Add MainThread.SetCustomImplementation for custom platform backends by Redth · Pull Request #34230 · dotnet/maui

Redth · 2026-02-25T02:01:47Z

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Description

Fixes #34101

On custom platform backends (e.g. Linux/GTK), MainThread.BeginInvokeOnMainThread() and MainThread.InvokeOnMainThreadAsync() throw NotImplementedInReferenceAssemblyException because there is no platform-specific partial class implementation for the netstandard TFM.

This PR adds a public MainThread.SetCustomImplementation() method that allows custom backends to register their own IsMainThread and BeginInvokeOnMainThread implementations.

Changes

MainThread.shared.cs: Added static callback fields and SetCustomImplementation(Func<bool>, Action<Action>) public method
MainThread.netstandard.cs: Modified to check for custom callbacks before throwing NotImplementedInReferenceAssemblyException
PublicAPI.Unshipped.txt: Added new public API entry for all targets
MainThread_Tests.cs: Added 6 unit tests covering the new functionality

Usage

Custom platform backends can register their implementation during startup:

MainThread.SetCustomImplementation(
    isMainThread: () => /* check if on UI thread */,
    beginInvokeOnMainThread: action => /* dispatch to UI thread */);

For example, a GTK backend might use:

MainThread.SetCustomImplementation(
    isMainThread: () => GLib.MainContext.Default.IsOwner,
    beginInvokeOnMainThread: action => GLib.Idle.Add(() => { action(); return false; }));

On standard platforms (iOS, Android, Windows, MacCatalyst), the existing platform-specific implementations continue to be used unchanged.

Fixes #34101. On custom platform backends (e.g. Linux/GTK), MainThread throws NotImplementedInReferenceAssemblyException because there is no platform-specific partial class implementation. This adds a public SetCustomImplementation method that allows custom backends to provide their own IsMainThread and BeginInvokeOnMainThread implementations. Changes: - Add SetCustomImplementation(Func<bool>, Action<Action>) to MainThread - Modify MainThread.netstandard.cs to use custom callbacks before throwing - Add unit tests verifying the new functionality - Update PublicAPI.Unshipped.txt for all targets Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Adds an opt-in mechanism for custom platform backends (e.g., Linux/GTK) to provide MainThread behavior when the portable (*.netstandard.cs) implementation would otherwise throw, addressing the “custom backend throws NotImplementedInReferenceAssemblyException” scenario.

Changes:

Added MainThread.SetCustomImplementation(Func<bool>, Action<Action>) and backing delegate fields.
Updated the netstandard MainThread implementation to use the custom delegates when present.
Added PublicAPI entries across TFMs and introduced new unit tests for the custom-implementation behavior.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
src/Essentials/src/MainThread/MainThread.shared.cs	Adds public API + stores delegates for custom main-thread detection/dispatch.
src/Essentials/src/MainThread/MainThread.netstandard.cs	Uses custom delegates before throwing the reference-assembly exception.
src/Essentials/src/PublicAPI/netstandard/PublicAPI.Unshipped.txt	Adds new API entry (but has BOM/format concerns).
src/Essentials/src/PublicAPI/net/PublicAPI.Unshipped.txt	Adds new API entry (but has BOM/format concerns).
src/Essentials/src/PublicAPI/net-windows/PublicAPI.Unshipped.txt	Adds new API entry (format appears inconsistent with shipped conventions).
src/Essentials/src/PublicAPI/net-tizen/PublicAPI.Unshipped.txt	Adds new API entry (format appears inconsistent with shipped conventions).
src/Essentials/src/PublicAPI/net-maccatalyst/PublicAPI.Unshipped.txt	Adds new API entry (format appears inconsistent with shipped conventions).
src/Essentials/src/PublicAPI/net-ios/PublicAPI.Unshipped.txt	Adds new API entry (format appears inconsistent with shipped conventions).
src/Essentials/src/PublicAPI/net-android/PublicAPI.Unshipped.txt	Adds new API entry (format appears inconsistent with shipped conventions).
src/Essentials/test/UnitTests/MainThread_Tests.cs	Adds unit tests validating custom-implementation behavior and argument validation.

Copilot · 2026-02-25T02:07:41Z

src/Essentials/src/PublicAPI/net-ios/PublicAPI.Unshipped.txt

@@ -1 +1,2 @@
 #nullable enable
+static Microsoft.Maui.ApplicationModel.MainThread.SetCustomImplementation(System.Func<bool>! isMainThread, System.Action<System.Action!>! beginInvokeOnMainThread) -> void


PublicAPI formatting here is inconsistent with the rest of this TFM's shipped APIs (which use the ~static prefix and omit nullable ! annotations for MainThread members). As written, this entry likely won't match the API as generated from the current (nullable-oblivious) source. Update the entry to the correct PublicAPI format for this assembly (likely ~static ... without !).

Suggested change

static Microsoft.Maui.ApplicationModel.MainThread.SetCustomImplementation(System.Func<bool>! isMainThread, System.Action<System.Action!>! beginInvokeOnMainThread) -> void

~static Microsoft.Maui.ApplicationModel.MainThread.SetCustomImplementation(System.Func<bool> isMainThread, System.Action<System.Action> beginInvokeOnMainThread) -> void

Copilot · 2026-02-25T02:07:41Z

src/Essentials/src/PublicAPI/net-maccatalyst/PublicAPI.Unshipped.txt

@@ -1 +1,2 @@
 #nullable enable
+static Microsoft.Maui.ApplicationModel.MainThread.SetCustomImplementation(System.Func<bool>! isMainThread, System.Action<System.Action!>! beginInvokeOnMainThread) -> void


PublicAPI formatting here is inconsistent with the rest of this TFM's shipped APIs (which use the ~static prefix and omit nullable ! annotations for MainThread members). As written, this entry likely won't match the API as generated from the current (nullable-oblivious) source. Update the entry to the correct PublicAPI format for this assembly (likely ~static ... without !).

Suggested change

static Microsoft.Maui.ApplicationModel.MainThread.SetCustomImplementation(System.Func<bool>! isMainThread, System.Action<System.Action!>! beginInvokeOnMainThread) -> void

~static Microsoft.Maui.ApplicationModel.MainThread.SetCustomImplementation(System.Func<bool> isMainThread, System.Action<System.Action> beginInvokeOnMainThread) -> void

Copilot · 2026-02-25T02:07:41Z

src/Essentials/src/MainThread/MainThread.shared.cs

+			s_isMainThreadImplementation = isMainThread ?? throw new ArgumentNullException(nameof(isMainThread));
+			s_beginInvokeOnMainThreadImplementation = beginInvokeOnMainThread ?? throw new ArgumentNullException(nameof(beginInvokeOnMainThread));


SetCustomImplementation mutates s_isMainThreadImplementation before validating beginInvokeOnMainThread. If beginInvokeOnMainThread is null, the method throws but leaves the class in a partially-updated state (IsMainThread may start returning a value instead of throwing). Validate both parameters first (or assign atomically) so the method is exception-safe and does not change state on failure.

Suggested change

s_isMainThreadImplementation = isMainThread ?? throw new ArgumentNullException(nameof(isMainThread));

s_beginInvokeOnMainThreadImplementation = beginInvokeOnMainThread ?? throw new ArgumentNullException(nameof(beginInvokeOnMainThread));

if (isMainThread is null)

throw new ArgumentNullException(nameof(isMainThread));

if (beginInvokeOnMainThread is null)

throw new ArgumentNullException(nameof(beginInvokeOnMainThread));

s_isMainThreadImplementation = isMainThread;

s_beginInvokeOnMainThreadImplementation = beginInvokeOnMainThread;

Copilot · 2026-02-25T02:07:42Z

src/Essentials/test/UnitTests/MainThread_Tests.cs

+			// Use reflection to clear the static fields since there's no public reset API
+			var type = typeof(MainThread);
+			var isMainThreadField = type.GetField("s_isMainThreadImplementation",
+				System.Reflection.BindingFlags.Static | System.Reflection.BindingFlags.NonPublic);
+			var beginInvokeField = type.GetField("s_beginInvokeOnMainThreadImplementation",
+				System.Reflection.BindingFlags.Static | System.Reflection.BindingFlags.NonPublic);
+			isMainThreadField?.SetValue(null, null);
+			beginInvokeField?.SetValue(null, null);


These tests reset MainThread state by reflecting on private field names. This is brittle (renames/ILLink changes can break tests) and bypasses InternalsVisibleTo that the Essentials assembly already provides. Consider adding an internal reset helper on MainThread (accessible to Microsoft.Maui.Essentials.UnitTests) and call that instead of reflection.

Suggested change

// Use reflection to clear the static fields since there's no public reset API

var type = typeof(MainThread);

var isMainThreadField = type.GetField("s_isMainThreadImplementation",

System.Reflection.BindingFlags.Static | System.Reflection.BindingFlags.NonPublic);

var beginInvokeField = type.GetField("s_beginInvokeOnMainThreadImplementation",

System.Reflection.BindingFlags.Static | System.Reflection.BindingFlags.NonPublic);

isMainThreadField?.SetValue(null, null);

beginInvokeField?.SetValue(null, null);

// Use the internal reset helper on MainThread instead of reflection on private fields

MainThread.Reset();

Copilot · 2026-02-25T02:07:42Z

src/Essentials/src/PublicAPI/net/PublicAPI.Unshipped.txt

@@ -1 +1,2 @@
-#nullable enable
+#nullable enable


The first line now contains a BOM character before #nullable enable (shows up as �#nullable enable). This frequently causes PublicAPI tooling diffs/failures. Please remove the BOM so the line is exactly #nullable enable.

Suggested change

#nullable enable

#nullable enable

Copilot · 2026-02-25T02:07:42Z

src/Essentials/src/PublicAPI/netstandard/PublicAPI.Unshipped.txt

@@ -1 +1,2 @@
-#nullable enable
+#nullable enable


The first line now contains a BOM character before #nullable enable. This frequently causes PublicAPI tooling diffs/failures. Please remove the BOM so the line is exactly #nullable enable.

Suggested change

#nullable enable

#nullable enable

Copilot · 2026-02-25T02:07:43Z

src/Essentials/src/PublicAPI/net-tizen/PublicAPI.Unshipped.txt

@@ -1 +1,2 @@
 #nullable enable
+static Microsoft.Maui.ApplicationModel.MainThread.SetCustomImplementation(System.Func<bool>! isMainThread, System.Action<System.Action!>! beginInvokeOnMainThread) -> void


PublicAPI formatting here is inconsistent with the rest of this TFM's shipped APIs (which use the ~static prefix and omit nullable ! annotations for MainThread members). As written, this entry likely won't match the API as generated from the current (nullable-oblivious) source. Update the entry to the correct PublicAPI format for this assembly (likely ~static ... without !).

Suggested change

static Microsoft.Maui.ApplicationModel.MainThread.SetCustomImplementation(System.Func<bool>! isMainThread, System.Action<System.Action!>! beginInvokeOnMainThread) -> void

~static Microsoft.Maui.ApplicationModel.MainThread.SetCustomImplementation(System.Func<bool> isMainThread, System.Action<System.Action> beginInvokeOnMainThread) -> void

Copilot · 2026-02-25T02:07:43Z

src/Essentials/src/PublicAPI/net-windows/PublicAPI.Unshipped.txt

@@ -1 +1,2 @@
 #nullable enable
+static Microsoft.Maui.ApplicationModel.MainThread.SetCustomImplementation(System.Func<bool>! isMainThread, System.Action<System.Action!>! beginInvokeOnMainThread) -> void


PublicAPI formatting here is inconsistent with the rest of this TFM's shipped APIs (which use the ~static prefix and omit nullable ! annotations for MainThread members). As written, this entry likely won't match the API as generated from the current (nullable-oblivious) source. Update the entry to the correct PublicAPI format for this assembly (likely ~static ... without !).

Suggested change

static Microsoft.Maui.ApplicationModel.MainThread.SetCustomImplementation(System.Func<bool>! isMainThread, System.Action<System.Action!>! beginInvokeOnMainThread) -> void

~static Microsoft.Maui.ApplicationModel.MainThread.SetCustomImplementation(System.Func<bool> isMainThread, System.Action<System.Action> beginInvokeOnMainThread) -> void

Copilot · 2026-02-25T02:07:43Z

src/Essentials/src/PublicAPI/net-android/PublicAPI.Unshipped.txt

@@ -1 +1,2 @@
 #nullable enable
+static Microsoft.Maui.ApplicationModel.MainThread.SetCustomImplementation(System.Func<bool>! isMainThread, System.Action<System.Action!>! beginInvokeOnMainThread) -> void


PublicAPI formatting here is inconsistent with the rest of this TFM's shipped APIs (which use the ~static prefix and omit nullable ! annotations for MainThread members). As written, this entry likely won't match the API as generated from the current (nullable-oblivious) source. Update the entry to the correct PublicAPI format for this assembly (likely ~static ... without !).

Suggested change

static Microsoft.Maui.ApplicationModel.MainThread.SetCustomImplementation(System.Func<bool>! isMainThread, System.Action<System.Action!>! beginInvokeOnMainThread) -> void

~static Microsoft.Maui.ApplicationModel.MainThread.SetCustomImplementation(System.Func<bool> isMainThread, System.Action<System.Action> beginInvokeOnMainThread) -> void

rmarinho · 2026-02-26T13:42:13Z

src/Essentials/src/MainThread/MainThread.shared.cs

 	/// </summary>
 	public static partial class MainThread
 	{
+		static Func<bool> s_isMainThreadImplementation;


do we use s_ ?

Copilot AI review requested due to automatic review settings February 25, 2026 02:01

Copilot started reviewing on behalf of Redth February 25, 2026 02:02 View session

Copilot AI reviewed Feb 25, 2026

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github-actions bot mentioned this pull request Feb 25, 2026

[repo-status] Daily Repo Status - February 25, 2026 🌟 #34232

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Add MainThread.SetCustomImplementation for custom platform backends#34230

Add MainThread.SetCustomImplementation for custom platform backends#34230
Redth wants to merge 1 commit intomainfrom
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		@@ -1 +1,2 @@
		#nullable enable
		static Microsoft.Maui.ApplicationModel.MainThread.SetCustomImplementation(System.Func<bool>! isMainThread, System.Action<System.Action!>! beginInvokeOnMainThread) -> void

	static Microsoft.Maui.ApplicationModel.MainThread.SetCustomImplementation(System.Func<bool>! isMainThread, System.Action<System.Action!>! beginInvokeOnMainThread) -> void
	~static Microsoft.Maui.ApplicationModel.MainThread.SetCustomImplementation(System.Func<bool> isMainThread, System.Action<System.Action> beginInvokeOnMainThread) -> void

		s_isMainThreadImplementation = isMainThread ?? throw new ArgumentNullException(nameof(isMainThread));
		s_beginInvokeOnMainThreadImplementation = beginInvokeOnMainThread ?? throw new ArgumentNullException(nameof(beginInvokeOnMainThread));

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Redth commented Feb 25, 2026

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