Block the List.Add() method

Question

I have a class that contains a private List. I have created a getter and a method to add elements to the list:

public class Test{

    private List<T> myList;
    public List<T> MyList
    {
        get { return myList; }
    }

    public Test()
    {
         myList = new List<T>();
    }

    public void AddElements(T element)
    {
         // Do dome stuff, subscribe to an event of T
         myList.Add(element);
    }
}

Since everytime an element is added to my list I want to do more things, I do not want that in some part of the code someone add an element directly:

Test test = new Test();

// Wrong
test.MyList.Add(element);

// Right
test.AddElements(element);

I have thought on creating a new class that implements the IList interface and overrides the Add() method, but I was wondering if there is a more simple/elegant way on "block" this Add() method.

Don't expose `List` if you don't want someone being able to modify it. You can expose [`ReadOnlyCollection`](https://msdn.microsoft.com/en-us/library/e78dcd75(v=vs.110).aspx), then no one can add (unless they use reflection). — Sinatr, Feb 26 '16 at 12:18

score 7 · Accepted Answer · answered Feb 26 '16 at 12:16

7

If you are using at least .NET 4.5, return a IReadOnlyList<T>.

public class Test{

    private List<T> myList;
    public IReadOnlyList<T> MyList
    {
        get { return myList; }
    }

    public Test()
    {
         myList = new List<T>();
    }

    public void AddElements(T element)
    {
         // Do dome stuff, subscribe to an event of T
         myList.Add(element);
    }
}

answered Feb 26 '16 at 12:16

Daniel A. White

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@JeroenvanLangen for sure, but there's always a way to get at the data. – Daniel A. White Feb 26 '16 at 12:55
I think this works fine enough and it doesn't create copies. Like you said. "there's always a way to get at the data" ;-) – Jeroen van Langen Feb 26 '16 at 12:59
Perhaps it's me but IReadOnlyList don't protect from cast, and it seems you can add with a cast. – Vonkel. Apr 22 '21 at 09:24

Jeroen van Langen · Answer 2 · 2016-02-26T13:36:04.847

I would return as ReadOnlyCollection<T>

Use it like:

public class Test<T> 
{

    private List<T> myList;
    public ReadOnlyCollection<T> MyList
    {
        get { return myList.AsReadOnly(); }
    }

    public Test()
    {
        myList = new List<T>();
    }

    public void AddElements(T element)
    {
        // Do dome stuff, subscribe to an event of T
        myList.Add(element);
    }
}

This way you prevent casting... It will Wrap your List<> with a ReadOnlyCollection class.

Or you could return it as an Array like:

public T[] MyList
{
    get { return myList.ToArray(); }
}

The ToArray() method will create a copy of the list

When it is Returned as an IReadOnlyList<T>. You could simply cast it back..

Test test = new Test<int>();

test.AddElements(10);

((List<int>)test.MyList).Add(20);

foreach(var i in test.MyList)
    Console.WriteLine(i);

score 3 · Answer 3 · edited May 23 '17 at 12:23

In Test class expose the List as IReadOnlyList<T> then you wont be allowed to use MyList.Add directly

public class Test{

    private List<T> myList;
    public IReadOnlyList<T> MyList
    {
        get { return myList; }
    }

    public Test()
    {
         myList = new List<T>();
    }

    public void AddElements(T element)
    {
         // Do dome stuff, subscribe to an event of T
         myList.Add(element);
    }
}

Edit updated it IEnumerable to IReadOnlyList. There are benifits in using IReadOnlyList - IEnumerable<T> vs IReadOnlyList<T>

MakePeaceGreatAgain · Answer 4 · 2016-02-26T12:26:25.663

1

If the only reason for your getter is to add elements you won´t need to directly access the list at all, but you can wrap the call to the Add-method into your own method:

class MyClass<T> {
    private List<T> _myList ...

    public void AddElements(T element)
    {
         // Do dome stuff, subscribe to an event of T
         _myList.Add(element);
    }
}

Exposing a generic list is furthermore avoided by CA1002

edited Feb 26 '16 at 12:26

answered Feb 26 '16 at 12:16

MakePeaceGreatAgain

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Unfortunately I have to use the elements of the list from outside, so I need access to it... – Ignacio Feb 26 '16 at 12:25

Block the List.Add() method

4 Answers4