Dictionary ContainsValue and get key in one step or efficient way to accomplish this task?

Question

Is there efficient way to check the value is in the Dictionary once know that value is there and then find the key for that value?

//check dictionary already has sortedWord (i.e key is word and value is sortedWord)

 if (anagramDict.ContainsValue(sortedWord))
     {
       //Get the key for matching the sortedWord 
       string keyWord = anagramDict.Single(kvp => kvp.Value == sortedWord).Key;

Basically reverse request of this question. Dictionary ContainsKey and get value in one function

Answer 1

You ask for "the" key corresponding to a value, but there may be multiple such keys. Therefore you should probably be using .First() rather than .Single().

And a dictionary's sort order is undefined, so looking for the "first" key is ambiguous.

You could do something like the following (also works for keys that are value types):

var keys = dict.Where(kvp => kvp.Value == sortedWord).Select(kvp => kvp.Key).ToList();
if (keys.Count > 0)
{
    // Now you need to decide which matching key you want.
    // If you don't care, you can take the first
    key = keys[0];
}

If you don't care which key you get, it is more efficient to stop at the first match:

var keys = dict.Where(kvp => kvp.Value == sortedWord).Select(kvp => kvp.Key).Take(1).ToList();
if (keys.Count > 0)
{
    key = keys[0];
}

For a dictionary whose keys are a reference type (which is your case: I assume your key is a string), and assuming you don't care which key you get, you can simplify this to:

var key = dict.Where(kvp => kvp.Value == sortedWord).Select(kvp => kvp.Key).FirstOrDefault();
if (key != null)
{
    ...
}