Create empty IAsyncEnumerable

Question

I have an interface which is written like this:

public interface IItemRetriever
{
    public IAsyncEnumerable

        

Answer 1

If for any reason you don't want to install the package which is mentioned in Jon's answer, you can create the method AsyncEnumerable.Empty<T>() like this:

using System;
using System.Collections.Generic;
using System.Threading.Tasks;
public static class AsyncEnumerable
{
    public static IAsyncEnumerator<T> Empty<T>() => EmptyAsyncEnumerator<T>.Instance;

    class EmptyAsyncEnumerator<T> : IAsyncEnumerator<T>
    {
        public static readonly EmptyAsyncEnumerator<T> Instance = 
            new EmptyAsyncEnumerator<T>();
        public T Current => default!;
        public ValueTask DisposeAsync() => default;
        public ValueTask<bool> MoveNextAsync() => new ValueTask<bool>(false);
    }
}

Note: The answer doesn't discourage using the System.Linq.Async package. This answer provides a brief implementation of AsyncEnumerable.Empty<T>() for cases that you need it and you cannot/don't want to use the package. You can find the implementation used in the package here.

Answer 2

If you install the System.Linq.Async package, you should be able to use AsyncEnumable.Empty<string>(). Here's a complete example:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        IAsyncEnumerable<string> empty = AsyncEnumerable.Empty<string>();
        var count = await empty.CountAsync();
        Console.WriteLine(count); // Prints 0
    }
}