Using IAsyncEnumerable with Dapper

When using IEnumerable in asynchronous context, you could not await it. With the introduction of IAsyncEnumerable it becomes possible to asynchronously iterate over a sequence of elements. It allows you to use the await keyword within the loop, which means you can perform asynchronous operations on each element as you iterate over them.

	await foreach (int item in RangeAsync(10, 3))
	Console.Write(item + " ");
	static async IAsyncEnumerable<int> RangeAsync(int start, int count)
	{
	for (int i = 0; i < count; i++)
	{
	await Task.Delay(i);
	yield return start + i;
	}
	}

view raw IAsyncEnumerable.cs hosted with ❤ by GitHub

In other words, IAsyncEnumerable provides an asynchronous version of the IEnumerable interface, making it easier to work with asynchronous data streams. You can use it to represent data streams that are produced asynchronously, such as a stream of data received over a network or a series of results returned from an asynchronous database query.

I was wondering if I could use this interface with Dapper, the open source micro-ORM for .NET. It turned out that there was no IAsyncEnumerable  support available out-of-the-box but maybe we could (easily) add it ourselves?

I created the following extension method:

	public static class IDbConnectionExtensions
	{
	public static async IAsyncEnumerable<T> QueryIncrementally<T>(this DbConnection connection, CommandDefinition commandDefinition, CommandBehavior behaviour = CommandBehavior.CloseConnection)
	{
	await using var reader = await connection.ExecuteReaderAsync(commandDefinition, behaviour);
	var rowParser = reader.GetRowParser<T>();
	while (await reader.ReadAsync())
	{
	yield return rowParser(reader);
	}
	}
	}

view raw IDbConnectionExtensions.cs hosted with ❤ by GitHub

Here is an example on how to use it:

	var connection = connectionProvider.GetActiveConnection() as DbConnection;
	var command = new CommandDefinition("SELECT * FROM CUSTOMERS");
	var customers = connection.QueryIncrementally<Customer>(command);
	await foreach (var customer in customers)
	{
	Console.WriteLine(customer.CustomerID);
	}

view raw AsyncCustomers.cs hosted with ❤ by GitHub

Don’t know if this is a foolproof implementation but maybe it can help you…

More information: Iterating with Async Enumerables in C# 8