To restrict or control what gets logged into Azure Application Insights, you can use several methods depending on your scenario. Here’s a complete guide for restricting or filtering telemetry being sent to Application Insights in an ASP.NET Core application:

✅ 1. Use Telemetry Processor
A Telemetry Processor allows you to filter out telemetry (like requests, dependencies, traces, exceptions) before it’s sent to Application Insights.

Example: Custom Telemetry Processor
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public class MyTelemetryProcessor : ITelemetryProcessor
{
private ITelemetryProcessor _next;

public MyTelemetryProcessor(ITelemetryProcessor next)
{
    _next = next;
}
public void Process(ITelemetry item)
{
 // Example: filter out requests to /health
 if (item is RequestTelemetry request && request.Url.AbsolutePath.Contains("/health"))
 {
 return; // Don't log this telemetry
 }
    _next.Process(item);
}

}
Register it in Startup.cs or Program.cs:

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services.AddApplicationInsightsTelemetry();
services.AddApplicationInsightsTelemetryProcessor();
✅ 2. Use Telemetry Initializer
A Telemetry Initializer adds or modifies data but doesn’t block it. You can use this to suppress sensitive fields or add custom properties (e.g., user ID, tenant ID).

Example: Mask PII
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public class CustomTelemetryInitializer : ITelemetryInitializer
{
public void Initialize(ITelemetry telemetry)
{
if (telemetry is RequestTelemetry request)
{
// Mask user-agent or add a tag
request.Context.User.Id = “Anonymous”;
}
}
}
Register it:

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services.AddSingleton();
✅ 3. Control Sampling
Sampling reduces the volume of telemetry sent to Application Insights, helping with cost and noise.

Example: Adaptive Sampling (default)
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services.AddApplicationInsightsTelemetry(options =>
{
options.EnableAdaptiveSampling = true;
});
Example: Fixed-rate Sampling
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services.Configure((config) =>
{
var processor = new SamplingTelemetryProcessor
(
config.DefaultTelemetrySink.TelemetryProcessorChainBuilder.Build()
)
{
SamplingPercentage = 25.0 // Keep 25% of telemetry
};
config.DefaultTelemetrySink.TelemetryProcessorChainBuilder.Use((next) => processor);
});
✅ 4. Disable Logging Completely for Specific Providers
You can control what logs are forwarded using LoggingBuilder filters.

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services.AddLogging(builder =>
{
builder.AddFilter(“”, LogLevel.Warning); // Log warnings and above
});
✅ 5. Disable Automatic Collection
You can disable certain automatic telemetry types like dependencies, performance counters, etc.

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services.AddApplicationInsightsTelemetry(options =>
{
options.EnableDependencyTrackingTelemetryModule = false;
options.EnableRequestTrackingTelemetryModule = false;
});
✅ 6. Environment-based Filtering
Only log in production, or restrict based on environment.

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if (env.IsProduction())
{
services.AddApplicationInsightsTelemetry();
}

It’s possible via configuration (appsettings.json/web.config/whatever you have).
Here’s an example of such configuration section from appsettings.json:

 "Logging": {
 "LogLevel": {
 "Default": "Information",
 "Microsoft": "Warning",
 "Microsoft.Hosting.Lifetime": "Information"
 },
 "ApplicationInsights": {
 "LogLevel": {
 "Default": "Warning",
 }
 }
 }

Also, if the intention is to save money by reducing logging you can specify sampling level for Application Insight instance in Azure. Having 10% sampling means only 10% of your logging entries will be sent to Application Insight.