C#•2mo ago

Managed callback to native code (reverse p/invoke)

If I pass a managed callback to native code, do I need to pin the stored delegate so it does not get moved to a new memory address or is this handled by the runtime?

public class MyTestClass
{
    private delegate void NativeTestCallback(int num);

    [DllImport("TestNative")]
    private static extern void NativeFunc(NativeTestCallback callback);

    private NativeTestCallback _callback;
    public MyTestClass()
    {
        // do I need to pin this?
        _callback = TestCallback;
        NativeFunc(_callback);
    }

    private void TestCallback(int num)
    {

    }
}

public class MyTestClass
{
    private delegate void NativeTestCallback(int num);

    [DllImport("TestNative")]
    private static extern void NativeFunc(NativeTestCallback callback);

    private NativeTestCallback _callback;
    public MyTestClass()
    {
        // do I need to pin this?
        _callback = TestCallback;
        NativeFunc(_callback);
    }

    private void TestCallback(int num)
    {

    }
}

8 Replies

Mąż Zuzanny Harmider Szczęście•2mo ago

all you need to do is make it static i think

nathanAjacobsOP•2mo ago

Yeah I could, but I'm curious specifically about the semantics of an instanced callback. For example, I could have an unmanaged and managed representation of a class and have an instance id to track it between creation and disposal on the native side

Mąż Zuzanny Harmider Szczęście•2mo ago

then ig u have to pin it, not really sure tho

nathanAjacobsOP•2mo ago

Yeah that's what I'm thinking too, but wanted to get confirmation

wasabi•2mo ago

You do not have to pin it. But you do have to keep a reference to it so it isn't collected.

nathanAjacobsOP•2mo ago

Yeah I found out the garbage collection part the hard way lol. Can you provide more info on why it doesn't have to be pinned? Couldn't the memory address change if not pinned? Okay I asked a LLM and it says this: "When you pass a managed delegate to native code via P/Invoke, the CLR automatically generates a thunk (a small, fixed bridge between native and managed code). This thunk has a fixed address in memory, even if the delegate or its target object (ManagedClass) is moved by the GC." Is this true?

wasabi•2mo ago

Uh huh.

nathanAjacobsOP•2mo ago

TIL what a thunk is

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Managed callback to native code (reverse p/invoke)

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