Memory Mapped Files

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I'm having a hard time writing a string value to a memory mapped file, I've tried char arrays, byte arrays, and I just can't figure out how to get it to work. The exception I get for all these types is

The specified Type must be a struct containing no references.
Parameter name: type

My code is:

 public struct Test
        {
            public int a;
            public int b;
            public int c;
            public int d;
            [MarshalAs(UnmanagedType.ByValTStr,SizeConst = 40)]
            public string s;

            public void Add(int i)
            {
                a += i;
                b += i;
                c += i;
                d += i;
            }
            public void Append(string str)
            {
                s += str;
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            int TestSize = Marshal.SizeOf(typeof(Test));
            MemoryMappedFile mmf = MemoryMappedFile.CreateOrOpen("Test", TestSize, MemoryMappedFileAccess.ReadWriteExecute);
            MemoryMappedViewAccessor accessor = mmf.CreateViewAccessor(0, TestSize);
            Test _test;
            for (long i = 0; i < TestSize; i += TestSize)
            {
                accessor.Read(i, out _test);
                _test.Add(100);
                _test.Append("hello");
                accessor.Write(i, ref _test);
            }
        }