? (nullable) operator in C# [duplicate]

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As others have said, “?” is just shorthand for changing it to Nullable<T>. This is just another value type with a Boolean flag to say whether or not there’s really a useful value, or whether it’s the null value for the type. In other words, Nullable<T> looks a bit like this:

public struct Nullable<T>
{
    private readonly bool hasValue;
    public bool HasValue { get { return hasValue; } }

    private readonly T value;
    public T value
    {
        get
        {
            if (!hasValue)
            {
                throw new InvalidOperationException();
            }
            return value;
        }
    }

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

    // Calling new Nullable<int>() or whatever will use the
    // implicit initialization which leaves value as default(T)
    // and hasValue as false.
}

Obviously in the real code there are more methods (like GetValueOrDefault()) and conversion operators etc. The C# compiler adds lifted operators which effectively proxy to the original operators for T.

At the risk of sounding like a broken record, this is still a value type. It doesn’t involve boxing… and when you write:

int? x = null;

that’s not a null reference – it’s the null value of Nullable<int>, i.e. the one where hasValue is false.

When a nullable type is boxed, the CLR has a feature whereby the value either gets boxed to a null reference, or a plain boxed T. So if you have code like this:

int? x = 5;
int y = 5;

object o1 = x;
object o2 = y;

The boxed values referred to by o1 and o2 are indistinguishable. You can’t tell that one is the result of boxing a nullable type.

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