dotnet · Copilot · Dec 18, 2025 · Dec 18, 2025 · Dec 18, 2025 · Dec 18, 2025
@@ -349,5 +349,30 @@ public void TypeScriptDefinitionsCopiedToWwwrootOnBuild(Configuration config, bo
                 Assert.False(fileExists, $"dotnet.d.ts should not exist at {dotnetDtsWwwrootPath} after the build with WasmEmitTypeScriptDefinitions={shouldEmit}");
             }
         }
+
+        [Theory]
+        [InlineData("true", true)]
+        [InlineData("false", false)]
+        [InlineData("", true)] // Default case
+        public void UseMonoRuntimeParameter(string useMonoRuntimeArg, bool expectUseMonoRuntime)
+        {
+            Configuration config = Configuration.Debug;
+            string extraArgs = string.IsNullOrEmpty(useMonoRuntimeArg) ? "" : $"--UseMonoRuntime {useMonoRuntimeArg}";
+            ProjectInfo info = CreateWasmTemplateProject(Template.WasmBrowser, config, aot: false, "usemonoruntime", extraArgs: extraArgs);
+
+            string projectDirectory = Path.GetDirectoryName(info.ProjectFilePath)!;
+            string projectFile = File.ReadAllText(info.ProjectFilePath);
+
+            // Verify UseMonoRuntime presence in the project file
+            bool containsUseMonoRuntime = projectFile.Contains("<UseMonoRuntime>true</UseMonoRuntime>");
+            if (expectUseMonoRuntime)
+            {
+                Assert.True(containsUseMonoRuntime, $"Expected <UseMonoRuntime>true</UseMonoRuntime> to be present in the project file when --UseMonoRuntime {useMonoRuntimeArg}");
+            }
+            else
+            {
+                Assert.False(containsUseMonoRuntime, $"Expected <UseMonoRuntime>true</UseMonoRuntime> to not be present in the project file when --UseMonoRuntime {useMonoRuntimeArg}");
+            }
+        }
     }
 }
@@ -48,6 +48,13 @@
       "defaultValue": "net11.0",
       "replaces": "netX.0",
       "displayName": "framework"
+    },
+    "UseMonoRuntime": {
+      "type": "parameter",
+      "datatype": "bool",
+      "defaultValue": "true",
+      "description": "Whether to use the Mono runtime for WebAssembly.",
+      "displayName": "Use Mono Runtime"
     }
   }
 }
@@ -0,0 +1,62 @@
+# WebAssembly Browser App Template
+
+This template creates a .NET app that runs on WebAssembly in a browser.
+
+## Template Options
+
+### UseMonoRuntime
+
+**Parameter:** `--UseMonoRuntime`  
+**Type:** boolean  
+**Default:** `true`
+
+Determines whether to use the Mono runtime for WebAssembly.
+
+When set to `true`, the generated project file will include:
+```xml
+<UseMonoRuntime>true</UseMonoRuntime>
+```
+
+#### Usage Examples
+
+Create a project with Mono runtime (default):
+```bash
+dotnet new wasmbrowser -n MyApp
+```
+
+Or explicitly specify:
+```bash
+dotnet new wasmbrowser -n MyApp --UseMonoRuntime true
+```
+
+Create a project without the Mono runtime property:
+```bash
+dotnet new wasmbrowser -n MyApp --UseMonoRuntime false
+```
+
+## Getting Started
+
+After creating your project:
+
+1. Navigate to your project directory:
+   ```bash
+   cd MyApp
+   ```
+
+2. Run the application:
+   ```bash
+   dotnet run
+   ```
+
+3. Open your browser to the URL displayed in the console output (typically `https://localhost:7000` or `http://localhost:5000`).
+
+## Project Structure
+
+- **Program.cs** - The entry point of your application
+- **wwwroot/** - Static web assets (HTML, CSS, JavaScript)
+- **Properties/** - Project properties and configuration
+
+## Learn More
+
+- [.NET WebAssembly Documentation](https://learn.microsoft.com/en-us/aspnet/core/blazor/webassembly)
+- [Mono Runtime](https://www.mono-project.com/)
@@ -3,6 +3,9 @@
     <TargetFramework>netX.0</TargetFramework>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <OverrideHtmlAssetPlaceholders>true</OverrideHtmlAssetPlaceholders>
+<!--#if (UseMonoRuntime) -->
+    <UseMonoRuntime>true</UseMonoRuntime>
+<!--#endif -->
   </PropertyGroup>
 
   <ItemGroup>