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

Question

Suppose I have code that looks like this:

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


        

Answer 1

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> ToDictionaryAsync(
        this IEnumerable enumerable,
        Func syncKeySelector,
        Func> asyncValueSelector)
    {
        Dictionary dictionary = new Dictionary();

        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>  DoIt()
{
    int[] numbers = new int[] { 1, 2, 3 };

    return await numbers.ToDictionaryAsync(
        x => x,
        x => DoSomethingReturnString(x));
}