CODEWITHSUNDEEP
c#listgenericsoverriding
JosephStyons
JosephStyons

Reputation: 58745

Mismatched List Types

I have a base class that has an abstract method which returns a list of itself.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    public abstract class baseclass
    {
        public abstract List<baseclass> somemethod();        
    }
}

And a descendant that tries to override the base class's method by returning a list of *it*self.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class childclass : baseclass
    {
        public override List<childclass> somemethod()
        {
            List<childclass> result = new List<childclass>();
            result.Add(new childclass("test"));
            return result;
        }

        public childclass(string desc)
        {
            Description = desc;
        }

        public string Description;
    }
}

But I get this error:

Error   1   'ConsoleApplication1.childclass.somemethod()':
return type must be 'System.Collections.Generic.List<ConsoleApplication1.baseclass>'
to match overridden member 'ConsoleApplication1.baseclass.somemethod()' 
C:\Users\josephst\AppData\Local\Temporary Projects\ConsoleApplication1childclass.cs 
0   42  ConsoleApplication1

What's the best approach to have a base class return a list of itself, overriding the base class's method that does the same thing?

Upvotes: 2

Views: 180

Answers (3)

GSerjo
GSerjo

Reputation: 4778

Generic i s nice solution, but don't use public abstract List<baseclass> somemethod(); it's bad practice

You should use non-virtual interface pattern

public abstract class BaseClass<T>
{
    protected abstract List<T> DoSomeMethod();

    public List<T> SomeMethod()
    {
        return DoSomeMethod();
    }
}

public class ChildClass : BaseClass<ChildClass>
{
    protected override List<ChildClass> DoSomeMethod(){ ... }
}

Upvotes: 2

Tim Schmelter
Tim Schmelter

Reputation: 460228

The error message is self-explanatory. To override the method you need to return a List<baseclass>.

public override List<baseclass> somemethod()
{
    List<childclass> result = new List<childclass>();
    result.Add(new childclass("test"));
    return result;
}

Upvotes: 1

Nikola Anusev
Nikola Anusev

Reputation: 7088

When overriding a method, a signature of overriding method must exactly match the signature of the method being overriden. You can achieve what you want with generics:

public abstract class BaseClass<T>
{
    public abstract List<T> SomeMethod();
}

public class ChildClass : BaseClass<ChildClass>
{
    public override List<ChildClass> SomeMethod() { ... }
}

Upvotes: 2

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