How to declare user defined Nullable type in C#

using System.IO;
using System;
struct MyStruct // Declare a struct.
{
    public int X; // Field
    public int Y; // Field
    public MyStruct(int xVal, int yVal) // Constructor
    {
        X = xVal;
        Y = yVal;
    }
}
class NullableType
{
    static void Main(string[] args)
    {
        MyStruct mSStruct = new MyStruct(6, 11); // Variable of struct
        MyStruct ? mSNull = new MyStruct(5, 10); // Variable of nullable type
        Console.WriteLine("mSStruct.X: {0}", mSStruct.X);
        Console.WriteLine("mSStruct.Y: {0}", mSStruct.Y);
        Console.WriteLine("mSNull.X: {0}", mSNull.Value.X);
        Console.WriteLine("mSNull.Y: {0}", mSNull.Value.Y);
    }
}