C#调用C++

extern "C" __declspec(dllexport) char* WINAPI base64_decode( char *data,char base[])
{
}

private void Form1_Load(object sender, EventArgs e)
{
    IntPtr ptrIn1 = Marshal.StringToHGlobalAnsi("这是一个中文base64加密的测试");
    IntPtr ptrIn2 = Marshal.StringToHGlobalAnsi("01234abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ56789+/=");
    IntPtr ptrIn3 = base64_encode(ptrIn1, ptrIn2);
    string retlust1 = Marshal.PtrToStringAnsi(ptrIn3);

    IntPtr ptrIn4 = Marshal.StringToHGlobalAnsi(retlust1);
    IntPtr ptrIn5 = Marshal.StringToHGlobalAnsi("01234abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ56789+/=");
    IntPtr ptrIn6 = base64_decode(ptrIn4, ptrIn5);
    string retlust2 = Marshal.PtrToStringAnsi(ptrIn6);
}
        
[DllImport("FwBase64.dll")]
private static extern IntPtr base64_encode(IntPtr a, IntPtr b);

[DllImport("FwBase64.dll")]
private static extern IntPtr base64_decode(IntPtr a, IntPtr b);