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?
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.
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