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up vote 1 down vote favorite

I am having trouble getting the key by specifying a value. What is the best way I can achieve this?

var st1= new List<string> { "NY", "CT", "ME" };
var st2= new List<string> { "KY", "TN", "SC" };
var st3= new List<string> { "TX", "OK", "MO" };
var statesToEmailDictionary = new Dictionary<string, List<string>>();
statesToEmailDictionary.Add("[email protected]", st1);
statesToEmailDictionary.Add("[email protected]", st2);
statesToEmailDictionary.Add("[email protected]", st3);

var emailAdd = statesToEmailDictionary.FirstOrDefault(x => x.Value.Where(y => y.Contains(state))).Key;
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5 Answers

up vote 2 down vote accepted

The return value from FirstOrDefault will be a KeyValuePair<string, List<string>>, so to get the key, simply use the Key property. Like this:

var emailAdd = statesToEmailDictionary
    .FirstOrDefault(x => x.Value.Contains(state))
    .Key;

Alternatively, here's the equivalent in query syntax:

var emailAdd = 
    (from p in statesToEmailDictionary
     where p.Value.Contains(state)
     select p.Key)
    .FirstOrDefault();
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Its throwing an error "Cannot convert expression type 'System.Collections.Generic.IEnumerable<string>' to return type 'bool' " – Krishh yesterday
@Krishh sorry, there was a mistake in my original code. See the updated answer. – p.s.w.g yesterday
up vote 2 down vote

I think you want:

var emailAdd = statesToEmailDictionary.FirstOrDefault(x => x.Value.Any(y => y.Contains(state))).Key;
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up vote 1 down vote 
var emailAdd = statesToEmailDictionary
    .FirstOrDefault(x => x.Value != null && x.Value.Contains(state))
    .Key;

But if you're looking for performance, I'd suggest reversing your dictionary and creating a dictionary of <state, email> to do what you're looking for.

// To handle when it's not in the results
string emailAdd2 = null;
foreach (var kvp in statesToEmailDictionary)
{
    if (kvp.Value != null && kvp.Value.Contains(state))
    {
        emailAdd2 = kvp.Key;
        break;
    }
}
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any way to handle NullReferenceException? – Ilya Ivanov yesterday
I don't think so, in a one-liner at least. Edited with a non-LINQ check. It does the same thing as the LINQ one does - loops through each value until it finds one, then breaks out of the loop. If it never finds one, the original value of null is kept. – Joe Enos yesterday
1  
Actually, if you were getting a NullReferenceException, then the list itself must have been null. You can handle that inside the lambda. Updated. – Joe Enos yesterday
up vote 0 down vote 
var temp = statesToEmailDictionary.Where( x => x.Value.Contains(state)).FirstOrDefault();
var emailAdd = temp != null ? temp.Key : string.Empty;
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1  
KeyValuePair<T, U> is a struct, so temp will never be null. – p.s.w.g yesterday
up vote 0 down vote

This is not typical usage for dictionary - i.e. your key is state and your value is email, so it should be like this:

  var dictionary = new Dictionary<string, string> {
    { "NY", "[email protected]" },
    { "CT", "[email protected]" },
    { "ME", "[email protected]" },
    { "KY", "[email protected]" },
    { "TN", "[email protected]" },
    { "SC", "[email protected]" },
    { "TX", "[email protected]" },
    { "OK", "[email protected]" },
    { "MO", "[email protected]" }
  };
  var emailAdd = dictionary[state];
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