It's very common to see code that does this:

Dictionary<TKey, TValue> data = ...;
...
if (!data.ContainsKey(key))
    data.Add(key, value);

This forces the dictionary to lookup the key twice when the key isn't yet in the dictionary.

It'd be great if we had an additional method on the type:

public bool TryAdd(TKey key, TValue value);

Functionally it would behave exactly as if it were implemented like:

public bool TryAdd(TKey key, TValue value)
{
    if (ContainsKey(key)) return false;
    Add(key, value);
    return true;
}

but the implementation internally would be done more efficiently, essentially implemented by calling a variant of Insert but that returns false instead of throwing when a duplicate key is found.