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OpenTelemetry .NET API

NuGet NuGet


dotnet add package OpenTelemetry.Api


Application developers and library authors use OpenTelemetry API to instrument their application/library. The API only surfaces necessary abstractions to instrument an application/library. It does not address concerns like how telemetry is exported to a specific telemetry backend, how to sample the telemetry, etc. The API consists of Tracing API, Logging API, Metrics API, Context and Propagation API, and a set of semantic conventions.

Tracing API

Tracing API allows users to generate Spans, which represent a single operation within a trace. Spans can be nested to form a trace tree. Each trace contains a root span, which typically describes the entire operation and, optionally one or more child-spans for its child-operations.

Logging API

OpenTelemetry .NET does not introduce its own API for logging. Instead it provides an integration with the well known Microsoft.Extensions.Logging API.

Metrics API

Metrics API allows users to capture measurements about the execution of a computer program at runtime. The Metrics API is designed to process raw measurements, generally with the intent to produce continuous summaries of those measurements.

Baggage API

Baggage API allows users to add context to metric, traces, and logs. Baggage can be propagated out of proc using Propagators. OpenTelemetry SDK ships a BaggagePropagator and enables it by default.

It is important to note that Baggage is not automatically attached to any telemetry. User can explicitly read Baggage and use it to enrich metrics, logs and traces. An example of doing this for traces is shown here.

// Use GetBaggage to get all the key/value pairs present in Baggage
foreach (var item in Baggage.GetBaggage())

// Use SetBaggage method to add a key/value pair in Baggage
Baggage.SetBaggage("AppName", "MyApp");
Baggage.SetBaggage("Region", "West US");

// Use RemoveBaggage method to remove a key/value pair in Baggage

// Use ClearBaggage method to remove all the key/value pairs in Baggage

The recommended way to add Baggage is to use the Baggage.SetBaggage() API. OpenTelemetry users should not use the Activity.AddBaggage method.

Introduction to OpenTelemetry .NET Tracing API

.NET runtime had Activity class for a long time, which was meant to be used for tracing purposes and represents the equivalent of the OpenTelemetry Span. OpenTelemetry .NET is reusing the existing Activity and associated classes to represent the OpenTelemetry Span. This means, users can instrument their applications/libraries to emit OpenTelemetry compatible traces by using just the .NET Runtime.

The Activity and associated classes are shipped as part of System.Diagnostics.DiagnosticSource nuget package. Version 5.0.0 of this package contains improvements to Activity class which makes it more closely aligned with OpenTelemetry API specification.

Even though Activity enables all the scenarios OpenTelemetry supports, users who are already familiar with OpenTelemetry terminology may find it easy to operate with that terminology. For instance, StartSpan may be preferred over StartActivity. To help with this transition, the OpenTelemetry.API package has shim classes to wrap around the .NET Activity classes.

The shim exist only in the API. OpenTelemetry SDK for .NET will be operating entirely with Activity only. Irrespective of whether shim classes or Activity is used for instrumentation, the end result would be same. i.e Processors/Exporters see the same data.

The recommended way of instrumenting is by using the .NET Activity API. Users are required to just take dependency on the DiagnosticSource. Adding dependency to OpenTelemetry.API is required only for the following scenarios:

  1. You want to use terminology matching OpenTelemetry spec (Span vs Activity). The shim can be useful for such users. Refer to the comparison of Activity API and OpenTelemetry Tracing API if you want to compare the differences.

  2. Your library performs communication with other libraries/components, and want to access Propagators, to inject and extract context data. Some of the most common libraries requiring this include HttpClient, ASP.NET Core. This or contrib repository already provides instrumentation for these common libraries. If your library is not built on top of these, and want to leverage propagators, follow the Context propagation section.

  3. You want to leverage Baggage API.

Instrumenting a library/application with .NET Activity API

Basic usage

As mentioned in the introduction, the instrumentation API for OpenTelemetry .NET is the .NET Activity API. Guidance for instrumenting using this API is documented fully in the TBD(dotnet activity user guide link), but is described here as well.

  1. Install the latest stable System.Diagnostics.DiagnosticSource to your application or library.

    dotnet add package System.Diagnostics.DiagnosticSource
  2. Create an ActivitySource, providing the name and version of the library/application doing the instrumentation. ActivitySource instance is typically created once and is reused throughout the application/library.

    static ActivitySource activitySource = new ActivitySource(

    The above requires import of the System.Diagnostics namespace.

  3. Use the ActivitySource instance from above to create Activity instances, which represent a single operation within a trace. The parameter passed is the DisplayName of the activity.

    var activity = activitySource.StartActivity("ActivityName");

    If there are no listeners interested in this activity, the activity above will be null. This happens when the final application does not enable OpenTelemetry (or other ActivityListeners), or when OpenTelemetry samplers chose not to sample this activity. Ensure that all subsequent calls using this activity are protected with a null check.

  4. Populate activity with tags following the OpenTelemetry semantic conventions, using the SetTag API. It is highly recommended to check activity.IsAllDataRequested, before populating any tags which are not readily available. IsAllDataRequested is the same as Span.IsRecording and will be false when samplers decide to not record the activity, and this can be used to avoid any expensive operation to retrieve tags.

    activity?.SetTag("http.method", "GET");
    if (activity != null && activity.IsAllDataRequested == true)
        activity.SetTag("http.url", "");
  5. Perform application/library logic.

  6. Stop the activity when done.


    Alternately, as Activity implements IDisposable, it can be used with a using block, which ensures activity gets stopped upon disposal. This is shown below.

    using (var activity = activitySource.StartActivity("ActivityName")
        activity?.SetTag("http.method", "GET");
    } // Activity gets stopped automatically at end of this block during dispose.

The above showed the basic usage of instrumenting using Activity. The following sections describes more features.

Activity creation options

Basic usage example above showed how StartActivity method can be used to start an Activity. The started activity will automatically becomes the Current activity. It is important to note that the StartActivity returns null, if no listeners are interested in the activity to be created. This happens when the final application does not enable OpenTelemetry, or when OpenTelemetry samplers chose not to sample this activity.

StartActivity has many overloads to control the activity creation.

  1. ActivityKind

    Activity has a property called ActivityKind which represents OpenTelemetry SpanKind. The default value will be Internal. StartActivity allows passing the ActivityKind while starting an Activity.

    var activity = activitySource.StartActivity("ActivityName", ActivityKind.Server);
  2. Parent using ActivityContext

    ActivityContext represents the OpenTelemetry SpanContext. While starting a new Activity, the currently active Activity is automatically taken as the parent of the new activity being created. StartActivity allows passing explicit ActivityContext to override this behavior.

    var parentContext = new ActivityContext(
    var activity = activitySource.StartActivity(

    As ActivityContext follows the W3C Trace-Context, it is also possible to provide the parent context as a single string matching the traceparent header of the W3C Trace-Context. This is shown below.

    var activity = activitySource.StartActivity(
  3. Initial Tags

    Tags in Activity represents the OpenTelemetry Span Attributes. Earlier sample showed the usage of SetTag method of Activity to add tags. Refer to the specification for best practices on naming tags. It is also possible to provide an initial set of tags during activity creation, as shown below. It is recommended to provide all available Tags during activity creation itself, as Samplers can only consider information present during activity creation time.

    var initialTags = new ActivityTagsCollection();
    initialTags["com.mycompany.product.mytag1"] = "tagValue1";
    initialTags["com.mycompany.product.mytag2"] = "tagValue2";
    var activity = activitySource.StartActivity(

    The above requires import of the System.Collections.Generic namespace.

  4. Activity Links

    Apart from the parent-child relation, activities can be linked using ActivityLinks which represent the OpenTelemetry Links. The linked activities must be provided during the creation time, as shown below.

    var activityLinks = new List<ActivityLink>();
    var linkedContext1 = new ActivityContext(
    var linkedContext2 = new ActivityContext(
    activityLinks.Add(new ActivityLink(linkedContext1));
    activityLinks.Add(new ActivityLink(linkedContext2));
    var activity = activitySource.StartActivity(

    Note that Activity above is created with default(ActivityContext) parent, which makes it child of implicit Activity.Current or orphan if there is no Current.

Adding Events

It is possible to add events to Activity using the AddEvent method as shown below.

activity?.AddEvent(new ActivityEvent("sample activity event."));

Apart from providing name, timestamp and attributes can be provided by using corresponding overloads of ActivityEvent.

Setting Status

OpenTelemetry defines a concept called Status to be associated with Activity. Starting with DiagnosticSource 6.0, SetStatus API on Activity can be used to set the status and description as shown below:

activity?.SetStatus(ActivityStatusCode.Error, "Error Description");

All the official Exporters shipped from this repo support the above shown mechanism of setting status.

Setting Status - DiagnosticSource version older than 6.0

Prior to DiagnosticSource 6.0 there was no Status field in Activity, and hence Status is set to an Activity using the following special tags:

otel.status_code is the Tag name used to store the StatusCode, and otel.status_description is the Tag name used to store the optional Description.


activity?.SetTag("otel.status_code", "ERROR");
activity?.SetTag("otel.status_description", "error status description");

Values for the StatusCode tag must be one of the strings "UNSET", "OK", or "ERROR", which correspond respectively to the enums Unset, Ok, and Error from StatusCode.

If using OpenTelemetry API shim, then you can leverage the SetStatus extension method on Activity as well.

Instrumenting using OpenTelemetry.API Shim

As mentioned in the introduction section, using OpenTelemetry.API Shim is only recommended if you want to use OpenTelemetry terminology like Tracer, Span instead of ActivitySource, Activity.

Follow this code for example usage of this shim.

Context propagation

OpenTelemetry.API must be used to access Propagators API which defines how to extract and inject context across process boundaries. This is typically required if you are not using any of the .NET communication libraries which has instrumentations already available which does the propagation (eg: Asp.Net Core or HttpClient). In such cases, context extraction and propagation is the responsibility of the library itself. An example would be a producer-consumer pattern using some queuing library like RabbitMQ. Follow the messaging example for examples on how to inject and extract context.


If you are using the instrumentation libraries shipped from this repo [e.g. ASP.NET Core or HttpClient], context propagation is done by using the default propagator. The default can be updated by calling Sdk.SetDefaultTextMapPropagator and passing the propagator of your choice.

Propagator Api used by the instrumentation libraries is different than DistributedContextPropagator available in System.Diagnostics. Implementing this will have no impact on the propagation, if used alongside instrumentation libraries.

Introduction to OpenTelemetry .NET Metrics API

Metrics in OpenTelemetry .NET are a somewhat unique implementation of the OpenTelemetry project, as the Metrics API is incorporated directly into the .NET runtime itself, as part of the System.Diagnostics.DiagnosticSource package. This means, users can instrument their applications/libraries to emit metrics by simply using the System.Diagnostics.DiagnosticSource package. This package can be used in applications targeting any of the officially supported versions of .NET and .NET Framework (an older Windows-based .NET implementation).

Instrumenting a library/application with .NET Metrics API

Basic metric usage

  1. Install the latest stable version of System.Diagnostics.DiagnosticSource to your application or library.

    dotnet add package System.Diagnostics.DiagnosticSource
  2. Create a Meter, providing the name and version of the library/application doing the instrumentation. The Meter instance is typically created once and is reused throughout the application/library.

    static Meter meter = new Meter(

    The above requires import of the System.Diagnostics.Metrics namespace.

    [!NOTE] It is important to note that Meter instances are created by using its constructor, and not by calling a GetMeter method on the MeterProvider. This is an important distinction from the OpenTelemetry specification, where Meters are obtained from MeterProvider.

  3. Use the Meter instance from above to create instruments, which can be used to report measurements. Just like meter instances, the instrument instances are to be created once and reused throughout the application/library.

    static Counter<long> MyFruitCounter = meter.CreateCounter<long>("MyFruitCounter");
  4. Use the instruments to report measurements, along with the attributes.

    MyFruitCounter.Add(1, new("name", "apple"), new("color", "red"));

The above showed the usage of a Counter instrument. The following sections describes more kinds of instruments.

Instrument types

See this.


This component uses an EventSource with the name "OpenTelemetry-Api" for its internal logging. Please refer to SDK troubleshooting for instructions on seeing these internal logs.