如何使泛型方法允许返回null并接受枚举？

Question

How to make the following extension works? I'm binding the ComboBoxe to an enum and in this case it doesn't compile because it returns null .

public static T GetSelectedValue<T>(this ComboBox control)
{
    if (control.SelectedValue == null)
        return null;

    return (T)control.SelectedValue;
}

Note: I want it te return null (instead of default(T)). The question is that what's the where expression that I've got to use?

Answer 1

Return a nullable instead of a plain T :

public static T? GetSelectedValue<T>(this ComboBox control) where T : struct
{
    if (control.SelectedValue == null)
        return null;

    return (T)control.SelectedValue;
}

Answer 2

That's impossible. Value types cannot be null. Your extension method returns an instance of T and if this T is an enum (value type), its value cannot be null. So without changing your return type such method signature simply cannot exist. As far as constraining the generic parameter to be an enum, that's also impossible in C# but possible in MSIL. Jon has blogged about it .

Answer 3

在这种情况下，最常见的方法是在枚举的所有其他成员中定义None ，在这种情况下，在您的逻辑中为None == null 。