OpenTelemetry.Proxy

Generate OpenTelemetry Activity instrumentation at compile time using Roslyn Source Generator + C# 14 Interceptors.

How it works

The Source Generator scans your code for attribute annotations ([ActivitySource], [Activity], etc.) and generates interceptor methods that wrap your method calls with OpenTelemetry instrumentation — Activity creation, tag setting, exception handling, and resource cleanup. All at compile time, zero runtime overhead.

Attributes

[ActivitySource]

Mark a type to automatically generate Activity for its methods.

IncludeAllMethods: By default, only async methods are auto-included (for interfaces/abstract classes, methods returning Task/ValueTask are considered async; async void methods are excluded). Set to true to include all public methods including async void.

[Activity]

Mark a method to generate Activity wrapping. Can specify ActivityName, Kind, and SuppressInstrumentation.

[NonActivity]

Exclude a method from Activity generation. If SuppressInstrumentation is true, downstream instrumentation will be suppressed.

[ActivityName]

Modify the DisplayName of an inner activity. Requires TracerProviderBuilder.AddActivityNameProcessor().

Priority: [NonActivity] > [Activity] > [ActivityName] (method) > [ActivitySource] > [ActivityName] (class)

[ActivityTag]

Add a tag to the Activity. Can be defined on parameters or return values.

[ActivityTags]

Add multiple tags to the Activity. Can be defined on types or methods. Tags are resolved against method parameters, instance fields/properties, and static fields/properties.

Setup

1. Install the NuGet package

dotnet add package PW.OpenTelemetry.Proxy

2. Register ActivitySources

The Source Generator creates an ActivitySourceHolder class with an extension method to register all generated ActivitySources:

builder.Services.AddOpenTelemetry()
    .WithTracing(b => b
        .AddMyAppSources()           // Generated: registers all ActivitySources in this assembly
        .AddActivityNameProcessor()   // Required for [ActivityName] support
        .AddOtlpExporter());

The method name is Add{AssemblyName}Sources(), unique per assembly.

3. Disable generation (optional)

<PropertyGroup>
  <DisableProxyGenerator>true</DisableProxyGenerator>
</PropertyGroup>

Tag expression

Must start with $ on [ActivityTag].

Any valid C# member access expression. For example:

• [ActivityTag(Expression = "$.Length")] on a string parameter generates param.Length
• [ActivityTag(Expression = "$.Hour")] on a DateTime return value generates @return.Hour

Example

[ActivitySource]
public class OrderService
{
    [Activity]
    public async Task<Order> CreateOrder([ActivityTag] int customerId, [ActivityTag] string product)
    {
        // Activity is automatically created with tags "customerId" and "product"
        return await SaveOrder(customerId, product);
    }

    [ActivityName(AdjustStartTime = true)]
    public async Task ProcessPayment([ActivityTag] decimal amount)
    {
        // Inner activity name is set to "OrderService.ProcessPayment"
        await ChargeCard(amount);
    }

    [NonActivity(true)]
    public void InternalCleanup()
    {
        // Downstream instrumentation is suppressed
    }
}

QA

How to get ActivitySource name?

ActivitySourceAttribute.GetActivitySourceName(typeof(YourType))