How to hydrate a Dictionary with the results of async calls?

Question

Suppose I have code that looks like this:

public async Task<string> DoSomethingReturnString(int n) { ... } int[] numbers = new int[] { 1, 2 , 3}; 

Suppose that I want to create a dictionary that contains the result of calling DoSomethingReturnString for each number similar to this:

Dictionary<int, string> dictionary = numbers.ToDictionary(n => n,     n => DoSomethingReturnString(n)); 

That won't work because DoSomethingReturnString returns Task<string> rather than string. The intellisense suggested that I try specifying my lambda expression to be async, but this didn't seem to fix the problem either.

Answer 1

If you insist on doing it with linq, Task.WhenAll is the key to "hydrate" the dictionary:

int[] numbers = new int[] { 1, 2 , 3};  KeyValuePair<int, string>[] keyValArray = //using KeyValuePair<,> to avoid GC pressure     await Task.WhenAll(numbers.Select(async p =>          new KeyValuePair<int, string>(p, await DoSomethingReturnString(p))));  Dictionary<int, string> dict = keyValArray.ToDictionary(p => p.Key, p => p.Value); 

Answer 2

LINQ methods do not support asynchronous actions (e.g., asynchronous value selectors), but you can create one yourself. Here is a reusable ToDictionaryAsync extension method that supports an asynchronous value selector:

public static class ExtensionMethods {     public static async Task<Dictionary<TKey, TValue>> ToDictionaryAsync<TInput, TKey, TValue>(         this IEnumerable<TInput> enumerable,         Func<TInput, TKey> syncKeySelector,         Func<TInput, Task<TValue>> asyncValueSelector)     {         Dictionary<TKey,TValue> dictionary = new Dictionary<TKey, TValue>();          foreach (var item in enumerable)         {             var key = syncKeySelector(item);              var value = await asyncValueSelector(item);              dictionary.Add(key,value);         }          return dictionary;     } } 

You can use it like this:

private static async Task<Dictionary<int,string>>  DoIt() {     int[] numbers = new int[] { 1, 2, 3 };      return await numbers.ToDictionaryAsync(         x => x,         x => DoSomethingReturnString(x)); }