C＃本机回调，带有指向struct参数的指针

Question

I'm writing a C# wrapper for a native library. It contains this callback function:

typedef application_event_result(*application_event_ptr)(application_request* request);

The parameter is defined as such:

typedef struct {
  uint32_t    query;
  const char* client;
  bool        isAuthenticated;
  bool        isGuest;
} application_request;

I have defined the C# callback delegate like this:

  [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
  public delegate application_event_result application_event([MarshalAs(UnmanagedType.Struct)]
        ref application_request request);

The structure in C#:

[StructLayout(LayoutKind.Sequential)]
public struct application_request
{
    public UInt32 query;

    [MarshalAs(UnmanagedType.LPStr)]
    public string client;

    [MarshalAs(UnmanagedType.I1)]
    public bool isAuthenticated;

    [MarshalAs(UnmanagedType.I1)]
    public bool isGuest; 
}

This all seems to work. The callback in C# is triggered, and the members of the struct have the expected values.

But upon return to native code, a heap corruption exception is triggered (0xc0000374).

Obviously, I would like to avoid that.

If I change the C# callback signature to use IntPtr instead of the "ref application_request" parameter, and then Marshal it manually using the following code it works.

var request = Marshal.PtrToStructure<application_request>(requestptr);

But I would like to have the signature be as precise as possible and not have to use the Marshaler myself.

Is there a away for me to change the callback delegate's signature so .net can convert the struct automatically?

Answer 1

Your problem is the char* member of the struct . The C# marshaler assumes that it is responsible for deallocating that memory. It does so by calling CoTaskMemFree . I think it is pretty clear that the memory is not meant to be destroyed by the C# code at all.

Marshal that member as IntPtr instead:

[StructLayout(LayoutKind.Sequential)]
public struct application_request
{
    public UInt32 query;

    public IntPtr client;

    [MarshalAs(UnmanagedType.I1)]
    public bool isAuthenticated;

    [MarshalAs(UnmanagedType.I1)]
    public bool isGuest; 
}

Inside your callback method you can read the value of the string by calling Marshal.PtrToStringAnsi .

You wrap this up by making the members private and exposing them via properties. That will allow you to encapsulate the conversion from pointer to string.