Composite Key Dictionary

Question

I have some objects in List, let\'s say List and MyClass has several properties. I would like to create an index of the list based on 3 properti

Answer 1

Now that VS2017/C#7 has come out, the best answer is to use ValueTuple:

// declare:
Dictionary<(string, string, int), MyClass> index;

// populate:
foreach (var m in myClassList) {
  index[(m.Name, m.Path, m.JobId)] = m;
}

// retrieve:
var aMyClass = index[("foo", "bar", 15)];

I chose to declare the dictionary with an anonymous ValueTuple (string, string, int). But I could have given them names (string name, string path, int id).

Perfwise, the new ValueTuple is faster than Tuple at GetHashCode but slower at Equals. I think you'd need to do complete end-to-end experiments to figure out which is really fastest for your scenario. But the end-to-end niceness and language syntax for ValueTuple makes it win out.

// Perf from https://gist.github.com/ljw1004/61bc96700d0b03c17cf83dbb51437a69
//
//              Tuple ValueTuple KeyValuePair
//  Allocation:  160   100        110
//    Argument:   75    80         80    
//      Return:   75   210        210
//        Load:  160   170        320
// GetHashCode:  820   420       2700
//      Equals:  280   470       6800