Alternative for java-HashMap in C# -Urgent help required

Question

Java HashMap equivalent in C# is HashTable.
But in java the HashMap can have a null key ie key is a null reference and there can be duplicate keys ie basically keys with the same name.
I want a class that provides the above feature ie
1)stores in key value pairs
2) key can be null
3)Keys with the same name came be added.

or if there is another util class that provided the above mentioned features.

Please help me with this.

Thanks a lot in advance.