How to select multiple values from a Dictionary using Linq as simple as possible

Question

I need to select a number of values (into a List) from a Dictionary based on a subset of keys.

I'm trying to do this in a single line of code using Linq but what I have found so far seems quite long and clumsy. What would be the shortest (cleanest) way to do this?

This is what I have now (the keys are Strings and keysToSelect is a List of keys to select):

List<ValueType> selectedValues = dictionary1.Where(x => keysToSelect.Contains(x.Key))
                                            .ToDictionary<String, valueType>(x => x.Key,
                                                                             x => x.Value)
                                            .Values.ToList;

Thank you.

Answer 1

Well you could start from the list instead of the dictionary:

var selectedValues = keysToSelect.Where(dictionary1.ContainsKey)
                     .Select(x => dictionary1[x])
                     .ToList();

If all the keys are guaranteed to be in the dictionary you can leave out the first Where:

var selectedValues = keysToSelect.Select(x => dictionary1[x]).ToList();

Note this solution is faster than iterating the dictionary, especially if the list of keys to select is small compared to the size of the dictionary, because Dictionary.ContainsKey is much faster than List.Contains.

Answer 2

A Dictionary<TKey,TValue> is IEnumerable<KeyValuePair<TKey,TValue>>, so you can simply Select the Value property:

 List<ValueType> selectedValues = dictionary1
           .Where(x => keysToSelect.Contains(x.Key))
           .Select(x => x.Value)
           .ToList();

or

 var selectValues = (from keyValuePair in dictionary1
                     where keysToSelect.Contains(keyValuePair.Key)
                     select keyValuePair.Value).ToList()

Answer 3

If you know that all the value that you want to select are in the dictionary, you can loop through the keys instead of looping through the dictionary:

List<ValueType> selectedValues = keysToSelect.Select(k => dictionary1[k]).ToList();

Answer 4

The accepted answer is not very efficient when keys are not guaranteed to exist b/c it performs two lookups.

Based on the accepted answer I came up with this extension method:

public static IEnumerable<TValue> GetMultiple<TKey, TValue>(this Dictionary<TKey, TValue> dictionary, IEnumerable<TKey> keysToSelect)
{
    foreach (var key in keysToSelect)
        if (dictionary.TryGetValue(key, out TValue value))
            yield return value;
}

Not exactly a "one liner" but I find it more readable than chaining four Linq methods.

Usage:

var results = dictionary.GetMultiple(keysToSelect).ToList()