.net - C# async await using LINQ ForEach()

Question

I have the following code that correctly uses async/await paradigm.

internal static async Task AddReferencseData(ConfigurationDbContext context)
{
    foreach (var sinkName in RequiredSinkTypeList)
    {
        var sinkType = new SinkType() { Name = sinkName };
        context.SinkTypeCollection.Add(sinkType);
        await context.SaveChangesAsync().ConfigureAwait(false);
    }
}

What is the equivalent way to write this if, instead of using foreach(), I want to use LINQ ForEach()? This one, for example, gives compile error.

internal static async Task AddReferenceData(ConfigurationDbContext context)
{
    RequiredSinkTypeList.ForEach(
        sinkName =>
        {
            var sinkType = new SinkType() { Name = sinkName };
            context.SinkTypeCollection.Add(sinkType);
            await context.SaveChangesAsync().ConfigureAwait(false);
        });
}

The only code I got to work without compile error is this.

internal static void AddReferenceData(ConfigurationDbContext context)
{
    RequiredSinkTypeList.ForEach(
        async sinkName =>
        {
            var sinkType = new SinkType() { Name = sinkName };
            context.SinkTypeCollection.Add(sinkType);
            await context.SaveChangesAsync().ConfigureAwait(false);
        });
}

I'm worried that this method has no async signature, only the body does. Is this the correct equivalent of my first block of code above?

Answer 1

No. It isn't. This ForEach doesn't support async-await and requires your lambda to be async void which should only be used for event handlers. Using it will run all your async operations concurrently and won't wait for them to complete.

You can use a regular foreach as you did but if you want an extension method you need a special async version.

You can create one yourself that iterates over the items, executes an async operation and awaits it:

public async Task ForEachAsync<T>(this IEnumerable<T> enumerable, Func<T, Task> action)
{
    foreach (var item in enumerable)
    {
        await action(item);
    }
}

Usage:

internal static async Task AddReferencseData(ConfigurationDbContext context)
{
    await RequiredSinkTypeList.ForEachAsync(async sinkName =>
    {
        var sinkType = new SinkType() { Name = sinkName };
        context.SinkTypeCollection.Add(sinkType);
        await context.SaveChangesAsync().ConfigureAwait(false);
    });
}

A different (and usually more efficient) implementation of ForEachAsync would be to start all the async operations and only then await all of them together but that's only possible if your actions can run concurrently which isn't always the case (e.g. Entity Framework):

public Task ForEachAsync<T>(this IEnumerable<T> enumerable, Func<T, Task> action)
{
    return Task.WhenAll(enumerable.Select(item => action(item)));
}

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As was noted in the comments you probably don't want to use SaveChangesAsync in a foreach to begin with. Preparing your changes and then saving them all at once will probably be more efficient.