Now that we have enum constraint, why doesn't compiler allow me to write this code?
public static TResult? ToEnum<TResult>(this String value, TResult? defaultValue)
where TResult : Enum
{
return String.IsNullOrEmpty(value) ? defaultValue : (TResult?)Enum.Parse(typeof(TResult), value);
}
The compiler says:
Error CS0453 The type 'TResult' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'Nullable'
You can, but you have to add another constraint: the struct constraint.
public static void DoSomething<T>(T? defaultValue) where T : struct, Enum
{
}
Nullable<T> requires T to be a struct type @thermite.
Nullable<T> show the where T : struct constraint. learn.microsoft.com/en-us/dotnet/api/… . Here is the constraint in the source code: github.com/microsoft/referencesource/blob/master/mscorlib/… / github.com/dotnet/runtime/blob/v5.0.0-preview.5.20278.1/src/…Because System.Enum is a class, you cannot declare a variable of type Nullable<Enum> (since Nullable<T> is only possible if T is a struct).
Thus:
Enum? bob = null;
won't compile, and neither will your code.
This is definitely strange (since Enum itself is a class, but a specific Enum that you define in your code is a struct) if you haven't run into it before, but it is clearly a class (not a struct) as per the docs and the source code.
System.Enumis a special class(which can't be used as type parameter constraint anyway). It is so special that it inherits fromSystem.ValueTypebut is a reference type. stackoverflow.com/questions/14561338/…Enumbase class is a weird case in the .NET type hierarchy, and all classes that inherit from it are value types. I thought the new constraint was akin to theclassorstructconstraints, not just allowing a base class constraint that was forbidden before.