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Backport dotnet-watch changes to main/.NET 11#10960

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jonathanpeppers merged 6 commits intomainfrom
dev/peppers/backport-dotnet-watch
Mar 19, 2026
Merged

Backport dotnet-watch changes to main/.NET 11#10960
jonathanpeppers merged 6 commits intomainfrom
dev/peppers/backport-dotnet-watch

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@jonathanpeppers jonathanpeppers commented Mar 17, 2026

@jonathanpeppers
[xabt] support @(RuntimeEnvironmentVariable) items (#10770)
d10c156 
Context: dotnet/sdk@bd5d3af

`dotnet run` now passes in `dotnet run -e FOO=BAR` as
`@(RuntimeEnvironmentVariable)` MSBuild items.

To opt in to this new feature, we need to add:

    <ProjectCapability Include="RuntimeEnvironmentVariableSupport" />

As well as update the `_GenerateEnvironmentFiles` MSBuild target:

    <!-- RuntimeEnvironmentVariable items come from 'dotnet run -e NAME=VALUE' -->
    <_GeneratedAndroidEnvironment Include="@(RuntimeEnvironmentVariable->'%(Identity)=%(Value)')" />

I added a new test to verify we have the env vars on-device at runtime.

Note that I tested this in combination with a local .NET SDK build:

* #10769

We won't be able to merge this until we have a .NET SDK here that
includes the above commit. Merging with nightly .NET 10.0.3xx SDK.
@jonathanpeppers
[xabt] dotnet watch support, based on env vars (#10778)
10a0d8d 
Context: dotnet/sdk#52492
Context: dotnet/sdk#52581

`dotnet-watch` now runs Android applications via:

    dotnet watch 🚀 [helloandroid (net10.0-android)] Launched 'D:\src\xamarin-android\bin\Debug\dotnet\dotnet.exe' with arguments 'run --no-build -e DOTNET_WATCH=1 -e DOTNET_WATCH_ITERATION=1 -e DOTNET_MODIFIABLE_ASSEMBLIES=debug -e DOTNET_WATCH_HOTRELOAD_WEBSOCKET_ENDPOINT=ws://localhost:9000 -e DOTNET_STARTUP_HOOKS=D:\src\xamarin-android\bin\Debug\dotnet\sdk\10.0.300-dev\DotnetTools\dotnet-watch\10.0.300-dev\tools\net10.0\any\hotreload\net10.0\Microsoft.Extensions.DotNetDeltaApplier.dll -bl': process id 3356

And so the pieces on Android for this to work are:

~~ Startup Hook Assembly ~~

Parse out the value:

    <_AndroidHotReloadAgentAssemblyPath>@(RuntimeEnvironmentVariable->WithMetadataValue('Identity', 'DOTNET_STARTUP_HOOKS')->'%(Value)'->Exists())</_AndroidHotReloadAgentAssemblyPath>

And verify this assembly is included in the app:

    <ResolvedFileToPublish Include="$(_AndroidHotReloadAgentAssemblyPath)" />

Then, for Android, we need to patch up `$DOTNET_STARTUP_HOOKS` to be
just the assembly name, not the full path:

    <_AndroidHotReloadAgentAssemblyName>$([System.IO.Path]::GetFileNameWithoutExtension('$(_AndroidHotReloadAgentAssemblyPath)'))</_AndroidHotReloadAgentAssemblyName>
    ...
    <RuntimeEnvironmentVariable Include="DOTNET_STARTUP_HOOKS" Value="$(_AndroidHotReloadAgentAssemblyName)" />

~~ Port Forwarding ~~

A new `_AndroidConfigureAdbReverse` target runs after deploying apps,
that does:

    adb reverse tcp:9000 tcp:9000

I parsed the value out of:

    <_AndroidWebSocketEndpoint>@(RuntimeEnvironmentVariable->WithMetadataValue('Identity', 'DOTNET_WATCH_HOTRELOAD_WEBSOCKET_ENDPOINT')->'%(Value)')</_AndroidWebSocketEndpoint>
    <_AndroidWebSocketPort>$([System.Text.RegularExpressions.Regex]::Match('$(_AndroidWebSocketEndpoint)', ':(\d+)').Groups[1].Value)</_AndroidWebSocketPort>

~~ Prevent Startup Hooks in Microsoft.Android.Run ~~

When I was implementing this, I keep seeing *two* clients connect to
`dotnet-watch` and I was pulling my hair to figure out why!

Then I realized that `Microsoft.Android.Run` was also getting
`$DOTNET_STARTUP_HOOKS`, and so we had a desktop process + mobile
process both trying to connect!

Easiest fix, is to disable startup hook support in
`Microsoft.Android.Run`. I reviewed the code in `dotnet run`, and it
doesn't seem correct to try to clear the env vars.

~~ Conclusion ~~

With these changes, everything is working!

    dotnet watch 🔥 C# and Razor changes applied in 23ms.
Copilot AI review requested due to automatic review settings March 17, 2026 13:35
Copilot AI reviewed Mar 17, 2026
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Pull request overview

Backport of .NET SDK dotnet run -e / dotnet watch behaviors into .NET for Android (.NET 11) by flowing @(RuntimeEnvironmentVariable) into the Android environment file, adding Hot Reload wiring (startup hook deployment + adb reverse), and extending device tests to validate the scenarios end-to-end.

Changes:

  • Add @(RuntimeEnvironmentVariable) support so dotnet run -e NAME=VALUE is written into the app’s generated environment file.
  • Add dotnet watch Hot Reload support by deploying the startup hook assembly, rewriting DOTNET_STARTUP_HOOKS for Android, and configuring adb reverse based on the websocket endpoint env var.
  • Add/extend device integration tests and test infrastructure (DotNetCLI, ProjectBuilder) to exercise dotnet run -e and dotnet watch hot reload.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 1 comment.

Show a summary per file
File Description
tests/MSBuildDeviceIntegration/Tests/InstallAndRunTests.cs Adds device tests for dotnet watch hot reload and dotnet run -e environment variable propagation; adjusts run property passing.
src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets Writes @(RuntimeEnvironmentVariable) into __environment__.txt and ensures Hot Reload env adjustments happen first.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Common/ProjectBuilder.cs Exposes BuiltBefore to support external build flows like dotnet watch while still using ProjectTools file update infrastructure.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Common/DotNetCLI.cs Adds StartWatch() and changes Run/StartRun parameter handling to allow passing non-MSBuild args (e.g. -e).
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.ProjectCapabilities.targets Advertises new project capabilities needed by the .NET SDK (RuntimeEnvironmentVariableSupport, HotReloadWebSockets).
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.HotReload.targets New targets to rewrite startup hook env var, deploy the delta applier assembly, and set up adb reverse.
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.BuildOrder.targets Hooks Hot Reload port forwarding and environment file generation into install/deploy target ordering.
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.AssemblyResolution.targets Ensures Hot Reload startup hook assembly is included in files to publish/deploy.
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.Application.targets Refactors _AdbToolPath computation into _AndroidAdbToolPath target for reuse.
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.After.targets Imports the new Hot Reload targets for Android application projects.
src/Microsoft.Android.Run/Microsoft.Android.Run.csproj Disables startup hook support to avoid host-side startup hooks interfering with device Hot Reload.
Documentation/docs-mobile/building-apps/build-items.md Documents @(RuntimeEnvironmentVariable) usage and the dotnet run -e integration.
.github/copilot-instructions.md Documents test best-practice for modifying project files via ProjectTools (Touch + Save).

jonathanpeppers and others added 4 commits March 17, 2026 13:23
@jonathanpeppers
Merge branch 'main' into dev/peppers/backport-dotnet-watch
b2d80a7
@jonathanpeppers
Include $(AdbTarget) in adb reverse command
536af42 
When multiple devices/emulators are connected, the adb reverse
command needs  (e.g. -s emulator-5554) to target
the correct device. Without it, the command fails with
'more than one device' or forwards on the wrong device.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@jonathanpeppers
Merge branch 'main' into dev/peppers/backport-dotnet-watch
9bb22be
@jonathanpeppers
Run test on MonoVM
5fc1442
jonathanpeppers
jonathanpeppers commented Mar 19, 2026
View reviewed changes
tests/MSBuildDeviceIntegration/Tests/InstallAndRunTests.cs
Comment on lines +210 to +211
var proj = new XamarinAndroidApplicationProject ();
proj.SetRuntime (AndroidRuntime.MonoVM); // MonoVM only for now, until we get: https://github.com/dotnet/sdk/pull/53501
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This works under Mono:

Image

Rerunning some failed lanes, but we can probably merge and parameterize for CoreCLR later.

@jonathanpeppers jonathanpeppers merged commit 24d6a92 into main Mar 19, 2026
6 checks passed
@jonathanpeppers jonathanpeppers deleted the dev/peppers/backport-dotnet-watch branch March 19, 2026 12:26
jonathanpeppers added a commit that referenced this pull request Mar 19, 2026
@jonathanpeppers
[xabt] dotnet watch support, based on env vars (#10960)
e1d3646 
Context: dotnet/sdk#52492
Context: dotnet/sdk#52581

`dotnet-watch` now runs Android applications via:

    dotnet watch 🚀 [helloandroid (net10.0-android)] Launched 'D:\src\xamarin-android\bin\Debug\dotnet\dotnet.exe' with arguments 'run --no-build -e DOTNET_WATCH=1 -e DOTNET_WATCH_ITERATION=1 -e DOTNET_MODIFIABLE_ASSEMBLIES=debug -e DOTNET_WATCH_HOTRELOAD_WEBSOCKET_ENDPOINT=ws://localhost:9000 -e DOTNET_STARTUP_HOOKS=D:\src\xamarin-android\bin\Debug\dotnet\sdk\10.0.300-dev\DotnetTools\dotnet-watch\10.0.300-dev\tools\net10.0\any\hotreload\net10.0\Microsoft.Extensions.DotNetDeltaApplier.dll -bl': process id 3356

And so the pieces on Android for this to work are:

~~ Startup Hook Assembly ~~

Parse out the value:

    <_AndroidHotReloadAgentAssemblyPath>@(RuntimeEnvironmentVariable->WithMetadataValue('Identity', 'DOTNET_STARTUP_HOOKS')->'%(Value)'->Exists())</_AndroidHotReloadAgentAssemblyPath>

And verify this assembly is included in the app:

    <ResolvedFileToPublish Include="$(_AndroidHotReloadAgentAssemblyPath)" />

Then, for Android, we need to patch up `$DOTNET_STARTUP_HOOKS` to be
just the assembly name, not the full path:

    <_AndroidHotReloadAgentAssemblyName>$([System.IO.Path]::GetFileNameWithoutExtension('$(_AndroidHotReloadAgentAssemblyPath)'))</_AndroidHotReloadAgentAssemblyName>
    ...
    <RuntimeEnvironmentVariable Include="DOTNET_STARTUP_HOOKS" Value="$(_AndroidHotReloadAgentAssemblyName)" />

~~ Port Forwarding ~~

A new `_AndroidConfigureAdbReverse` target runs after deploying apps,
that does:

    adb reverse tcp:9000 tcp:9000

I parsed the value out of:

    <_AndroidWebSocketEndpoint>@(RuntimeEnvironmentVariable->WithMetadataValue('Identity', 'DOTNET_WATCH_HOTRELOAD_WEBSOCKET_ENDPOINT')->'%(Value)')</_AndroidWebSocketEndpoint>
    <_AndroidWebSocketPort>$([System.Text.RegularExpressions.Regex]::Match('$(_AndroidWebSocketEndpoint)', ':(\d+)').Groups[1].Value)</_AndroidWebSocketPort>

~~ Prevent Startup Hooks in Microsoft.Android.Run ~~

When I was implementing this, I keep seeing *two* clients connect to
`dotnet-watch` and I was pulling my hair to figure out why!

Then I realized that `Microsoft.Android.Run` was also getting
`$DOTNET_STARTUP_HOOKS`, and so we had a desktop process + mobile
process both trying to connect!

Easiest fix, is to disable startup hook support in
`Microsoft.Android.Run`. I reviewed the code in `dotnet run`, and it
doesn't seem correct to try to clear the env vars.

~~ Conclusion ~~

With these changes, everything is working!

    dotnet watch 🔥 C# and Razor changes applied in 23ms.

* Include $(AdbTarget) in adb reverse command

When multiple devices/emulators are connected, the adb reverse
command needs  (e.g. -s emulator-5554) to target
the correct device. Without it, the command fails with
'more than one device' or forwards on the wrong device.

NOTE: we run the test on MonoVM-only for now, until we get:
* dotnet/sdk#53501

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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