Read-Only List in C#

Question

I have some class with List-property:

class Foo {
  private List<int> myList;
}

I want provide access to this field only for read.

I.e. I want property with access to Enumerable, Count, etc. and without access to Clear, Add, Remove, etc. How I can do it?

I'm going to go a head and change the title of this question to read-only because read-only and immutability are two very different albeit related topics, I'd prefer it if we did not mix them. If you're looking for immutability check out http://stackoverflow.com/a/9009748/58961. — John Leidegren, Mar 09 '13 at 17:04
Possible duplicate of [Read-only list or unmodifiable list in .NET 4.0](https://stackoverflow.com/questions/984042/read-only-list-or-unmodifiable-list-in-net-4-0) — StayOnTarget, Sep 27 '18 at 12:08
Starting with .NET Core 1.0 (and .NET 5), the library includes all sorts of immutable collections. See [System.Collections.Immutable Namespace](https://docs.microsoft.com/en-us/dotnet/api/system.collections.immutable?view=net-6.0#classes). The difference between immutable and read-only collections in short: an instance of an immutable collection is read-only *and* unchangeable. Instances of read-only collections are wrappers that allow the owner of the wrapped collection to change it. From the consumer perspective, a read-only collection could change at any time while an immutable never will. — BionicCode, Aug 27 '22 at 10:47

FearlessHyena · Answer 1 · 2022-08-29T09:46:10.343

77

You can expose a List<T> as a ReadOnlyCollection<T> by using the AsReadOnly() method

C# 6.0 and later (using Expression Bodied Properties)

class Foo { 

  private List<int> myList;

  public ReadOnlyCollection<int> ReadOnlyList => myList.AsReadOnly();

}

C# 5.0 and earlier

class Foo {

  private List<int> myList;

  public ReadOnlyCollection<int> ReadOnlyList {
     get {
         return myList.AsReadOnly();
     }
  }
}

edited Aug 29 '22 at 09:46

answered Nov 18 '13 at 17:51

FearlessHyena

3,527
33
39

1

Nice and clean, exactly what I wanted. – AnthonyVO Jan 08 '14 at 04:48
1

I think that there should be a `return` before `myList` – Jeffrey Stewart Mar 05 '20 at 20:58

score 43 · Accepted Answer · edited Mar 04 '16 at 20:23

43

If you want a read-only view of a list you could use ReadOnlyCollection<T>.

class Foo {
    private ReadOnlyCollection<int> myList;
}

edited Mar 04 '16 at 20:23

Community

1
1

answered Jan 13 '11 at 12:32

Darin Dimitrov

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271
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5

Or, more accurately, a private `List` variable and a public `ReadOnlyCollection` property with a `get {}` that returns the variable. – Victor Nicollet Jan 13 '11 at 12:34
5

@Victor: that would give calling code access to the mutable list by a simple cast. Probably not what the OP wants. The readonly property could `return new ReadOnlyCollection(myList);` though. – Fredrik Mörk Jan 13 '11 at 12:35
10

ReadOnlyCollection is not really immutable as it contains a pointer to the original list - if you add something to your list, the readOnly-instance can access this new item either... but, you are right, when it comes down to modifications-methods - these are missing! – Jan 13 '11 at 12:36
@Andreas: the OP does not state that the list content must not change; the only requirement given is that the calling code must not be able to change it. – Fredrik Mörk Jan 13 '11 at 12:39
@Fredrik: this is what I meant (and expressed quite awkwardly, I admit) ;-) – Victor Nicollet Jan 13 '11 at 12:42
@Fredrik Mörk: my comment was just for mentioning a side-effect – Jan 13 '11 at 13:35
1

@FredrikMörk: because the title says "immutable". Perhaps it should be changed to "readonly". – David Schmitt Sep 25 '12 at 07:45

K. R. · Answer 3 · 2015-02-27T01:20:36.597

9

There is now an Immutable Collections library that does exactly that. Which you can install via nuget.

The immutable collection classes are supported starting with the .NET Framework 4.5.

https://msdn.microsoft.com/en-us/library/dn385366%28v=vs.110%29.aspx

The System.Collections.Immutable namespace provides generic immutable collection types that you can use for these scenarios, including: ImmutableArray<T>, ImmutableDictionary<Tkey,TValue>, ImmutableSortedDictionary<T>, ImmutableHashSet<T>, ImmutableList<T>, ImmutableQueue<T>, ImmutableSortedSet<T>, ImmutableStack<T>

Example of Usage:

class Foo
{
    public ImmutableList<int> myList { get; private set; }

    public Foo(IEnumerable<int> list)
    {
        myList = list.ToImmutableList();
    }
}

edited Feb 27 '15 at 01:20

answered Feb 27 '15 at 00:20

K. R.

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ImmutableList DOES allow you to add new elements, it just creates another list object for that. `Note that these methods return a new object. When you add or remove items from an immutable list, a copy of the original list is made with the items added or removed, and the original list is unchanged.` This answer shouldn't be upvoted. – AymenDaoudi Oct 08 '21 at 17:52
@AymenDaoudi The terms "immutable" or "read-only" refer to the actual instance. *"[...] return a new object."* means "return a new instance", where the original instance remains in its original state. Distributing an accessible immutable list in fact prevents the consumer to mutate it. They will always modify a copy. A read-only collection on the other hand is not immutable as it can be modified via the underlying i.e. wrapped collection. In other words: retrieving an immutable collection instance guarantees the consumer that this instance will never change. – BionicCode Aug 27 '22 at 09:54
1

@AymenDaoudi This means, an instance of `ImmutableList` is read-only (it implements `IReadOnlyList` and *in addition* it is immutable (unchangeable). Whether the OP needs the list to be immutable or not is not stated in the question. Since I believe that it is really worth to consider to make the returned instance immutable, I find this a valid answer which gives the OP or any reader that looks for a solution regarding a similar problem a valuable choice. Hence this answer qualifies for an upvote. – BionicCode Aug 27 '22 at 09:57
1

`ImmutableList` is available without installing additional packages. It is now included in .NET Core and .NET 5 or later. See [System.Collections.Immutable Namespace](https://learn.microsoft.com/en-us/dotnet/api/system.collections.immutable?view=net-6.0#classes). – BionicCode Aug 27 '22 at 10:00
@AymenDaoudi The original title of this question even explicitly asked for an immutable list. – BionicCode Aug 27 '22 at 10:02

score 9 · Answer 4 · 2011-01-13T13:33:43.220

9

i would go for

public sealed class Foo
{
    private readonly List<object> _items = new List<object>();

    public IEnumerable<object> Items
    {
        get
        {
            foreach (var item in this._items)
            {
                yield return item;
            }
        }
    }
}

edited Jan 13 '11 at 13:33

answered Jan 13 '11 at 12:38

5

Why bother with the yield break at the end? – Jon Skeet Jan 13 '11 at 12:40
Also the Foo could implement IEnumerable – Wojtek Turowicz Jan 13 '11 at 12:53
@Jon Skeet: just copy past ... at first i had a `// TODO`, but replaced it with a foreach ... thanks for that hint! – Jan 13 '11 at 13:33
@Turek: jep, indeed ... but as not stated i did not take that into account - maybe the OP needs some more properties (lists) – Jan 13 '11 at 13:34

score 3 · Answer 5 · answered Jan 13 '11 at 12:35

If you declare a readonly list in your class, you'll still be able to add items to it.

If you don't want to add or change anything, you should be using ReadOnlyCollection<T> as Darin suggested.

If you want to add, remove items from the list but don't want to change the content, you can use a readonly List<T>.

Read-Only List in C#

5 Answers5

C# 6.0 and later (using Expression Bodied Properties)

C# 5.0 and earlier

Linked

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