C# Hashtable Class

Represents a collection of key/value pairs that are organized based on the hash code of the key.

using System.Collections;

Hashtable openWith = new Hashtable();

openWith.Add("txt", "notepad.exe");
openWith.Add("bmp", "paint.exe");

// error
openWith.Add("txt", "winword.exe");

Sources:

https://docs.microsoft.com/en-us/dotnet/api/system.collections.hashtable?view=netframework-4.8