Different ways of adding to Dictionary

Question

What is the difference in Dictionary.add(key, value) and Dictionary[key] = value?

I've noticed that the last version does not throw an ArgumentException when inserting a duplicate key, but is there any reason to prefer the first version?

Edit: Does anyone have an authoritative source of information about this? I've tried MSDN, but it is as always a wild goose chase :(

Answer 1

The performance is almost a 100% identical. You can check this out by opening the class in Reflector.net

This is the This indexer:

public TValue this[TKey key]
{
    get
    {
        int index = this.FindEntry(key);
        if (index >= 0)
        {
            return this.entries[index].value;
        }
        ThrowHelper.ThrowKeyNotFoundException();
        return default(TValue);
    }
    set
    {
        this.Insert(key, value, false);
    }
}

And this is the Add method:

public void Add(TKey key, TValue value)
{
    this.Insert(key, value, true);
}

I won't post the entire Insert method as it's rather long, however the method declaration is this:

private void Insert(TKey key, TValue value, bool add)

And further down in the function, this happens:

if ((this.entries[i].hashCode == num) && this.comparer.Equals(this.entries[i].key, key))
{
    if (add)
    {
        ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_AddingDuplicate);
    }

Which checks if the key already exists, and if it does and the parameter add is true, it throws the exception.

So for all purposes and intents the performance is the same.

Like a few other mentions, it's all about whether you need the check, for attempts at adding the same key twice.

Sorry for the lengthy post, I hope it's okay.