StringDictionary. This is a specialized collection. It is found in the System.Collections Specialized namespace. It only allows string keys and string values.

Some drawbacks. The StringDictionary type is limited. It also suffers from performance problems. Usually the Dictionary generic type is a better choice.

This program shows some methods on the StringDictionary. The constructor is called, and then Add is invoked. Various other features of the type are then used.

using System;
using System.Collections;
using System.Collections.Specialized;

class Program
{
    static void Main()
    {
        // Create new StringDictionary.
        StringDictionary dict = new StringDictionary();
        dict.Add("cat", "feline");
        dict.Add("dog", "canine");

        // Try the indexer.
        Console.WriteLine(dict["cat"]);
        Console.WriteLine(dict["test"] == null);

        // Use ContainsKey.
        Console.WriteLine(dict.ContainsKey("cat"));
        Console.WriteLine(dict.ContainsKey("puppet"));

        // Use ContainsValue.
        Console.WriteLine(dict.ContainsValue("feline"));
        Console.WriteLine(dict.ContainsValue("perls"));

        // See if it is thread-safe.
        Console.WriteLine(dict.IsSynchronized);

        // Loop through pairs.
        foreach (DictionaryEntry entry in dict)
        {
            Console.WriteLine("{0} = {1}", entry.Key, entry.Value);
        }

        // Loop through keys.
        foreach (string key in dict.Keys)
        {
            Console.WriteLine(key);
        }

        // Loop through values.
        foreach (string value in dict.Values)
        {
            Console.WriteLine(value);
        }

        // Use Remove method.
        dict.Remove("cat");
        Console.WriteLine(dict.Count);

        // Use Clear method.
        dict.Clear();
        Console.WriteLine(dict.Count);
    }
}
feline
True
True
False
True
False
False
dog = canine
cat = feline
dog
cat
canine
feline
1
0

Internals. StringDictionary uses a Hashtable member field to store all the data you add. The Hashtable is not a generic type so it will have some overhead.

Summary. The StringDictionary is mostly useless. Its underlying collection, the Hashtable, is also obsolete because of the faster and cleaner Dictionary generic type.