Non-nullable reference types. It's from Jon Skeet

NonNullable.cs

public struct NonNullable<T> where T : class
{
    private readonly T value;

    public NonNullable(T value)
    {
        if (value == null)
        {
            throw new ArgumentNullException("value");
        }
        this.value = value;
    }

    public T Value
    {
        get
        {
            if (value == null)
            {
                throw new NullReferenceException();
            }
            return value;
        }
    }

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

    public static implicit operator T(NonNullable<T> wrapper)
    {
        return wrapper.Value;
    }
}