CODEWITHSUNDEEP
c#clr
Wei Ma
Wei Ma

Reputation: 3155

Customized Nullable struct

I am reading the CLR Via C# book and trying out the example provided. 

[Serializable, StructLayout(LayoutKind.Sequential)]
public struct MyNullable<T> where T : struct
{
    private readonly  bool _hasValue ; //Should be false
    private readonly T _value; //Really should be value=default(T)

    public MyNullable(T value)
    {
        _value = value;
        _hasValue = true;
    }

    public bool HasValue {
        get { return _hasValue; }
    }

    public T Value
    {
        get
        {
            if (!_hasValue)
            {
                throw new InvalidOperationException("Nullable object must have a value");
            }
            return _value;
        }
    }

    public T GetValueOrDefault()
    {
        return _value;
    }

    public T GetValueOrDefault(T defaultValue)
    {
        return !_hasValue ? defaultValue : _value;
    }

    public override bool Equals(object obj)
    {
        if (!_hasValue) return obj == null;
        if (obj == null) return false;
        return _value.Equals(obj);
    }

    public override int GetHashCode()
    {
        return !_hasValue ? 0 : _value.GetHashCode();
    }
    public override string ToString()
    {
        return _hasValue ? _value.ToString() : "";
    }


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

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

This code is supposed to let me assign a null value to MyNullable as

MyNullable i = null;

but Visual Studio is giving me error message as "Cannot convert source type null to target type MyNullable"

I thought public static implicit operator MyNullable(T value) and 

   **public static explicit operator T(MyNullable<T> value)**

Did the trick but seemed it's not the case.

Is there any way to make this code work?

Thanks.

Upvotes: 0

Views: 154

Answers (1)

leppie
leppie

Reputation: 117260

As @EricLippert stated in one of his recent blog posts, it is not possible to implement in user code. There are quite a few things happening behind the scenes.

See: http://ericlippert.com/2012/12/20/nullable-micro-optimizations-part-one/

Upvotes: 1

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