Why can't I pass a Type variable to the keyword "default" in c#?

Question

I was to dynamically return the default value of a type, but I can't pass the default keyword a variable of type Type.

Why not?

Eg:

    private object GetSpecialDefaultValue(Type theType)
    {
        if (theType == typeof (string))
        {
            return String.Empty;
        }
        if (theType == typeof (int))
        {
            return 1;
        }
        return default(theType);
    }

Gives me the compile time error:

The type or namespace name 'theType' could not be found (are you missing a using directive or an assembly reference?)

possible duplicate of [Programmatic equivalent of default(Type)](http://stackoverflow.com/questions/325426/programmatic-equivalent-of-defaulttype) — Mitch, Feb 24 '14 at 06:03
Because default is resolved at compile time. Since the type you're giving it is not known at compile, it doesn't work. — Yev, Feb 24 '14 at 06:06

score 5 · Answer 1 · answered Feb 24 '14 at 05:55

5

You can only use default with generic type parameter.

The default keyword can be used in the switch statement or in generic code:

from default (C# Reference)

How about that one?

private object GetSpecialDefaultValue<T>()
{
    var theType = typeof(T);

    if (theType == typeof (string))
    {
        return String.Empty;
    }
    if (theType == typeof (int))
    {
        return 1;
    }
    return default(T);
}

Update

You can try following instead of default, but I'm not 100% sure it will work.

return theType.IsValueType ? (object)(Activator.CreateInstance(theType)) : null;

answered Feb 24 '14 at 05:55

MarcinJuraszek

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+1. Note that `default(SomeType)` can be used anywhere, but makes not much sense in non-generic code where `null`/`new SomeValueType` can give more clarity. – Alexei Levenkov Feb 24 '14 at 06:44
I would definitely make note of Alexi's comment in your post if only for the sake of accuracy. – Hardrada Feb 24 '14 at 06:59

score 2 · Accepted Answer · edited May 23 '17 at 12:26

The reason that default does not take an instance of Type is that there is no default instruction in IL. The syntax of default translates to ldnull or initobj T depending upon whether the type is a value type or not.

If you want to get a default value from a Type, perform the same logic as given in this other question:

public static object GetDefaultValue(Type t)
{
    if (!t.IsValueType || Nullable.GetUnderlyingType(t) != null)
        return null;

    return Activator.CreateInstance(t);
}

Why can't I pass a Type variable to the keyword "default" in c#?

2 Answers2