Start a journey into the .NET Profiling APIs

Christophe Nasarre
5 min readAug 7, 2021



When I want to dig into a new API, I implement a real world scenario. This is exactly what I did for the .NET native profiling API. I want to know how to get parameters and return value of any method call during any .NET application life. The expected result would be something like :

Enter PublicClass::ClassParamReturnClass
this = 0x6f97e190 (8)
ClassType obj = 0x6f97e488 (8)
| int32 <IntProperty>k__BackingField = 84
| int32 intField = 42
| String stringField = 43
ClassType obj = 0x0000023A475BBAD8
Leave PublicClass::ClassParamReturnClass
| int32 <IntProperty>k__BackingField = 170
| int32 intField = 85
| String stringField = 86
returns 0x0000023A475BBBB0

when the following method is executed:

You will have to write native C/C++ code to leverage the .NET Profiling API as John Robbins explained in his 2003 Debugging Applications book. Even though Microsoft is providing a code sample for that, it does just show how to get notified when a function is called or exited but nothing about its type, its name, what are its parameters value and its return value. This series will detail both the .NET profiling API and the metadata API (i.e. the native reflection API).

Let’s start with the basics of .NET Profiling. As shown in the following figure from the Microsoft Profiling documentation, with the right environment configuration, the CLR will load a COM-like object implementing ICorProfilerCallback interface to notify almost everything happening in a .NET application from startup to shutdown.

Since .NET was launched, more and more notifications have been added by versioning the interface up to ICorProfilerCallback9. Welcome to the usual COM world! I recommend that you watch Pavel Yosifovich session about writing a CLR Profiler in an hour to get an overview. I will use his sample solution as a starting point.

For performance sake, you tell the runtime which events you are interested in; i.e. which ICorProfilerCallback functions will be called and you simply have to return S_OK from all other functions. This setup is done in your implementation of ICorProfilerCallback::Initialize. This first function called by the CLR provides a parameter from which you need to QueryInterface a version of ICorProfilerInfo interface (the current one is ICorProfilerInfo10). This interface provides functions to query information about parameters passed to your ICorProfilerCallback functions (such as AppDomain, assembly, type, function, thread and so on).

The first ICorProfilerInfo function you will use is SetEventMask to filter the ICorProfilerCallback functions that will be called by the runtime. It accepts a flag combination of values from the COR_PRF_MONITOR enumeration.

Assembly code is needed for Enter/Leave/TailCall

To get notified when a managed method is called or exits, you should pass:


to ICorProfilerInfo::SetEventMask. The first flag tells the runtime to call static callbacks (i.e. not exposed as functions of ICorProfilerCallback) when a managed method gets executed or returns. The other three flags ensure that these callbacks will receive enough information to extract method arguments and return value.

Unlike the other notifications that end up calling your ICorProfilerCallback functions, you need to register three special callbacks to the runtime via ICorProfilerInfo3::SetEnterLeaveFunctionHooks3WithInfo. For performance reasons, the .NET team asks you to write the prolog and epilog (i.e. saving/restoring CPU registers on/from the stack) yourself in assembly code instead of relying on well-defined calling conventions supported by the C/C++ compiler.

This is why you have to call ICorProfilerInfo3:: SetEnterLeaveFunctionHooks3WithInfo and pass pointers to these “naked” functions. The Microsoft ELTProfiler sample implements the stubs both for x86 (as inlined assembly embedded in a C++ file) and x64 (defined in .asm file). In x64, you need to update your project file to add the following:

The nakedcallbacks.asm file contains the assembly code to call stub functions wrapped by the expected prolog and epilog written in assembly code. Here is the signature of the functions from where you will be able to start working in C++:

A function is identified by a FunctionID and this is where you start your adventure (I will come back later to the COR_PRF_ELT_INFO parameter). Note that for the EnterStub function, you need to get the FunctionID from the FunctionIDOrClientID.functionId field.

Once your hook callbacks have been registered via ICorProfilerInfo3:: SetEnterLeaveFunctionHooks3WithInfo, it is still possible to decide whether or not a managed method call should trigger them. For that, it is needed to register a “mapper” function that is called once per managed method with one of these functions:

The latter allows you to pass some “client data” to the mapper function such as a helper class to manipulate the received FunctionID or your profiler state.

If pHookFunction is set to TRUE, your enter/leave functions will be called and the returned UINT_PTR will be passed as the FunctionID parameter. This allows you to handle function name or signature computation at one single place outside of the real profiling work done each time a method is called. If pHookFunction is set to FALSE, the enter/leave functions will never be called for that FunctionID. The mapper callback is called only once per FunctionID: this could be a good way to avoid performance impact if you just want to profile a small subset of methods.

How to debug your profiler

Before going any further, it is needed to know how to debug your C++ profiler code with Visual Studio. The first step is to write a simple .NET application to execute the method calls you want to intercept. The second natural step would be to setup the environment variables needed to inject your profiler:

and also check the Enable native code debugging option:

If you start a debug session, the Visual Studio debugger with use both managed and native debugging APIs. Unfortunately, the managed debugging API does not allow breakpoints sets in ICorProfilerCallback functions.

Instead, you need to Debug | Start Without Debugging (CTRL+F5), not Debug | Start Debugging (F5) the C# test application with the same environment variables and attach the debugger via Debug | Attach to Process. Select the process and click Select button in the Attach to section:

To make attachment simple, add a Console.ReadLine in a console application before starting the method calls to test.

The next post will show you how to extract information from a FunctionID.




Christophe Nasarre

Loves to understand how things work (MVP Developer Technologies)