C＃指针：错误CS0254：固定语句赋值的右侧可能不是强制转换表达式

Question

I have a c++ server and now a trying to make ac# client.

In c++, to send a packet I make this:

PMessage *msg = (PMessage*)&buf[0];
msg->id = PID_LOGIN;
sendto(sServer, buf, sizeof(buf), 0, (SOCKADDR*)&from, fromlen);

in c# I trying this:

unsafe
{
    try
    {
        byte[] rawdata = new byte[Marshal.SizeOf(typeof(PMessage))];

        fixed (PMessage* p = (PMessage*)&rawdata[0])
        {

        //     p->id = 1001;
        }
        udpClient.Send(rawdata, rawdata.Length); ;
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.ToString());
    }
}

The compiler show this error:

error CS0254: The right hand side of a fixed statement assignment may not be a cast expression.

c#:

[StructLayout(LayoutKind.Sequential, Pack = 1)]
 unsafe struct PMessage
{
    public  fixed byte msg[256];
    public int id;
    public int size;
}

c++:

   struct PMessage
    {
        char msg[256];
        unsigned int id;
        unsigned int size;
    };

Answer 1

Try this:

PMessage p = new PMessage();
// p.id = 1001;
byte[] rawdata = new byte[Marshal.SizeOf(typeof(PMessage))];
IntPtr mem = Marshal.AllocHGlobal(rawdata.Length);
try{
    Marshal.StructureToPtr(p, mem, true);
    Marshal.Copy(mem, rawdata, 0, rawdata.Length);
}finally{
    Marshal.FreeHGlobal(mem);
}
udpClient.Send(rawdata, rawdata.Length);

Also don't mix unsafe ( fixed ) and unmanaged ( SizeOf ) environments.

Answer 2

You can't do this, because the compiler loses track of which object needs to be pinned.

fixed (PMessage* p = (PMessage*)&rawdata[0])

However this should work

fixed (byte* pby = &rawdata[0]) {
    PMessage* p = (PMessage*)pby;

Please note that your C++ code was probably violating strict aliasing, and both C++ and C# may not have correct alignment.