C# Java HashMap equivalent

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.