C#中的只读列表

Question

I have some class with List -property:

class Foo {
  private List<int> myList;
}

I want provide access to this field only for read.

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

Answer 1

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();
     }
  }
}

Answer 2

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

class Foo {
    private ReadOnlyCollection<int> myList;
}

Answer 3

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;
            }
        }
    }
}

Answer 4

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();
    }
}

Answer 5

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> .