Debug ASP.NET Core Blazor WebAssembly

Daniel Roth

Blazor WebAssembly apps can be debugged using the browser dev tools in Chromium-based browsers (Edge/Chrome). You can also debug your app using the following integrated development environments (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.Main (Program.cs) and breakpoints in the OnInitialized{Async} methods of components that are loaded by the first page requested from the app.

Prerequisites

Debugging requires either of the following browsers:

  • Google Chrome (version 70 or later) (default)
  • Microsoft Edge (version 80 or later)

Visual Studio for Mac requires version 8.8 (build 1532) or later:

  1. Install the latest release of Visual Studio for Mac by selecting the Download Visual Studio for Mac button at Microsoft: Visual Studio for Mac.
  2. Select the Preview channel from within Visual Studio. For more information, see Install a preview version of Visual Studio for Mac.

Note

Apple Safari on macOS isn't currently supported.

Enable debugging

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 WebSockets protocol (wsProtocol), host (url.hostname), port (url.port), and inspector URI on the launched browser (browserInspectUri) are provided by the framework.

To debug a Blazor WebAssembly app in Visual Studio:

  1. Create a new ASP.NET Core hosted Blazor WebAssembly app.

  2. Press F5 to run the app in the debugger.

    Note

    Start Without Debugging (Ctrl+F5) 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.Main (Program.cs) and breakpoints in the OnInitialized{Async} methods of components that are loaded by the first page requested from the app.

If the app is hosted at a different app base path than /, update the following properties in Properties/launchSettings.json to reflect the app's base path:

  • applicationUrl:

    "iisSettings": {
      ...
      "iisExpress": {
        "applicationUrl": "http://localhost:{INSECURE PORT}/{APP BASE PATH}/",
        "sslPort": {SECURE PORT}
      }
    },
    
  • inspectUri of each profile:

    "profiles": {
      ...
      "{PROFILE 1, 2, ... N}": {
        ...
        "inspectUri": "{wsProtocol}://{url.hostname}:{url.port}/{APP BASE PATH}/_framework/debug/ws-proxy?browser={browserInspectUri}",
        ...
      }
    }
    

The placeholders in the preceding settings:

  • {INSECURE PORT}: The insecure port. A random value is provided by default, but a custom port is permitted.
  • {APP BASE PATH}: The app's base path.
  • {SECURE PORT}: The secure port. A random value is provided by default, but a custom port is permitted.
  • {PROFILE 1, 2, ... N}: Launch settings profiles. Usually, an app specifies more than one profile by default (for example, a profile for IIS Express and a project profile, which is used by Kestrel server).

In the following examples, the app is hosted at /OAT with an app base path configured in wwwroot/index.html as <base href="/OAT/">:

"applicationUrl": "http://localhost:{INSECURE PORT}/OAT/",
"inspectUri": "{wsProtocol}://{url.hostname}:{url.port}/OAT/_framework/debug/ws-proxy?browser={browserInspectUri}",

For information on using a custom app base path for Blazor WebAssembly apps, see Host and deploy ASP.NET Core Blazor.

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 (for example, https://localhost:5001).

  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.

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.

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 method 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.