Dictionary(Hashmap)

08 Apr 2019

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Dictionary in C# is very close to hashmap in Java. There are some notable differences that:

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.