Copydotnet.d.ts to wwwroot on build/publish. (#120097)

- WebAssembly builds now expose an opt-in
`WasmEmitTypeScriptDefinitions` MSBuild property (default `false`) so
projects can request the generated `dotnet.d.ts` typings.
- A new `_EnsureDotnetTypeScriptDefinitions` target copies `dotnet.d.ts`
from the runtime pack into the project’s `wwwroot` on build or publish
when that property is enabled.
- Added a test (`TypeScriptDefinitionsCopiedToWwwrootOnBuild`) to cover
Debug/Release and verify the file is produced only when requested.

Fixes #77174.

Author: ilonatommy

src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.targets

@@ -191,6 +191,8 @@ Copyright (c) .NET Foundation. All rights reserved.
       <_WasmCopyOutputSymbolsToOutputDirectory Condition="'$(_WasmCopyOutputSymbolsToOutputDirectory)'==''">true</_WasmCopyOutputSymbolsToOutputDirectory>
       <_WasmEnableThreads>$(WasmEnableThreads)</_WasmEnableThreads>
       <_WasmEnableThreads Condition="'$(_WasmEnableThreads)' == ''">false</_WasmEnableThreads>
+      <_WasmEmitTypeScriptDefinitions>$(WasmEmitTypeScriptDefinitions)</_WasmEmitTypeScriptDefinitions>
+      <_WasmEmitTypeScriptDefinitions Condition="'$(WasmEmitTypeScriptDefinitions)' == ''">false</_WasmEmitTypeScriptDefinitions>
 
       <_WasmEnableWebcil>$(WasmEnableWebcil)</_WasmEnableWebcil>
       <_WasmEnableWebcil Condition="'$(_TargetingNET80OrLater)' != 'true'">false</_WasmEnableWebcil>
@@ -885,4 +887,23 @@ Copyright (c) .NET Foundation. All rights reserved.
       <WasmAssembliesToBundle Include="@(ResolvedFileToPublish)" Exclude="@(_Exclude)" Condition="%(Extension) == '.dll'" />
     </ItemGroup>
   </Target>
+
+  <!-- Ensure dotnet.d.ts is available in wwwroot for TypeScript development experience when enabled -->
+  <Target Name="_EnsureDotnetTypeScriptDefinitions" 
+          AfterTargets="_ResolveWasmConfiguration" 
+          Condition="'$(_WasmEmitTypeScriptDefinitions)' == 'true'">
+    <PropertyGroup>
+      <_RuntimePackDir>$(MicrosoftNetCoreAppRuntimePackDir)</_RuntimePackDir>
+      <_RuntimePackDir Condition="'$(_RuntimePackDir)' == ''">%(ResolvedRuntimePack.PackageDirectory)</_RuntimePackDir>
+      <_RuntimePackNativeDir>$([MSBuild]::NormalizeDirectory($(_RuntimePackDir), 'runtimes', 'browser-wasm', 'native'))</_RuntimePackNativeDir>
+      <_DotnetTypesSourcePath>$([MSBuild]::NormalizePath($(_RuntimePackNativeDir), 'dotnet.d.ts'))</_DotnetTypesSourcePath>
+      <_DotnetTypesDestPath>$(MSBuildProjectDirectory)\wwwroot\dotnet.d.ts</_DotnetTypesDestPath>
+    </PropertyGroup>
+    
+    <!-- Copy dotnet.d.ts if source exists. MSBuild will skip if files are identical. -->
+    <Copy SourceFiles="$(_DotnetTypesSourcePath)"
+          DestinationFiles="$(_DotnetTypesDestPath)"
+          Condition="Exists('$(_DotnetTypesSourcePath)')"
+          SkipUnchangedFiles="true" />
+  </Target>
 </Project>

src/mono/wasm/Wasm.Build.Tests/Templates/WasmTemplateTests.cs

@@ -317,5 +317,37 @@ public async Task LibraryModeBuild(bool useWasmSdk)
             }
 
         }
+
+        [Theory]
+        [InlineData(Configuration.Debug, true)]
+        [InlineData(Configuration.Release, true)]
+        [InlineData(Configuration.Debug, false)]
+        [InlineData(Configuration.Release, false)]
+        public void TypeScriptDefinitionsCopiedToWwwrootOnBuild(Configuration config, bool emitTypeScriptDts)
+        {
+            string shouldEmit = emitTypeScriptDts ? "true" : "false";
+            string emitTypeScriptDtsProp = $"<WasmEmitTypeScriptDefinitions>{shouldEmit}</WasmEmitTypeScriptDefinitions>";
+            ProjectInfo info = CreateWasmTemplateProject(Template.WasmBrowser, config, aot: false, "tsdefs", extraProperties: emitTypeScriptDtsProp);
+
+            string projectDirectory = Path.GetDirectoryName(info.ProjectFilePath)!;
+            string dotnetDtsWwwrootPath = Path.Combine(projectDirectory, "wwwroot", "dotnet.d.ts");
+
+            // Verify dotnet.d.ts is not in wwwroot after creation
+            Assert.False(File.Exists(dotnetDtsWwwrootPath), $"dotnet.d.ts should not exist at {dotnetDtsWwwrootPath} after creation of the project");
+
+            // Build to trigger the _EnsureDotnetTypeScriptDefinitions target during the build phase
+            BuildProject(info, config, new BuildOptions());
+
+            // Verify dotnet.d.ts presence in the project's wwwroot directory after build
+            bool fileExists = File.Exists(dotnetDtsWwwrootPath);
+            if (emitTypeScriptDts)
+            {
+                Assert.True(fileExists, $"dotnet.d.ts should be created at {dotnetDtsWwwrootPath} after the build with WasmEmitTypeScriptDefinitions={shouldEmit}");
+            }
+            else
+            {
+                Assert.False(fileExists, $"dotnet.d.ts should not exist at {dotnetDtsWwwrootPath} after the build with WasmEmitTypeScriptDefinitions={shouldEmit}");
+            }
+        }
     }
 }

src/mono/wasm/build/WasmApp.Common.targets

@@ -93,6 +93,8 @@
                                               - AppBundle/_content contains web files from nuget packages (css, js, etc)
       - $(WasmStripILAfterAOT)              - Set to true to enable trimming away AOT compiled methods body (IL code)
                                               Defaults to true.
+      - $(WasmEmitTypeScriptDefinitions)    - Controls whether TypeScript definitions (dotnet.d.ts) should be copied to the project's `wwwroot`. 
+                                              Defaults to false.
 
       Public items:
       - @(WasmExtraFilesToDeploy) - Files to copy to $(WasmAppDir).