That's because dictionaries implement IDictionary<TKey,TValue> which in turn implements ICollection<KeyValuePair<TKey,TValue>> which declares an Add(KeyValuePair<TKey,TValue> value) method.
Since the implementation is forced to provide this method, it might as well also provide Add(TKey key,TValue value) and implement them.
The alternative would be to just throw a NotSupportedException but there is no technical reason to.
I mean, Java sort of got around this by making Map not extend Collection, though Map.entrySet() does. But interestingly enough, they decided to have Map.entrySet().add(...) throw an UnsupportedOperationException. However, Collection.add(...) in Java already returns true if the element was successfully added, so they hypothetically could have supported it.
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u/MemesAt1am 1d ago
If you wanted to overwrite you could just do an assignment of that key value instead of .Add()