Similarly, moved the class to the `.Extensions` namespace
- Renamed the APIs to `EnqueueAsync` to mirror the `TryEnqueue` name of the actual API in the `DispatcherQueue` class. This better follows the naming convention of the class, and it's also clearer as in case users have multiple windows and different dispatching queues, there isn't really a single "UI thread" - the API is literally just enqueueing an operation on a given dispatcher queue.
- Added a check in case `TryEnqueue` fails, so if that happens we now return a wrapped `InvalidOperationException` in the `Task`, whereas the previous behavior would've just caused that task to never be completed, leaving the caller just waiting forever.

## PR Checklist

Please check if your PR fulfills the following requirements:

- [X] Tested code with current [supported SDKs](../readme.md#supported)
- [ ] ~~Pull Request has been submitted to the documentation repository [instructions](..\contributing.md#docs). Link: <!-- docs PR link -->~~
- [ ] ~~Sample in sample app has been added / updated (for bug fixes / features)~~
    - [ ] ~~Icon has been created (if new sample) following the [Thumbnail Style Guide and templates](https://github.com/windows-toolkit/WindowsCommunityToolkit-design-assets)~~
- [ ] ~~Tests for the changes have been added (for bug fixes / features) (if applicable)~~
- [X] Header has been added to all new source files (run *build/UpdateHeaders.bat*)
- [ ] Contains **NO** breaking changes

Author: msftbot[bot]

Microsoft.Toolkit.Uwp.Connectivity/BluetoothLEHelper/BluetoothLEHelper.cs

@@ -8,7 +8,7 @@
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
-using Microsoft.Toolkit.Uwp.Helpers;
+using Microsoft.Toolkit.Uwp.Extensions;
 using Windows.Devices.Bluetooth;
 using Windows.Devices.Bluetooth.Advertisement;
 using Windows.Devices.Enumeration;
@@ -197,7 +197,7 @@ private async Task Init()
         /// <param name="args">The advertisement.</param>
         private async void AdvertisementWatcher_Received(BluetoothLEAdvertisementWatcher sender, BluetoothLEAdvertisementReceivedEventArgs args)
         {
-            await DispatcherQueue.ExecuteOnUIThreadAsync(
+            await DispatcherQueue.EnqueueAsync(
                 () =>
                 {
                     if (_readerWriterLockSlim.TryEnterReadLock(TimeSpan.FromSeconds(1)))
@@ -281,7 +281,7 @@ private async void DeviceWatcher_Removed(DeviceWatcher sender, DeviceInformation
             // Protect against race condition if the task runs after the app stopped the deviceWatcher.
             if (sender == _deviceWatcher)
             {
-                await DispatcherQueue.ExecuteOnUIThreadAsync(
+                await DispatcherQueue.EnqueueAsync(
                     () =>
                     {
                         if (_readerWriterLockSlim.TryEnterWriteLock(TimeSpan.FromSeconds(1)))
@@ -331,7 +331,7 @@ private async Task AddDeviceToList(DeviceInformation deviceInfo)
 
                 if (connectable)
                 {
-                    await DispatcherQueue.ExecuteOnUIThreadAsync(
+                    await DispatcherQueue.EnqueueAsync(
                         () =>
                         {
                             if (_readerWriterLockSlim.TryEnterWriteLock(TimeSpan.FromSeconds(1)))

Microsoft.Toolkit.Uwp.Connectivity/BluetoothLEHelper/ObservableBluetoothLEDevice.cs

@@ -11,12 +11,11 @@
 using System.Runtime.CompilerServices;
 using System.Threading;
 using System.Threading.Tasks;
-using Microsoft.Toolkit.Uwp.Helpers;
+using Microsoft.Toolkit.Uwp.Extensions;
 using Windows.Devices.Bluetooth;
 using Windows.Devices.Bluetooth.GenericAttributeProfile;
 using Windows.Devices.Enumeration;
 using Windows.System;
-using Windows.UI.Core;
 using Windows.UI.Xaml.Media.Imaging;
 
 namespace Microsoft.Toolkit.Uwp.Connectivity
@@ -404,7 +403,7 @@ private void ObservableBluetoothLEDevice_PropertyChanged(object sender, Property
         /// <exception cref="Exception">Throws Exception when no permission to access device</exception>
         public async Task ConnectAsync()
         {
-            await DispatcherQueue.ExecuteOnUIThreadAsync(
+            await DispatcherQueue.EnqueueAsync(
                 async () =>
             {
                 if (BluetoothLEDevice == null)
@@ -478,7 +477,7 @@ public async Task DoInAppPairingAsync()
         /// <returns>The task of the update.</returns>
         public async Task UpdateAsync(DeviceInformationUpdate deviceUpdate)
         {
-            await DispatcherQueue.ExecuteOnUIThreadAsync(
+            await DispatcherQueue.EnqueueAsync(
                 () =>
                 {
                     DeviceInfo.Update(deviceUpdate);
@@ -521,7 +520,7 @@ protected virtual void OnPropertyChanged([CallerMemberName] string propertyName
         /// <param name="args">The arguments.</param>
         private async void BluetoothLEDevice_NameChanged(BluetoothLEDevice sender, object args)
         {
-            await DispatcherQueue.ExecuteOnUIThreadAsync(() => { Name = BluetoothLEDevice.Name; }, DispatcherQueuePriority.Normal);
+            await DispatcherQueue.EnqueueAsync(() => { Name = BluetoothLEDevice.Name; }, DispatcherQueuePriority.Normal);
         }
 
         /// <summary>
@@ -531,7 +530,7 @@ private async void BluetoothLEDevice_NameChanged(BluetoothLEDevice sender, objec
         /// <param name="args">The arguments.</param>
         private async void BluetoothLEDevice_ConnectionStatusChanged(BluetoothLEDevice sender, object args)
         {
-            await DispatcherQueue.ExecuteOnUIThreadAsync(
+            await DispatcherQueue.EnqueueAsync(
                 () =>
                 {
                     IsPaired = DeviceInfo.Pairing.IsPaired;
@@ -544,7 +543,7 @@ await DispatcherQueue.ExecuteOnUIThreadAsync(
         /// </summary>
         private async void LoadGlyph()
         {
-            await DispatcherQueue.ExecuteOnUIThreadAsync(
+            await DispatcherQueue.EnqueueAsync(
                 async () =>
                 {
                     var deviceThumbnail = await DeviceInfo.GetGlyphThumbnailAsync();

Microsoft.Toolkit.Uwp.Connectivity/BluetoothLEHelper/ObservableGattCharacteristics.cs

@@ -7,7 +7,7 @@
 using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Threading.Tasks;
-using Microsoft.Toolkit.Uwp.Helpers;
+using Microsoft.Toolkit.Uwp.Extensions;
 using Windows.Devices.Bluetooth;
 using Windows.Devices.Bluetooth.GenericAttributeProfile;
 using Windows.Security.Cryptography;
@@ -469,7 +469,7 @@ protected virtual void OnPropertyChanged([CallerMemberName] string propertyName
         /// <param name="args">The <see cref="GattValueChangedEventArgs"/> instance containing the event data.</param>
         private async void Characteristic_ValueChanged(GattCharacteristic sender, GattValueChangedEventArgs args)
         {
-            await DispatcherQueue.ExecuteOnUIThreadAsync(() => { SetValue(args.CharacteristicValue); }, DispatcherQueuePriority.Normal);
+            await DispatcherQueue.EnqueueAsync(() => { SetValue(args.CharacteristicValue); }, DispatcherQueuePriority.Normal);
         }
 
         /// <summary>

Microsoft.Toolkit.Uwp.SampleApp/SamplePages/DispatcherQueueHelper/DispatcherQueueHelperPage.xaml.cs

@@ -6,6 +6,7 @@
 using Microsoft.Toolkit.Uwp.Helpers;
 using Windows.System;
 using Windows.UI.Xaml;
+using Microsoft.Toolkit.Uwp.Extensions;
 
 namespace Microsoft.Toolkit.Uwp.SampleApp.SamplePages
 {
@@ -21,7 +22,7 @@ private async void ExecuteFromDifferentThreadButton_Click(object sender, RoutedE
             var dispatcherQueue = DispatcherQueue.GetForCurrentThread();
             int crossThreadReturnedValue = await Task.Run<int>(async () =>
             {
-                int returnedFromUIThread = await dispatcherQueue.ExecuteOnUIThreadAsync<int>(() =>
+                int returnedFromUIThread = await dispatcherQueue.EnqueueAsync<int>(() =>
                 {
                     NormalTextBlock.Text = "Updated from a random thread!";
                     return 1;

Microsoft.Toolkit.Uwp.UI/Cache/ImageCache.cs

@@ -7,9 +7,8 @@
 using System.IO;
 using System.Reflection;
 using System.Threading.Tasks;
-using Microsoft.Toolkit.Uwp.Helpers;
+using Microsoft.Toolkit.Uwp.Extensions;
 using Windows.Storage;
-using Windows.Storage.Streams;
 using Windows.System;
 using Windows.UI.Xaml.Media.Imaging;
 
@@ -63,7 +62,7 @@ protected override async Task<BitmapImage> InitializeTypeAsync(Stream stream, Li
                 throw new FileNotFoundException();
             }
 
-            return await DispatcherQueue.ExecuteOnUIThreadAsync(async () =>
+            return await DispatcherQueue.EnqueueAsync(async () =>
             {
                 BitmapImage image = new BitmapImage();
 

Microsoft.Toolkit.Uwp.UI/Extensions/ScrollViewer/ScrollViewerExtensions.MiddleClickScrolling.cs

@@ -4,7 +4,7 @@
 
 using System;
 using System.Threading;
-using Microsoft.Toolkit.Uwp.Helpers;
+using Microsoft.Toolkit.Uwp.Extensions;
 using Windows.Devices.Input;
 using Windows.Foundation;
 using Windows.System;
@@ -369,7 +369,7 @@ private static bool IsCursorResourceAvailable()
 
         private static async void RunInUIThread(DispatcherQueue dispatcherQueue, Action action)
         {
-            await dispatcherQueue.ExecuteOnUIThreadAsync(action, DispatcherQueuePriority.Normal);
+            await dispatcherQueue.EnqueueAsync(action, DispatcherQueuePriority.Normal);
         }
     }
 }

Microsoft.Toolkit.Uwp.UI/Helpers/ThemeListener.cs

@@ -3,10 +3,9 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
-using System.Diagnostics;
 using System.Runtime.CompilerServices;
 using System.Threading.Tasks;
-using Microsoft.Toolkit.Uwp.Helpers;
+using Microsoft.Toolkit.Uwp.Extensions;
 using Windows.Foundation.Metadata;
 using Windows.System;
 using Windows.UI.ViewManagement;
@@ -84,7 +83,7 @@ public ThemeListener(DispatcherQueue dispatcherQueue = null)
         private void Accessible_HighContrastChanged(AccessibilitySettings sender, object args)
         {
 #if DEBUG
-            Debug.WriteLine("HighContrast Changed");
+            System.Diagnostics.Debug.WriteLine("HighContrast Changed");
 #endif
 
             UpdateProperties();
@@ -100,15 +99,15 @@ private async void Settings_ColorValuesChanged(UISettings sender, object args)
         internal Task OnColorValuesChanged()
         {
             // Getting called off thread, so we need to dispatch to request value.
-            return DispatcherQueue.ExecuteOnUIThreadAsync(
+            return DispatcherQueue.EnqueueAsync(
                 () =>
                 {
                     // TODO: This doesn't stop the multiple calls if we're in our faked 'White' HighContrast Mode below.
                     if (CurrentTheme != Application.Current.RequestedTheme ||
                         IsHighContrast != _accessible.HighContrast)
                     {
 #if DEBUG
-                        Debug.WriteLine("Color Values Changed");
+                        System.Diagnostics.Debug.WriteLine("Color Values Changed");
 #endif
 
                         UpdateProperties();
@@ -122,7 +121,7 @@ private void CoreWindow_Activated(Windows.UI.Core.CoreWindow sender, Windows.UI.
                 IsHighContrast != _accessible.HighContrast)
             {
 #if DEBUG
-                Debug.WriteLine("CoreWindow Activated Changed");
+                System.Diagnostics.Debug.WriteLine("CoreWindow Activated Changed");
 #endif
 
                 UpdateProperties();

Microsoft.Toolkit.Uwp/Deferred/EventDeferral.cs

@@ -3,8 +3,6 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
-using System.Collections.Generic;
-using System.Text;
 using System.Threading;
 using System.Threading.Tasks;