Use HashSet instead of "seen" Hashtable.

In our code (originally created in .Net 1.1)  there are a few examples of "seen" Hashtablepattern

 

Hashtable seen = new Hashtable();

for (int i = 0; i < nCount; i++)

{

    if (seen[key] == null)

    {

        //Do the changes

        seen.Add(key, key);

    }

}

It's better to use type-safe HashSet:

HashSet<string> seen = new HashSet<string>();

for (int i = 0; i < nCount; i++)

{

    if (!seen.Contains(key))

    {

        //Do the changes

        seen.Add(key);

    }

}