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ASP.NET Core Blazor WebAssembly Event Pipe diagnostics

This article describes diagnostic tools and how to use them in Blazor WebAssembly apps.

Prerequisite for all scenarios

Install the .NET WebAssembly build tools:

dotnet workload install wasm-tools

How a WebAssembly app uses memory and how to troubleshoot memory leaks

In the app's project file (.csproj), add following properties for the duration of the investigation:

<PropertyGroup>
  <EnableDiagnostics>true</EnableDiagnostics>
</PropertyGroup>

Warning

Don't enable diagnostics in production because it has a negative performance impact.

Build your app with the wasm-tools workload.

Open the app in a browser and navigate to problematic pages or components.

Take a managed memory dump by calling collectGcDump JavaScript API:

globalThis.getDotnetRuntime(0).collectGcDump();

Call the preceding API from either a browser developer tools console or JavaScript code of the app.

A .nettrace file is downloaded from the browser into a local folder, usually the Downloads folder on Windows.

Convert the dump to .gcdump format using the dotnet-gcdump tool. To view the converted .gcdump file, use Visual Studio or PrefView.

For more information, see View the GC dump captured from dotnet-gcdump.

How a WebAssembly app uses CPU and how to find slow or hot methods

In the app's project file (.csproj), add following properties for the duration of the investigation:

<PropertyGroup>
  <EnableDiagnostics>true</EnableDiagnostics>
  <!-- Disable debugger -->
  <WasmDebugLevel>0</WasmDebugLevel>
  <!-- Sampling in all methods, see below for filtering options -->
  <WasmPerformanceInstrumentation>all</WasmPerformanceInstrumentation>
</PropertyGroup>

Warning

Don't enable diagnostics in production because it has a negative performance impact.

Build the app with the wasm-tools workload.

Open the app in a browser and navigate to problematic pages or components.

Start colllecting CPU samples for 60 seconds by calling the collectCpuSamples JavaScript API:

globalThis.getDotnetRuntime(0).collectCpuSamples({durationSeconds: 60});

Call the preceding API from either a browser developer tools console or JavaScript code of the app.

Start using the app to run problematic code.

After the predefined period, the browser downloads a .nettrace file into a local folder, usually the Downloads folder on Windows. To view the .nettrace file, use Visual Studio or PrefView.

For more information, see Use EventPipe to trace your .NET application.

The Timing-Allow-Origin HTTP header allows for more precise time measurements.

How to observe metrics emmited by a WebAssembly app

In the app's project file (.csproj), add following properties for the duration of the investigation:

<PropertyGroup>
  <EnableDiagnostics>true</EnableDiagnostics>
  <MetricsSupport>true</MetricsSupport>
  <EventSourceSupport>true</EventSourceSupport>
</PropertyGroup>

Warning

Don't enable diagnostics in production because it has a negative performance impact.

Build the app with the wasm-tools workload.

Open the app in a browser and navigate to problematic pages or components.

Start colllecting metrics for 60 seconds by calling the collectMetrics JavaScript API:

globalThis.getDotnetRuntime(0).collectMetrics({durationSeconds: 60});

Call the preceding API from either a browser developer tools console or JavaScript code of the app.

After the predefined period, the browser downloads a .nettrace file into a local folder, usually the Downloads folder on Windows. To view the .nettrace file, use Visual Studio or PrefView.

For more information, see Use EventPipe to trace your .NET application.

MSBuild properties that enable diagnostic integration

Property Default Set value to… Description
<EnableDiagnostics> false true Enables support for WebAssembly performance tracing.
<WasmPerformanceInstrumentation> No value See table† Enables instrumentation necessary for the sampling profiler. The property follows the callspec syntax. †For permissible values, see the following table.
<MetricsSupport> false true Enables System.Diagnostics.Metrics support. For more information, see the System.Diagnostics.Metrics namespace.
<EventSourceSupport> false true Enables EventPipe support. For more information, see Diagnostics and instrumentation: Observability and telemetry.

The following table describes permissable <WasmPerformanceInstrumentation> values.

<WasmPerformanceInstrumentation> value Description
all All assemblies
program Entry point assembly
{ASSEMBLY} Specifies an assembly ({ASSEMBLY})
M:Type:{METHOD} Specifies a method ({METHOD})
N:{NAMESPACE} Specifies a namespace ({NAMESPACE})
T:{TYPE} Specifies a type ({TYPE})
+EXPR Includes expression
-EXPR Excludes expression

Your code should yield to main browser loop often to allow the trace to be collected. When executing long running loops, the internal diagnostic buffers could overflow.

Caution

Enabling profilers and diagnostic tools has negative size and performance impacts, so don't publish an app for production with profilers enabled.

Additional resources