Debug ASP.NET Core Blazor WebAssembly

This article describes how to debug Blazor WebAssembly with browser developer tools and an integrated development environment (IDE).

Blazor WebAssembly apps can be debugged using the browser developer tools in Chromium-based browsers (Edge/Chrome). You can also debug your app using the following IDEs:

• Visual Studio
• Visual Studio for Mac
• Visual Studio Code

Available scenarios include:

• Set and remove breakpoints.
• Run the app with debugging support in IDEs.
• Single-step through the code.
• Resume code execution with a keyboard shortcut in IDEs.
• In the Locals window, observe the values of local variables.
• See the call stack, including call chains between JavaScript and .NET.

For now, you can't:

• Break on unhandled exceptions.
• Hit breakpoints during app startup before the debug proxy is running. This includes breakpoints in Program.cs and breakpoints in the OnInitialized{Async} lifecycle methods of components that are loaded by the first page requested from the app.
• Debug in non-local scenarios (for example, Windows Subsystem for Linux (WSL) or Visual Studio Codespaces).
• Automatically rebuild the backend Server app of a hosted Blazor WebAssembly solution during debugging, for example by running the app with dotnet watch run.
• Use a symbol server for debugging.

Prerequisites

Debugging requires the latest version of the following browsers:

• Google Chrome (default)
• Microsoft Edge

Ensure that firewalls or proxies don't block communication with the debug proxy (NodeJS process). For more information, see the Firewall configuration section.

Visual Studio Code users require the C# for Visual Studio Code Extension.

Visual Studio for Mac requires version 8.8 (build 1532) or later. Install the latest release of Visual Studio for Mac by selecting the Download Visual Studio for Mac button at Microsoft: Visual Studio for Mac.

Note

Apple Safari on macOS isn't currently supported.

Packages

Standalone Blazor WebAssembly: Microsoft.AspNetCore.Components.WebAssembly.DevServer: Development server for use when building Blazor apps. Calls WebAssemblyNetDebugProxyAppBuilderExtensions.UseWebAssemblyDebugging internally to add middleware for debugging Blazor WebAssembly apps inside Chromium developer tools.

Hosted Blazor WebAssembly:

• Client project: Microsoft.AspNetCore.Components.WebAssembly.DevServer: Development server for use when building Blazor apps. Calls WebAssemblyNetDebugProxyAppBuilderExtensions.UseWebAssemblyDebugging internally to add middleware for debugging Blazor WebAssembly apps inside Chromium developer tools.
• Server project: Microsoft.AspNetCore.Components.WebAssembly.Server: References an internal package (Microsoft.NETCore.BrowserDebugHost.Transport) for assemblies that share the browser debug host.

Note

For guidance on adding packages to .NET apps, see the articles under Install and manage packages at Package consumption workflow (NuGet documentation). Confirm correct package versions at NuGet.org.

Debug a standalone Blazor WebAssembly app

To enable debugging for an existing Blazor WebAssembly app, update the launchSettings.json file in the startup project to include the following inspectUri property in each launch profile:

"inspectUri": "{wsProtocol}://{url.hostname}:{url.port}/_framework/debug/ws-proxy?browser={browserInspectUri}"

Once updated, the launchSettings.json file should look similar to the following example:

{
    "iisSettings": {
      "windowsAuthentication": false,
      "anonymousAuthentication": true,
      "iisExpress": {
        "applicationUrl": "http://localhost:50454",
        "sslPort": 44399
      }
    },
    "profiles": {
      "IIS Express": {
        "commandName": "IISExpress",
        "launchBrowser": true,
        "inspectUri": "{wsProtocol}://{url.hostname}:{url.port}/_framework/debug/ws-proxy?browser={browserInspectUri}",
        "environmentVariables": {
          "ASPNETCORE_ENVIRONMENT": "Development"
        }
      },
      "BlazorApp1.Server": {
        "commandName": "Project",
        "launchBrowser": true,
        "inspectUri": "{wsProtocol}://{url.hostname}:{url.port}/_framework/debug/ws-proxy?browser={browserInspectUri}",
        "applicationUrl": "https://localhost:5001;http://localhost:5000",
        "environmentVariables": {
          "ASPNETCORE_ENVIRONMENT": "Development"
        }
      }
    }
  }

The inspectUri property:

• Enables the IDE to detect that the app is a Blazor WebAssembly app.
• Instructs the script debugging infrastructure to connect to the browser through Blazor's debugging proxy.

The placeholder values for the WebSocket protocol (wsProtocol), host (url.hostname), port (url.port), and inspector URI on the launched browser (browserInspectUri) are provided by the framework.

Visual Studio

To debug a Blazor WebAssembly app in Visual Studio:

1. Create a new hosted Blazor WebAssembly solution.

2. With the Server project selected in Solution Explorer, press F5 to run the app in the debugger.

   Note

   When debugging with a Chromium-based browser, such as Google Chrome or Microsoft Edge, a new browser window might open with a separate profile for the debugging session instead of opening a tab in an existing browser window with the user's profile. If debugging with the user's profile is a requirement, adopt one of the following approaches:

   • Close all open browser instances before pressing F5 to start debugging.
   • Configure Visual Studio to launch the browser with the user's profile. For more information on this approach, see Blazor WASM Debugging in VS launches Edge with a separate user data directory (dotnet/aspnetcore #20915).

   Note

   Start Without Debugging [Ctrl+F5 (Windows) or ⌘+F5 (macOS)] isn't supported. When the app is run in Debug configuration, debugging overhead always results in a small performance reduction.

3. In the Client app, set a breakpoint on the currentCount++; line in Pages/Counter.razor.

4. In the browser, navigate to Counter page and select the Click me button to hit the breakpoint.

5. In Visual Studio, inspect the value of the currentCount field in the Locals window.

6. Press F5 to continue execution.

While debugging a Blazor WebAssembly app, you can also debug server code:

1. Set a breakpoint in the Pages/FetchData.razor page in OnInitializedAsync.
2. Set a breakpoint in the WeatherForecastController in the Get action method.
3. Browse to the Fetch Data page to hit the first breakpoint in the FetchData component just before it issues an HTTP request to the server.
4. Press F5 to continue execution and then hit the breakpoint on the server in the WeatherForecastController.
5. Press F5 again to let execution continue and see the weather forecast table rendered in the browser.

Note

Breakpoints are not hit during app startup before the debug proxy is running. This includes breakpoints in Program.cs and breakpoints in the OnInitialized{Async} lifecycle methods of components that are loaded by the first page requested from the app.

Debug standalone Blazor WebAssembly

For information on configuring VS Code assets in the .vscode folder, see the Linux operating system guidance in Tooling for ASP.NET Core Blazor.

1. Open the standalone Blazor WebAssembly app in VS Code.

   You may receive a notification that additional setup is required to enable debugging:

   Additional setup is required to debug Blazor WebAssembly applications.

   If you receive the notification:

   • Confirm that the latest C# for Visual Studio Code Extension is installed. To inspect the installed extensions, open View > Extensions from the menu bar or select the Extensions icon in the Activity sidebar.
   • When using the C# for Visual Studio Code Extension version 1.23.9 or later, confirm that the latest Blazor WASM Debugging Extension is installed. To inspect the installed extensions, open View > Extensions from the menu bar or select the Extensions icon in the Activity sidebar.
   • Reload the window.

2. Create a .vscode/launch.json file with the following configuration. Replace the {PORT} placeholder with the port configured in Properties/launchSettings.json:

   {
     "name": "Launch and Debug",
     "type": "blazorwasm",
     "request": "launch",
     "url": "http://localhost:{PORT}"
   }
   

   If the app is in a subfolder of the workspace root, include the current working directory (cwd) property with the path to the app. In the following property value, replace the {PATH} placeholder with the path to the app:

   "cwd": "${workspaceFolder}/{PATH}"
   

   In the following example, the app is in a subfolder named blazorwasm:

   "cwd": "${workspaceFolder}/blazorwasm"
   

3. Start debugging using the F5 keyboard shortcut or the menu command.

   Note

   Start Without Debugging [Ctrl+F5 (Windows) or ⌘+F5 (macOS)] isn't supported. When the app is run in Debug configuration, debugging overhead always results in a small performance reduction.

4. The standalone app is launched, and a debugging browser is opened.

5. In the Client app, set a breakpoint on the currentCount++; line in Pages/Counter.razor.

6. In the browser, navigate to Counter page and select the Click me button to hit the breakpoint.

Note

Breakpoints are not hit during app startup before the debug proxy is running. This includes breakpoints in Program.cs and breakpoints in the OnInitialized{Async} lifecycle methods of components that are loaded by the first page requested from the app.

Debug hosted Blazor WebAssembly

For guidance on configuring VS Code assets in the .vscode folder and where to place the .vscode folder in the solution, see the Linux operating system guidance in Tooling for ASP.NET Core Blazor.

Note

Only browser debugging is supported at this time.

You can't automatically rebuild the backend Server app of a hosted Blazor WebAssembly solution during debugging, for example by running the app with dotnet watch run.

To debug a published, hosted Blazor WebAssembly app, configure debugger support (DebuggerSupport) and copy output symbols to the publish directory (CopyOutputSymbolsToPublishDirectory) in the Client app's project file:

<DebuggerSupport>true</DebuggerSupport>
<CopyOutputSymbolsToPublishDirectory>true</CopyOutputSymbolsToPublishDirectory>

By default, publishing an app disables the preceding properties by setting them to false.

Warning

Published, hosted Blazor WebAssembly apps should only enable debugging and copying output symbols when deploying published assets locally. Do not* deploy a published app into production with the DebuggerSupport and CopyOutputSymbolsToPublishDirectory properties set to true.

Attach to an existing debugging session

To attach to a running Blazor app, create a .vscode/launch.json file with the following configuration. Replace the {URL} placeholder with the URL where the app is running:

{
  "name": "Attach and Debug"
  "type": "blazorwasm",
  "request": "attach",
  "url": "{URL}"
}

Note

Attaching to a debugging session is only supported for standalone apps. To use full-stack debugging, you must launch the app from VS Code.

Launch configuration options

The following launch configuration options are supported for the blazorwasm debug type (.vscode/launch.json).

Option	Description
request	Use launch to launch and attach a debugging session to a Blazor WebAssembly app or attach to attach a debugging session to an already-running app.
url	The URL to open in the browser when debugging.
browser	The browser to launch for the debugging session. Set to edge or chrome. Defaults to edge.
trace	Used to generate logs from the JS debugger. Set to true to generate logs.
hosted	Must be set to true if launching and debugging a hosted Blazor WebAssembly app.
webRoot	Specifies the absolute path of the web server. Should be set if an app is served from a sub-route.
timeout	The number of milliseconds to wait for the debugging session to attach. Defaults to 30,000 milliseconds (30 seconds).
program	A reference to the executable to run the server of the hosted app. Must be set if hosted is true.
cwd	The working directory to launch the app under. Must be set if hosted is true.
env	The environment variables to provide to the launched process. Only applicable if hosted is set to true.

Visual Studio for Mac

To debug a Blazor WebAssembly app in Visual Studio for Mac:

1. Create a new hosted Blazor WebAssembly app.

2. Press ⌘+↩ to run the app in the debugger.

   Note

   Start Without Debugging (⌥+⌘+↩) isn't supported. When the app is run in Debug configuration, debugging overhead always results in a small performance reduction.

   Important

   Google Chrome or Microsoft Edge must be the selected browser for the debugging session.

3. In the Client app, set a breakpoint on the currentCount++; line in Pages/Counter.razor.

4. In the browser, navigate to Counter page and select the Click me button to hit the breakpoint:

5. In Visual Studio, inspect the value of the currentCount field in the Locals window.

6. Press ⌘+↩ to continue execution.

While debugging a Blazor WebAssembly app, you can also debug server code:

1. Set a breakpoint in the Pages/FetchData.razor page in OnInitializedAsync.
2. Set a breakpoint in the WeatherForecastController in the Get action method.
3. Browse to the Fetch Data page to hit the first breakpoint in the FetchData component just before it issues an HTTP request to the server.
4. Press ⌘+↩ to continue execution and then hit the breakpoint on the server in the WeatherForecastController.
5. Press ⌘+↩ again to let execution continue and see the weather forecast table rendered in the browser.

Note

Breakpoints are not hit during app startup before the debug proxy is running. This includes breakpoints in Program.cs and breakpoints in the OnInitialized{Async} lifecycle methods of components that are loaded by the first page requested from the app.

For more information, see Debugging with Visual Studio for Mac.

Debug in the browser

The guidance in this section applies to Google Chrome and Microsoft Edge running on Windows.

1. Run a Debug build of the app in the Development environment.

2. Launch a browser and navigate to the app's URL.

3. In the browser, attempt to commence remote debugging by pressing Shift+Alt+d.

   The browser must be running with remote debugging enabled, which isn't the default. If remote debugging is disabled, an Unable to find debuggable browser tab error page is rendered with instructions for launching the browser with the debugging port open. Follow the instructions for your browser, which opens a new browser window. Close the previous browser window.

1. Once the browser is running with remote debugging enabled, the debugging keyboard shortcut in the previous step opens a new debugger tab.

2. After a moment, the Sources tab shows a list of the app's .NET assemblies within the file:// node.

3. In component code (.razor files) and C# code files (.cs), breakpoints that you set are hit when code executes. After a breakpoint is hit, single-step (F10) through the code or resume (F8) code execution normally.

Blazor provides a debugging proxy that implements the Chrome DevTools Protocol and augments the protocol with .NET-specific information. When debugging keyboard shortcut is pressed, Blazor points the Chrome DevTools at the proxy. The proxy connects to the browser window you're seeking to debug (hence the need to enable remote debugging).

Browser source maps

Browser source maps allow the browser to map compiled files back to their original source files and are commonly used for client-side debugging. However, Blazor doesn't currently map C# directly to JavaScript/WASM. Instead, Blazor does IL interpretation within the browser, so source maps aren't relevant.

Firewall configuration

If a firewall blocks communication with the debug proxy, create a firewall exception rule that permits communication between the browser and the NodeJS process.

Warning

Modification of a firewall configuration must be made with care to avoid creating security vulnerabilities. Carefully apply security guidance, follow best security practices, and respect warnings issued by the firewall's manufacturer.

Permitting open communication with the NodeJS process:

• Opens up the Node server to any connection, depending on the firewall's capabilities and configuration.
• Might be risky depending on your network.
• Is only recommended on developer machines.

If possible, only allow open communication with the NodeJS process on trusted or private networks.

For Windows Firewall configuration guidance, see Create an Inbound Program or Service Rule. For more information, see Windows Defender Firewall with Advanced Security and related articles in the Windows Firewall documentation set.

Troubleshoot

If you're running into errors, the following tips may help:

• In the Debugger tab, open the developer tools in your browser. In the console, execute localStorage.clear() to remove any breakpoints.
• Confirm that you've installed and trusted the ASP.NET Core HTTPS development certificate. For more information, see Enforce HTTPS in ASP.NET Core.
• Visual Studio requires the Enable JavaScript debugging for ASP.NET (Chrome, Edge and IE) option in Tools > Options > Debugging > General. This is the default setting for Visual Studio. If debugging isn't working, confirm that the option is selected.
• If your environment uses an HTTP proxy, make sure that localhost is included in the proxy bypass settings. This can be done by setting the NO_PROXY environment variable in either:
  • The launchSettings.json file for the project.
  • At the user or system environment variables level for it to apply to all apps. When using an environment variable, restart Visual Studio for the change to take effect.
• Ensure that firewalls or proxies don't block communication with the debug proxy (NodeJS process). For more information, see the Firewall configuration section.

Breakpoints in OnInitialized{Async} not hit

The Blazor framework's debugging proxy takes a short time to launch, so breakpoints in the OnInitialized{Async} lifecycle methods might not be hit. We recommend adding a delay at the start of the method body to give the debug proxy some time to launch before the breakpoint is hit. You can include the delay based on an if compiler directive to ensure that the delay isn't present for a release build of the app.

OnInitialized:

protected override void OnInitialized()
{
#if DEBUG
    Thread.Sleep(10000);
#endif

    ...
}

OnInitializedAsync:

protected override async Task OnInitializedAsync()
{
#if DEBUG
    await Task.Delay(10000);
#endif

    ...
}

Visual Studio (Windows) timeout

If Visual Studio throws an exception that the debug adapter failed to launch mentioning that the timeout was reached, you can adjust the timeout with a Registry setting:

VsRegEdit.exe set "<VSInstallFolder>" HKCU JSDebugger\Options\Debugging "BlazorTimeoutInMilliseconds" dword {TIMEOUT}

The {TIMEOUT} placeholder in the preceding command is in milliseconds. For example, one minute is assigned as 60000.