SingleItemReadOnlyList.cs

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namespace System.Collections.Specialized;
 
internal sealed class SingleItemReadOnlyList : IList, ICollection, IEnumerable
{
    private readonly object _item;
 
    public object this[int index]
    {
        get
        {
            if (index != 0)
            {
                throw new ArgumentOutOfRangeException("index");
            }
            return _item;
        }
        set
        {
            throw new NotSupportedException(System.SR.NotSupported_ReadOnlyCollection);
        }
    }
 
    public bool IsFixedSize => true;
 
    public bool IsReadOnly => true;
 
    public int Count => 1;
 
    public bool IsSynchronized => false;
 
    public object SyncRoot => this;
 
    public SingleItemReadOnlyList(object item)
    {
        _item = item;
    }
 
    public IEnumerator GetEnumerator()
    {
        yield return _item;
    }
 
    public bool Contains(object value)
    {
        if (_item != null)
        {
            return _item.Equals(value);
        }
        return value == null;
    }
 
    public int IndexOf(object value)
    {
        if (!Contains(value))
        {
            return -1;
        }
        return 0;
    }
 
    public void CopyTo(Array array, int index)
    {
        CollectionHelpers.ValidateCopyToArguments(1, array, index);
        array.SetValue(_item, index);
    }
 
    public int Add(object value)
    {
        throw new NotSupportedException(System.SR.NotSupported_ReadOnlyCollection);
    }
 
    public void Clear()
    {
        throw new NotSupportedException(System.SR.NotSupported_ReadOnlyCollection);
    }
 
    public void Insert(int index, object value)
    {
        throw new NotSupportedException(System.SR.NotSupported_ReadOnlyCollection);
    }
 
    public void Remove(object value)
    {
        throw new NotSupportedException(System.SR.NotSupported_ReadOnlyCollection);
    }
 
    public void RemoveAt(int index)
    {
        throw new NotSupportedException(System.SR.NotSupported_ReadOnlyCollection);
    }
}