CODEWITHSUNDEEP
c#
Jallrich
Jallrich

Reputation: 469

abstract List<T>.Add not working on base constructor

I have a simple class which overrides a list getter with predefined values, but, when calling the add method on the base constructor, it doesn't add the new value.

This is my code showing the error:

using System.Collections.Generic;
using NUnit.Framework;

public class SimpleTest
{
    [Test]
    public void Test()
    {
        var child = new ChildClass();
        Assert.AreEqual(4, child.myList.Count);
    }

    private abstract class MyBaseClass
    {
        public abstract List<string> myList { get; }

        public MyBaseClass(string addThis)
        {
            myList.Add(addThis);
        }
    }

    private class ChildClass : MyBaseClass
    {
        public override List<string> myList => new List<string> { "one", "two", "three" };

        public ChildClass() : base("four")
        {
            myList.Add("dsada");
        }
    }
}

Why is this caused? By not having a setter the list can't be replaced by a different one but the Add method should still work for this case.

Upvotes: 2

Views: 151

Answers (2)

MakePeaceGreatAgain
MakePeaceGreatAgain

Reputation: 37050

When you write public override List<string> myList => new List<string> { "one", "two", "three" }; this is just a short-cut for the following:

public override List<string> myList { get { return new List<string> { "one", "two", "three" }; }

So you´re allways returning a completely new list.

Use this instead:

public override List<string> myList { get; } = new List<string> { "one", "two", "three" };

which will just set the initial value once for your property.

Alternativly use a backing-field:

private readonly List<string> _myList = new List<string> { "one", "two", "three" };
public override List<string> myList => _myList;

Upvotes: 3

Nico Schertler
Nico Schertler

Reputation: 32627

public override List<string> myList => new List<string> { "one", "two", "three" };

You are always returning a new list when you call the getter. Either create a field that stores the list and return that one or use an auto-implemented property, which you fill in the constructor.

Upvotes: 7

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