C＃泛型方法值

Question

I need to write method, that returns children of generic class in List . I wrote something like this :

static List<Model<T>> Get<T>( string value ) where T : Model<T>
{
    switch( value )
    {
        case "ROLE":
        return GetRoles();
    }
    return new List<Model<T>>();
}

GetRoles() returns List<Role> , where Role : Model<Role> .
But visual studio tells me that it cannot convert List<Role> to List<Model<T>> .

Answer 1

The error here is that List<Role> is not the same as List<Model<T>> even if Role is a Model<Role>

Example, you have two classes:

public class Bar : Model<Foo>
{

}

public class Foo : Model<Foo>
{

}

and create a list:

var fooList = new List<Foo>();

if you could cast it to List<Model<Foo>> :

var modelList = (List<Model<Foo>>) fooList ; // not possible

than you should be able to add new Bars to list:

modelList.Add(new Bar());

But that is still a list of Foo and it could have only Foo objects.

As a solution you could try to cast your Role to Model:

static List<Model<T>> Get<T>(string value)  where T:Model<T>
{
    switch (value)
    {
        case "ROLE":
            return GetRoles().Cast<Model<T>>().ToList();
            break;
    }
    return new List<Model<T>>();
}

Answer 2

The problem here is that List<T> is not covariant . What that means is that if you were allowed to do this, the caller would think it had a List<Model<T>> but in fact would have a List<Role> - if you then tried adding an instance of another class derived from Model<T> to your list it would fail. C# won't let you do that.

Instead, you could explicitly convert your result to List<Model<T>> using:

return new List<Model<T>>(GetRoles());