c# - 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);