Fix TelemetryClient.Flush/FlushAsync NullReferenceException in DI scenarios

[rajkumar-rangaraj]

Fix Issue #3115 .

Changes

TelemetryClient.Flush() and FlushAsync() throw System.NullReferenceException when called from DI scenarios because this.sdk is null.

In DI scenarios, TelemetryClient is constructed with isFromDependencyInjection: true, which intentionally skips calling TelemetryConfiguration.Build() — the OTel SDK is owned by AddOpenTelemetry() in the DI container, not by TelemetryConfiguration. However, both Flush() and FlushAsync() unconditionally access this.sdk.TracerProvider, causing:

System.NullReferenceException: Object reference not set to an instance of an object.
at Microsoft.ApplicationInsights.TelemetryClient.<>c__DisplayClass36_0.<FlushAsync>b__0()

• Added an IServiceProvider field and a new internal constructor that accepts it
• Updated the DI TelemetryClient factory in AddTelemetryConfigAndClient to pass the IServiceProvider
• Flush() and FlushAsync() now check: if this.sdk is available (non-DI), flush via the SDK; otherwise if this.serviceProvider is available (DI), resolve TracerProvider/MeterProvider/LoggerProvider from the DI container and flush those

[Copilot]

Pull request overview

This pull request fixes a NullReferenceException in TelemetryClient.Flush() and FlushAsync() methods when called in dependency injection (DI) scenarios. The issue occurred because in DI scenarios, TelemetryClient is constructed with isFromDependencyInjection: true, which intentionally skips building the OpenTelemetry SDK (since it's managed by the DI container via AddOpenTelemetry()). However, both flush methods unconditionally accessed this.sdk.TracerProvider, causing a NullReferenceException when sdk was null.

Changes:

• Added IServiceProvider field to TelemetryClient and a new internal constructor that accepts it
• Updated DI registration to pass the IServiceProvider to TelemetryClient
• Modified Flush() and FlushAsync() to check if SDK is available first, then fall back to resolving providers from DI container

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
BASE/src/Microsoft.ApplicationInsights/TelemetryClient.cs	Added serviceProvider field, new constructor accepting IServiceProvider, and updated Flush()/FlushAsync() to resolve OTel providers from DI when sdk is null
NETCORE/src/Shared/Extensions/ApplicationInsightsExtensions.cs	Updated TelemetryClient factory registration to pass IServiceProvider to the new constructor

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[harsimar]

How was this tested? Consider adding some automated testing

[rajkumar-rangaraj]

How was this tested? Consider adding some automated testing

We don't have an automated infra built yet. It's outside the scope of the PR. We might need to consider building one.
I tested using the repro provided by customer.

[maarten-kieft]

I'm running in this issue; what is the expected release date of this fix?

[rajkumar-rangaraj]

I'm running in this issue; what is the expected release date of this fix?

Within 2 weeks.