问题
Inspired by Phil Haack's attempt on null or empty coalescing, I'm trying to write a couple of extension methods for the string object, as well as on the IEnumerable<T> interface, to simplify null or emtpy ckecking. However, I'm running into problems: when I'm attempting to call the string version of AsNullIsEmpty, the compiler treats my string as an IEnumerable<char>, and of course gives the wrong return type.
Is there any way to put an "anti-constraint" on the definition of the IEnumerable version, so that I can tell the compiler to use that one whenever the type of T is not string? Something like
public static IEnumerable<T> AsNullIfEmpty(this IEnumerable<T> items)
where T !: string
I know that I could just change the name of one of them, but I want to have the same name for consistency.
Update: It turns out my problem with the extension methods was solved another way, by fixing a simple and stupid error (I was using str.IsNullOrEmpty(), the extension method on IEnumerable<T>, instead of string.IsNullOrEmpty(str)...) but since the question of anti-constraints on generics is still an interesting one, I won't delete it.
回答1:
The only way to do this would be to create an overload of this extension that accepts a string as its this parameter.
public static string AsNullIfEmpty(this string value)
Doing this will cause the specifically-typed version to be considered a better overload match than the generic version.
As to your specific question ("Can I specify an 'anti-constraint' on a generic type parameter?"), the answer is no. You could get very close, though, with the Obsolete attribute.
[Obsolete("AsNullIfEmpty is not supported for strings.", true)]
public static string AsNullIfEmpty(this string value)
This would cause the compiler to report an error for this overload.
来源：https://stackoverflow.com/questions/3255450/anti-constraint-on-c-sharp-generics