Compiler Error CS1954

The best overloaded method match 'method' for the collection initializer element cannot be used. Collection initializer 'Add' methods cannot have ref or out parameters.

For a collection class to be initialized by using a collection initializer, the class must have a publicAdd method that is not a ref or out parameter.

To correct this error

• If you can modify the source code of the collection class, provide an Add method that does not take a ref or out parameter.

• If you cannot modify the collection class, initialize it with the constructors it provides. You cannot use a collection initializer with it.

Example

The following example produces CS1954 because the only available overload of the Add list in MyList has a ref parameter.

// cs1954.cs
using System.Collections.Generic;
class MyList<T> : IEnumerable<T>
{
    List<T> _list;
    public void Add(ref T item)
    {
        _list.Add(item);
    }

    public System.Collections.Generic.IEnumerator<T> GetEnumerator()
    {
        int index = 0;
        T current = _list[index];
        while (current != null)
        {
            yield return _list[index++];
        }
    }

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

public class MyClass
{
    public string tree { get; set; }
}
class Program
{
    static void Main(string[] args)
    {
        MyList<MyClass> myList = new MyList<MyClass> { new MyClass { tree = "maple" } }; // CS1954
    }
}

See Also

Reference

Object and Collection Initializers (C# Programming Guide)