C# Extension Method for Null or value

问题

How to write an extension method that should check value of the object,if object is null then it should return null otherwise value{without doing casting at receiving end}.

something like...

public static object GetDefault(this object obj)
{
    if (obj == null) return null;
    else return obj;
}

I mean without casting can i check for null?

int? a=a.GetDefault();

ContactType type=type.GetDefault();   [For EnumType]

string  s=a.GetDefault()

回答1:

This should work:

public static class ExtensionMethods
{
    public static T GetObject<T>(this T obj, T def)
    {
        if (default(T).Equals(obj))
            return def;
        else
            return obj;
    }
}

I've added a parameter def because I expected you to want to return this default value when obj is null. Otherwise, you can always leave out the T def parameter and return null instead.

来源：https://stackoverflow.com/questions/3987986/c-sharp-extension-method-for-null-or-value