Represents a reference type that may never be assigned a value of null.

NotNull.cs

public struct NotNull<T> where T : class
{
    private T mValue;
    public T Value
    {
        get { return this.mValue; }
        set
        {
            if( value == null )
                throw new ArgumentNullException( "value" );
            
            this.mValue = value;
        }
    }
    
    // Prior to C# 6.0, structs were not allowed to contain user-defined,
    // parameterless constructors. Remove the preprocessor directive
    // or define 'CSHARP6' in your preferred way to enable this constructor.
#if CSHARP6
    public NotNull()
    {
        throw new InvalidOperationException( "NotNull must be initialized with a value." );
    }
#endif

    public NotNull( T value )
    {
        Value = value;    
    }

    public static implicit operator T( NotNull<T> instance )
    {
        return instance.Value;
    }

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

    public override string ToString()
    {
        return this.Value.ToString();    
    }

    public static bool operator ==( NotNull<T> left, T right )
    {
        return right == null ? false : left.Equals( right );    
    }

    public static bool operator !=( NotNull<T> left, T right )
    {
        return !( left == right );  
    }

    public override bool Equals( object other )
    {
        if( other == null )
            return false;
        
        if( other is NotNull<T> )
            return this.Equals( (NotNull<T>)other );
        
        if( other is T )
            return this.Equals( (T)other );

        return false;
    }

    public bool Equals( NotNull<T> other )
    {
        return this.Value.Equals( other.Value );
    }

    public override int GetHashCode()
    {
        return this.Value.GetHashCode();    
    }
}