Adds an element with the specified key and value into the Hashtable.
Type Reason ArgumentNullException key is null. ArgumentException An entry with the same key already exists in the current instance. NotSupportedException The current instance is read-only or has a fixed size.
A key cannot be null, but a value can be.
An object that has no correlation between its state and its hash code value should typically not be used as the key. For example, String objects are better than StringBuilder objects for use as keys.
You can also use the Hashtable.Item(object) property to add new elements by setting the value of a key that does not exist in the Hashtable; for example, myCollection["myNonexistentKey"] = myValue. However, if the specified key already exists in the Hashtable, setting the Hashtable.Item(object) property overwrites the old value. In contrast, the Hashtable.Add(object, object) method does not modify existing elements.
If Hashtable.Count is less than the capacity of the Hashtable, this method is an O(1) operation. If the capacity needs to be increased to accommodate the new element, this method becomes an O(n) operation, where n is Hashtable.Count.