C# Java HashMap equivalent

Coming from a Java world into a C# one is there a HashMap equivalent? If not what would you recommend?

Answer

Dictionary is probably the closest. System.Collections.Generic.Dictionary implements the System.Collections.Generic.IDictionary interface (which is similar to Java’s Map interface).

Some notable differences that you should be aware of:

• Adding/Getting items
  • Java’s HashMap has the put and get methods for setting/getting items
    • myMap.put(key, value)
    • MyObject value = myMap.get(key)
  • C#’s Dictionary uses [] indexing for setting/getting items
    • myDictionary[key] = value
    • MyObject value = myDictionary[key]
• null keys
  • Java’s HashMap allows null keys
  • .NET’s Dictionary throws an ArgumentNullException if you try to add a null key
• Adding a duplicate key
  • Java’s HashMap will replace the existing value with the new one.
  • .NET’s Dictionary will replace the existing value with the new one if you use [] indexing. If you use the Add method, it will instead throw an ArgumentException.
• Attempting to get a non-existent key
  • Java’s HashMap will return null.
  • .NET’s Dictionary will throw a KeyNotFoundException. You can use the TryGetValue method instead of the [] indexing to avoid this:
    MyObject value = null; if (!myDictionary.TryGetValue(key, out value)) { /* key doesn't exist */ }

Dictionary‘s has a ContainsKey method that can help deal with the previous two problems.