该类型不能在泛型类型或方法中用作类型“T”。 没有隐式的引用转换

Question

I'm trying to write a generic method to check if List is null or empty and throw an exception.

This is what I have:

public static void IsNullOrEmpty<T>(T argument) where T : List<T>
{
   if (argument == null || argument.Count == 0)
   {
        throw new ArgumentException("Argument " + nameof(argument) + " can't be null or empty list.");
   }
}

The problem is with calling this method; I trying to call this method like this:

ThrowIf.Argument.IsNullOrEmpty<List<ParameterServiceReference.Parameter>>(options.Parameters);

Where options.Parameters is type of: List<ParameterServiceReference.Parameter>.

I'm getting this error:

There is no implicit reference conversion from 'System.Collections.Generic.List<ParameterServiceReference.Parameter>' to 'System.Collections.Generic.List<System.Collections.Generic.List<ParameterServiceReference.Parameter>>

If I call method like this:

ThrowIf.Argument.IsNullOrEmpty<ParameterServiceReference.Parameter>(options.Parameters);

I get this error:

cannot convert from 'System.Collections.Generic.List<Project.Layer.ParameterServiceReference.Parameter>' to 'Project.Layer.ParameterServiceReference.Parameter'

Is this kind of generic method possible with generic collection or not, in the solution I will have many generic lists of different classes and I think it's not smart to write so many overloaded methods.

Answer 1

You generic constraint specifically says that the parameter needs to be some type that is a List where the generic type of the list is itself . When you say where T : List<T> you're saying that the generic argument, T , needs to be a List<T> where T is the type you're adding the constraint for.

What you should really do here is just have the method accept a List<T> , not a T where T is a List :

public static void IsNullOrEmpty<T>(List<T> argument) { //...

Answer 2

You can't have a constraint that's T : List<T> , just like you can't have a type MyList that inherits from MyList (or type T that inherits from List<T> ).

Just use public static void IsNullOrEmpty<T>(List<T> argument)

Answer 3

You can have something like this:

public static void IsNullOrEmpty<S,T>(T argument) where T : List<S>
{
    if (argument == null || argument.Count == 0)
    {
        throw new ArgumentException("Argument " + nameof(argument) + " can't be null or empty list.");
    }
}

Now you can call your method:

ThrowIf.Argument.IsNullOrEmpty<ParameterServiceReference.Parameter, List<ParameterServiceReference.Parameter>>(options.Parameters);