Application Insights Extension Generator

February 18, 2026 ยท View on GitHub

When your test project references Microsoft.ApplicationInsights, this generator creates WithApplicationInsights() and related extension methods for AutoMocker that enable testing of telemetry tracking code without making real API calls.

The generator is version-aware: it produces different code depending on whether you are using Application Insights 2.x or 3.x.

Features

  • Automatically generates when Microsoft.ApplicationInsights is referenced
  • Provides a TelemetryClient that doesn't make real API calls
  • Application Insights 2.x: Uses a FakeTelemetryChannel to capture ITelemetry items
  • Application Insights 3.x: Uses OpenTelemetry in-memory exporters to capture log records, metrics, and activities
  • Application Insights 3.x without OpenTelemetry.Exporter.InMemory: Generates an [Obsolete] stub that throws.
  • Allows verification of telemetry calls in tests

Usage

Application Insights 3.x (with OpenTelemetry)

Application Insights 3.x uses OpenTelemetry under the hood. To use the generated extension methods, your test project must reference both Microsoft.ApplicationInsights (3.x) and OpenTelemetry.Exporter.InMemory:

<ItemGroup>
  <PackageReference Include="Microsoft.ApplicationInsights" Version="3.0.0" />
  <PackageReference Include="OpenTelemetry.Exporter.InMemory" Version="1.15.0" />
</ItemGroup>

Verifying Tracked Events (Log Records)

[TestClass]
public class MyServiceTests
{
    [TestMethod]
    public void DoWork_TracksEvents()
    {
        AutoMocker mocker = new();
        mocker.WithApplicationInsights();

        var service = mocker.CreateInstance<MyService>();
        service.DoWork();

        var logRecords = mocker.GetApplicationInsightsLogRecords();
        Assert.HasCount(2, logRecords);
    }
}

Verifying Tracked Activities (Traces)

[TestMethod]
public void ProcessRequest_TracksOperations()
{
    AutoMocker mocker = new();
    mocker.WithApplicationInsights();

    var service = mocker.CreateInstance<MyService>();
    service.ProcessRequest("TestRequest");

    var activities = mocker.GetApplicationInsightsActivities();
    Assert.HasCount(1, activities);
    Assert.AreEqual("TestRequest", activities[0].OperationName);
}

Verifying Tracked Metrics

[TestMethod]
public void TrackMetrics_CapturesValues()
{
    AutoMocker mocker = new();
    mocker.WithApplicationInsights();

    var service = mocker.CreateInstance<MyService>();
    service.RecordResponseTime(123.45);

    // Flush to ensure metrics are exported
    mocker.Get<TelemetryClient>().Flush();

    var metrics = mocker.GetApplicationInsightsMetrics();
    Assert.HasCount(1, metrics);
}

Application Insights 2.x

With Application Insights 2.x, the generator uses a FakeTelemetryChannel to intercept telemetry. No additional packages are needed beyond Microsoft.ApplicationInsights 2.x:

[TestMethod]
public void DoWork_TracksEvents()
{
    AutoMocker mocker = new();
    mocker.WithApplicationInsights();

    var service = mocker.CreateInstance<MyService>();
    service.DoWork();

    // V2 uses ITelemetry items via GetSentTelemetry()
    var telemetry = mocker.GetSentTelemetry();
    Assert.HasCount(2, telemetry);
}

Application Insights 3.x without OpenTelemetry.Exporter.InMemory

If your project references Application Insights 3.x but does not reference OpenTelemetry.Exporter.InMemory, the generator produces a stub method marked [Obsolete] that throws InvalidOperationException at runtime:

// Compiler warning: "To use WithApplicationInsights with Application Insights 3.x,
// add a reference to the OpenTelemetry.Exporter.InMemory NuGet package."
mocker.WithApplicationInsights(); // Throws InvalidOperationException

To fix this, add OpenTelemetry.Exporter.InMemory to your project and rebuild.

Generated Extension Methods

Application Insights 3.x (with InMemory exporter)

/// <summary>
/// Sets up AutoMocker with Application Insights 3.x using OpenTelemetry in-memory exporters.
/// </summary>
public static AutoMocker WithApplicationInsights(this AutoMocker mocker)

/// <summary>
/// Gets captured log records from the in-memory exporter.
/// </summary>
public static IList<LogRecord> GetApplicationInsightsLogRecords(this AutoMocker mocker)

/// <summary>
/// Gets captured metrics from the in-memory exporter.
/// </summary>
public static IList<OpenTelemetry.Metrics.Metric> GetApplicationInsightsMetrics(this AutoMocker mocker)

/// <summary>
/// Gets captured activities (traces) from the in-memory exporter.
/// </summary>
public static IList<Activity> GetApplicationInsightsActivities(this AutoMocker mocker)

Application Insights 2.x

/// <summary>
/// Sets up AutoMocker with Application Insights using a FakeTelemetryChannel.
/// </summary>
public static AutoMocker WithApplicationInsights(this AutoMocker mocker)

/// <summary>
/// Gets telemetry items captured by the FakeTelemetryChannel.
/// </summary>
public static IReadOnlyList<ITelemetry> GetSentTelemetry(this AutoMocker mocker)

How It Works

The generator detects the major version of Microsoft.ApplicationInsights referenced by your project and generates version-appropriate code.

Application Insights 3.x (with OpenTelemetry)

  1. Creates a TelemetryCollector instance containing lists for log records, metrics, and activities
  2. Creates a TelemetryConfiguration with a dummy connection string
  3. Configures the OpenTelemetry builder on the configuration to export to the in-memory collector lists
  4. Creates a TelemetryClient with the configuration
  5. Registers the TelemetryCollector, TelemetryConfiguration, and TelemetryClient with the AutoMocker

The GetApplicationInsightsLogRecords(), GetApplicationInsightsMetrics(), and GetApplicationInsightsActivities() methods retrieve the TelemetryCollector from the AutoMocker and return the corresponding list.

Application Insights 2.x

  1. Creates a FakeTelemetryChannel that captures ITelemetry items instead of sending them
  2. Creates a TelemetryConfiguration with the fake channel and a dummy instrumentation key
  3. Creates a TelemetryClient with the fake configuration
  4. Registers everything with the AutoMocker

The GetSentTelemetry() method retrieves the FakeTelemetryChannel and returns the captured items.

Application Insights 3.x (without InMemory exporter)

Generates a method marked [Obsolete] that throws InvalidOperationException, providing a clear message to add OpenTelemetry.Exporter.InMemory.

Advanced Usage

Method Chaining

The WithApplicationInsights() extension method returns the AutoMocker instance, allowing fluent chaining:

[TestMethod]
public void Test_CombinedSetup()
{
    AutoMocker mocker = new();

    mocker.WithApplicationInsights()
          .WithFakeLogging()
          .WithOptions<MySettings>(s => s.Value = "test");

    var service = mocker.CreateInstance<MyService>();
    // ... test assertions ...
}

Combining Telemetry Types

With Application Insights 3.x, you can verify multiple types of telemetry in a single test:

[TestMethod]
public void ComplexOperation_TracksAllTelemetry()
{
    AutoMocker mocker = new();
    mocker.WithApplicationInsights();

    var service = mocker.CreateInstance<MyService>();
    service.ComplexOperation();

    // Flush to ensure metrics are exported
    mocker.Get<TelemetryClient>().Flush();

    var logRecords = mocker.GetApplicationInsightsLogRecords();
    var metrics = mocker.GetApplicationInsightsMetrics();
    var activities = mocker.GetApplicationInsightsActivities();

    Assert.IsTrue(logRecords.Count > 0);
    Assert.IsTrue(metrics.Count > 0);
    Assert.IsTrue(activities.Count > 0);
}

Disabling the Generator

You can disable this generator using an MSBuild property in your test project's .csproj file:

<PropertyGroup>
  <EnableMoqAutoMockerApplicationInsightsGenerator>false</EnableMoqAutoMockerApplicationInsightsGenerator>
</PropertyGroup>

Example: Disabling in Project File

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net10.0</TargetFramework>

    <!-- Disable Application Insights Extension Generator -->
    <EnableMoqAutoMockerApplicationInsightsGenerator>false</EnableMoqAutoMockerApplicationInsightsGenerator>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Moq.AutoMock" Version="3.5.0" />
    <PackageReference Include="Microsoft.ApplicationInsights" Version="3.0.0" />
  </ItemGroup>
</Project>

Troubleshooting

Extension Method Not Available

  1. Verify Microsoft.ApplicationInsights is referenced in your test project
  2. Check that the generator is not disabled in your .csproj
  3. Rebuild the project to trigger generator execution
  4. Ensure you're using the Moq.AutoMock namespace

Obsolete Warning / InvalidOperationException with Application Insights 3.x

If you see a compiler warning about WithApplicationInsights being obsolete, or get an InvalidOperationException at runtime:

  1. Add a reference to OpenTelemetry.Exporter.InMemory in your test project
  2. Rebuild the project so the generator detects the new reference

TelemetryClient Not Configured

  1. Make sure you're calling WithApplicationInsights() before creating your instance
  2. Verify the TelemetryClient is being injected properly
  3. Check that you're using the same AutoMocker instance throughout the test

Metrics List Is Empty

OpenTelemetry metrics require an explicit flush before they appear in the in-memory exporter. Call mocker.Get<TelemetryClient>().Flush() before reading metrics:

mocker.Get<TelemetryClient>().Flush();
var metrics = mocker.GetApplicationInsightsMetrics();

Best Practices

  • Always flush for metrics: Call mocker.Get<TelemetryClient>().Flush() before asserting on metrics with 3.x
  • Migrate to 3.x APIs: If upgrading from Application Insights 2.x, switch from GetSentTelemetry() to the type-specific methods (GetApplicationInsightsLogRecords(), GetApplicationInsightsMetrics(), GetApplicationInsightsActivities())
  • Add OpenTelemetry.Exporter.InMemory early: When using Application Insights 3.x, add this package to avoid the obsolete stub