How to use async lambda with SelectMany?

问题

I'm getting the following error when trying to use an async lambda within IEnumerable.SelectMany:

var result = myEnumerable.SelectMany(async (c) => await Functions.GetDataAsync(c.Id));

The type arguments for method 'IEnumerable System.Linq.Enumerable.SelectMany(this IEnumerable, Func>)' cannot be inferred from the usage. Try specifying the type arguments explicitly

Where GetDataAsync is defined as:

public interface IFunctions {
    Task<IEnumerable<DataItem>> GetDataAsync(string itemId);
}

public class Functions : IFunctions {
    public async Task<IEnumerable<DataItem>> GetDataAsync(string itemId) {
        // return await httpCall();
    }
}

I guess because my GetDataAsync method actually returns a Task<IEnumerable<T>>. But why does Select work, surely it should throw the same error?

var result = myEnumerable.Select(async (c) => await Functions.GetDataAsync(c.Id));

Is there any way around this?

回答1:

async lambda expression cannot be converted to simple Func<TSource, TResult>.

So, select many cannot be used. You can run in synchronized context:

myEnumerable.Select(c => Functions.GetDataAsync(c.Id)).SelectMany(task => task.Result);

List<DataItem> result = new List<DataItem>();

foreach (var ele in myEnumerable)
{
    result.AddRange(await Functions.GetDataAsyncDo(ele.Id));
}

You cannot neither use yield return - it is by design. f.e.:

public async Task<IEnuemrable<DataItem>> Do() 
{
    ...
    foreach (var ele in await Functions.GetDataAsyncDo(ele.Id)) 
    {
        yield return ele; // compile time error, async method 
                          // cannot be used with yield return
    }

}

回答2:

This is an extension:

public static async Task<IEnumerable<T1>> SelectManyAsync<T, T1>(this IEnumerable<T> enumeration, Func<T, Task<IEnumerable<T1>>> func)
{
    return (await Task.WhenAll(enumeration.Select(func))).SelectMany(s => s);
}

That allows you to run:

var result = await myEnumerable.SelectManyAsync(c => Functions.GetDataAsync(c.Id));

Explanation: you have a list of tasks, each returns Task<IEnumerable<T>>. So you need to fire them all, then await all, and then squash the result via SelectMany.

回答3:

Select works because it will return an IEnumerable<Task<T>>, which can then be awaited with e.g. Task.WhenAll.

So, an easy workaround to this problem is:

IEnumerable<Task<IEnumerable<T>>> tasks = source.Select(GetNestedEnumerableTask);
IEnumerable<T>[] nestedResults = await Task.WhenAll(tasks);
IEnumerable<T> results = nestedResults.SelectMany(nr => nr);

来源：https://stackoverflow.com/questions/33495617/how-to-use-async-lambda-with-selectmany

标签

linq

lambda

linq-to-objects