泛型：可为空类型的约束

Question

The following doesn't compile

public static T Retrieve<T>(this NameValueCollection collection, String key) where T : Object
{

    if (collection.AllKeys.Contains(key))
    {
        try
        {
            val = (T)Convert.ChangeType((object)collection[key], typeof(T));
        }
        catch { }
    }

    return val;            
}

because the Constraint cannot be the object class. So is there a way to contrain T for anything that can be set to a null?

Answer 1

where T : class

Your current constraint, where T : Object says "anything which is or inherits from System.Object", which is: everything. All types, including Int32 and String, inherit from System.Object. So constraining on Object would do nothing.

Edit: as usual, Eric shines a light on this in a far more accurate way :

"in C# every type derives from object". Not true! The way to correct this myth is to simply replace "derives from" with "is convertible to", and to ignore pointer types: every non-pointer type in C# is convertible to object.

Answer 2

I don't believe it is possible to constrain your generic argument purely to a nullable type. You can easily constrain it to a reference type (as in previous answer), but, while all reference types are nullable, not all nullables are reference types.