Marshalling a C# structure

I am trying to serialize the following c# structure:

[Serializable]
[StructLayout(LayoutKind.Sequential, Size = 70, CharSet = CharSet.Ansi)]
public struct USSDContinueModel
{
    [MarshalAs(UnmanagedType.U4)]
    public uint Command_Length;

    [MarshalAs(UnmanagedType.U4)]
    public uint Command_ID;

    [MarshalAs(UnmanagedType.U4)]
    public uint Command_Status;

    [MarshalAs(UnmanagedType.U4)]
    public uint Sender_ID;

    [MarshalAs(UnmanagedType.U4)]
    public uint Receiver_ID;

    [MarshalAs(UnmanagedType.U1)]
    public uint Ussd_Version;

    [MarshalAs(UnmanagedType.U1)]
    public uint Ussd_Op_Type;

    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 21)]
    public string MsIsdn;

    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 21)]
    public string Service_Code;

    [MarshalAs(UnmanagedType.U1)]
    public uint Code_Scheme;

    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 182)]
    public string Ussd_Content;

    // Calling this method will return a byte array with the contents
    // of the struct ready to be sent via the tcp socket.
    public byte[] Serialize()
    {
        // allocate a byte array for the struct data
        var buffer = new byte[Marshal.SizeOf(typeof(USSDContinueModel))];

        // Allocate a GCHandle and get the array pointer
        var gch = GCHandle.Alloc(buffer, GCHandleType.Pinned);
        var pBuffer = gch.AddrOfPinnedObject();

        // copy data from struct to array and unpin the gc pointer
        Marshal.StructureToPtr(this, pBuffer, false);
        gch.Free();

        return buffer;
    }

    // this method will deserialize a byte array into the struct.
    public void Deserialize(ref byte[] data)
    {
        var gch = GCHandle.Alloc(data, GCHandleType.Pinned);
        this = (USSDContinueModel)Marshal.PtrToStructure(gch.AddrOfPinnedObject(), typeof(USSDContinueModel));
        gch.Free();
    }
}

when i try to serialize an instance of the structure say:

        public USSDContinueModel continueModel;
        continueModel.Command_Length = 174;
        continueModel.Command_ID = 0x00000070;
        continueModel.Command_Status = 0;
        continueModel.Sender_ID = 0x01000005;
        continueModel.Receiver_ID = 0x2900AB12;
        continueModel.Ussd_Version = 0x20;
        continueModel.Ussd_Op_Type = 0x01;
        continueModel.MsIsdn = "08098765476";
        continueModel.Service_Code = "*308";
        continueModel.Code_Scheme = 0x44;
        continueModel.Ussd_Content = "1. Continue if you are 18+ 2. Exit i";

I keep getting the error "Type: USSDcontinueModel cannot be marshalled as an unmanaged structure; no meaningful size or offset can be computed".

I noticed this happens when i set the Ussd_Version, Ussd_Op_Type and Code_Scheme as [MarshalAs(UnmanagedType.U1)] but it works fine with [MarshalAs(UnmanagedType.U4)].

Is [MarshalAs(UnmanagedType.U1)] unmarshallable? what do i do?

The problem is that the definition:

[MarshalAs(UnmanagedType.U1)]
public uint Ussd_Op_Type;

is ambiguous as to what size the members could be. If the interop marshal uses the member's size, it will calculate 4 bytes, whereas if it uses the MarshalAs attribute it will calculate 1 byte. Changing the member to use the correct size type,

[MarshalAs(UnmanagedType.U1)]
public byte Ussd_Op_Type;

should fix the issue.