Beware - Parallel.ForEach And Async Don't Play Well Together

I was recently troubleshooting some database interfacing code that would persistently throw an error to the effect that I had run out of SQL Server connections.

The exact error was as follows:

'Timeout expired.  The timeout period elapsed prior to obtaining a connection from the pool.  This may have occurred because all pooled connections were in use and max pool size was reached.'

The code in question was as follows:

using Dapper;
using Microsoft.Data.SqlClient;

string[] collection = ["One", "Two", "Three"];

Parallel.ForEach(collection, async (element) =>
{
  // Create database connection
  await using (var cn = new SqlConnection("......"))
  {
    // Execute query asynchronously
    await cn.ExecuteAsync("Query", new { ID = element });
  }
});

Which looks innocent enough.

The problem is this - Parallel.ForEach is a synchronous construct, and introducing async into the construct does not do what you think it does.

The culprit is above.

Introducing async does not change the fact that the delegate from the Parallel.ForEach returns void.

This means that the query execution is not awaited at all.

What will happen is lots and lots of database connections will be created as fast as the thread pool can create them, and depending on the size of the collection, you will run out of connections from the database pool because the query execution is not awaited before the next connection is created.

TLDR

**Parallel.ForEach does not play well with async.**

The code is in my GitHub.

Happy hacking!