Profiler .NET APIs FAQ

The .NET Profiler API is a set of interfaces that allow you to write custom profilers to instrument and analyze the execution of .NET applications. It enables you to gather data about method calls, object allocations, and other runtime events.

You can use the Profiler API to:

• Measure execution time of methods.
• Track object allocations and garbage collection behavior.
• Monitor thread activity and synchronization.
• Implement custom code coverage tools.
• Profile managed code execution within native applications.

The Profiler API itself is not directly part of the .NET SDK in the same way that the base class libraries are. It's a COM-based interface provided by the .NET runtime. You typically interact with it by implementing COM interfaces and loading your profiler as a DLL.

To start, you'll need to:

1. Understand the COM interfaces involved (e.g., ICorProfilerCallback, ICorProfilerInfo).
2. Implement the ICorProfilerCallback interface in a COM-visible DLL.
3. Register your profiler DLL correctly.
4. Attach your profiler to a running .NET process or start a process with your profiler attached.

Microsoft provides sample profilers and documentation that are excellent starting points.

Since the Profiler API is COM-based, you can use any language that supports COM interop. C++ is a common choice due to its direct control and performance, but C# with COM interop or other .NET languages can also be used.

Attaching a profiler is typically done using the CorRuntimeHost.StartInMonitoringMode or CorRuntimeHost.AttachProfiler methods via COM, or by setting environment variables like COR_ENABLE_PROFILING and COR_PROFILER when launching a process.

The .NET runtime notifies your profiler about events by calling methods on your implementation of the ICorProfilerCallback interface. These are known as callbacks. Examples include ModuleLoadFinished, FunctionEnter, and ObjectAllocated.

ICorProfilerInfo provides the basic set of methods to query information about the runtime and its objects. Later versions like ICorProfilerInfo2, ICorProfilerInfo3, and ICorProfilerInfo4 introduce new features and information, often related to newer .NET runtime capabilities such as generics, dynamic assemblies, and specific GC generations. Always try to query for the latest interface version your runtime supports.

You can use methods provided by ICorProfilerInfo (and its successors) such as GetModuleMetaDataInterface, GetFunctionInfo, GetClassFromObject, and GetMemberName to retrieve metadata and information about code elements.

Function hooking (or instrumentation) involves modifying the execution flow to intercept calls to specific functions. The Profiler API allows you to achieve this. For example, you can receive notifications when a function is entered or exited, and with appropriate permissions, you can insert your own code at these points using techniques like function redirection or inlining.

Profiling inherently adds overhead. The impact depends heavily on what events you subscribe to and how efficiently your profiler code is written. Frequent callbacks (e.g., for every object allocation or method call) can significantly slow down the application. It's crucial to:

• Only subscribe to necessary events.
• Implement callback handlers efficiently.
• Avoid blocking operations in callbacks.
• Minimize memory allocations within your profiler.

You can use the Profiler API to profile managed code that is called from native applications. This involves creating a native DLL that implements the profiler interfaces and then attaching it to the native process that hosts the CLR.

Profilers have deep access into the running application. Be mindful of security if you distribute your profiler. Ensure it's well-tested and doesn't introduce vulnerabilities. When running profilers with elevated privileges, be extra cautious.

You can use callbacks like GarbageCollectionStarted, GarbageCollectionFinished, and ObjectAllocated to monitor GC behavior. The ICorProfilerInfo2 (and later) interface provides more detailed information about GC generations.

The .NET runtime supports Just-In-Time (JIT) compilation, and code can be recompiled (reJIT). The Profiler API provides callbacks like JITCompilationStarted and GetILToNativeMapping to help you understand and profile dynamically generated code. Newer versions of the API offer more robust support for reJIT.