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function foo(pcResultBuffer: pchar; iResultBufferSize: integer): integer; stdcall;                    

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[UnmanagedFunctionPointer(CallingConvention.StdCall)]
private delegate int FooCallbackType([MarshalAs(UnmanagedType.LPStr)] string resultBuffer, int resultBufferSize);                    

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private FooCallbackType fooCallback = null;                    

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private void GetProcAddresses()
{
    IntPtr funcAddr = IntPtr.Zero;
    
    funcAddr = GetProcAddress(dllHandle, "foo");
    if (funcAddr != IntPtr.Zero)
        fooCallback = (FooCallbackType)Marshal.GetDelegateForFunctionPointer(
            funcAddr, typeof(FooCallbackType));
}                    

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string resultBuffer;
int bufferSize;
int charsInBuffed;

//hier Buffer-Initialiserierung ????

charsInBuffed = foo(resultBuffer, bufferSize);