C# .NET – Implementation of Nullable

I am trying to implement Nullable type. But below mentioned code doesn't support null value for valuetype datatypes.

using System;
using System.Runtime;
using System.Runtime.InteropServices;

namespace Nullable
{

    [Serializable, StructLayout(LayoutKind.Sequential)]    
    public struct Nullable<T> where T : struct
    {
        private bool hasValue;

        public bool HasValue
        {
            get { return hasValue; }
        }

        internal T value;

        public Nullable(T value)
        {
            this.value = value;
            this.hasValue = true;
        }

        public T Value
        {
            get
            {
                if (!this.hasValue)
                {
                    new InvalidOperationException("No value assigned");
                }
                return this.value;
            }
        }

        public T GetValueOrDefault()
        {
            return this.value;
        }

        public T GetValueOrDefault(T defaultValue)
        {
            if (!this.HasValue)
            {
                return defaultValue;
            }
            return this.value;

        }

        public override bool Equals(object obj)
        {
            if (!this.HasValue)
            {
                return obj == null;
            }
            if (obj == null)
            {
                return false;
            }
            return this.value.Equals(obj);
        }

        public override int GetHashCode()
        {
            if (!this.hasValue)
            {
                return 0;
            }
            return this.value.GetHashCode();
        }

        public override string ToString()
        {
            if (!this.hasValue)
            {
                return string.Empty;
            }
            return this.value.ToString();
        }

        public static implicit operator Nullable<T>(T value)
        {
            return new Nullable<T>(value);
        }

        public static explicit operator T(Nullable<T> value)
        {
            return value.Value;
        }
    }
}

When I trying to assign the value as null, it's throwing an error "Cannot convert null to 'Nullable.Nullable' because it is a non-nullable value type"

What I have to do to resolve this issue?