SampleUnion.cs

using System.Runtime.InteropServices;

namespace ConsoleApp1
{
    [StructLayout(LayoutKind.Explicit)]
    struct SampleUnion
    {
        [FieldOffset(0)]
        public uint x;

        [FieldOffset(0)]
        public byte byte1;

        [FieldOffset(1)]
        public byte byte2;

        [FieldOffset(2)]
        public byte byte3;

        [FieldOffset(3)]
        public byte byte4;
    }
}