Exception Type thown on dupicate collection item adding

Question

Following code should throw exception to prevent adding duplicate collection item.

ICollection<T> collection = new List<T>();

public void Add(T item)
{
    if (collection.Contain(item))
    {
          throw new SomeExceptoinType()
    }

    collection.Add(item);
}

What standard exception type is the most apropriate?

Answer 1

I would use InvalidOperationException:

The exception that is thrown when a method call is invalid for the object's current state.

Since the validity of the argument's value is contingent upon the state of the object (that is whether or not collection.Contains(item) is true) I think this is the best exception to use.

Make sure that you add a good message to the exception that makes it clear what the problem was to the caller.

Answer 2

ArgumentException would probably be the best. This is the exception thrown when an argument is invalid.

Answer 3

ArgumentException would be the proper exception (Dictionary uses that exception as well)

Answer 4

System.ArgumentException

Answer 5

Well, Dictionary<,>.Add() throws ArgumentException if such key already exists, so I guess this could be a precedent.

Answer 6

I'd say InvalidOperationException, because it is not valid to add an object that's already in the collection

Answer 7

I would throw an ArgumentException. That's what the generic System.Collections.Generic.SortedList<> does in its Add method.

From the .NET Framework 2.0 code:

    public void Add(TKey key, TValue value)
    {
        if (key == null)
        {
            System.ThrowHelper.ThrowArgumentNullException(System.ExceptionArgument.key);
        }
        int num = Array.BinarySearch<TKey>(this.keys, 0, this._size, key, this.comparer);
        if (num >= 0)
        {
            System.ThrowHelper.ThrowArgumentException(System.ExceptionResource.Argument_AddingDuplicate);
        }
        this.Insert(~num, key, value);
    }