In C# how do you pass a function pointer to a C++ dll using DllImport

Question

I have a dll which takes in a function pointer with a const char * as an argument.
I am trying to call this from my C# code using DllImport.
It works in the method where I set it but not if I call it again.

C: Code
The function format: void Log( const char * msg );
To set the function: SetLogger( &Log);
To call the log: RunLog(); // This runs Log("Test")

C# code
public delegate void LogDelegate([MarshalAs(UnmanagedType.LPStr)]string msg);
[DllImport(DllName, CallingConvention = CallingConvention.Cdecl)]
private static extern void SetLogger([MarshalAs(UnmanagedType.FunctionPtr)] LogDelegate logFunction);

[DllImport(DllName, CallingConvention = CallingConvention.Cdecl)]
private static extern void RunLog();

public void Logger( string msg )
{
// save msg to a file.
}

// Run first
public void Initialize()
{
SetLogger( Logger );
RunLog(); // This works.
}

// Run second.
public void ReRun()
{
RunLog(); // Fails sometimes does nothing some times null pointer exception.
}

Answer 1

Hi Hans-7890,

I found a similar issue on stack overflow: How to pass function pointer from C# to a C++ Dll?, maybe it could help you.

And since this issue is more related to Visual C#, which is not supported in Microsoft Q&A forum currently. We recommend you could go to msdn forum: Visual C#, and the community members will provide dedicated support for you.

Best Regards,
Dylan

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