How do I avoid "Access Violation" during asynchronous database connections?

Goal: Using ExecuteAsync correctly.

I know my application itself is at least pretty close to correct because it executes correctly once, so I know I can eliminate the usual suspects of firewalls, wrong database passwords, procedure names, etc.

private async Task<T> executeAsyncHelper<T>(Func<IDataReader, T> then)
{
    using (SqlConnection conn = new SqlConnection(connectionString))
    {
        conn.Open();
        using (SqlCommand cmd = new SqlCommand(name, conn))
        {
            parameters.Select(p => cmd.Parameters.Add(p))
                .ToImmutableList(); 
            cmd.CommandType = CommandType.StoredProcedure;
            using (IDataReader rdr = await cmd.ExecuteReaderAsync())
            {
                return then(rdr);
            }
        }
    }
}

The Issue: Access Violation on the Second SqlConnection.Open()

The method gets called twice on the same procedure. The first time, it successfully updates the database. The second time, the whole thing dumps out on conn.Open() with "Access violation." Importantly, it's not throwing an exception. Rather, the whole debugger exits with a negative status code.

These are the last two lines of the debugger output (The rest of the debugger output looks normal):

The program '[6904] dotnet.exe' has exited with code -1073741819 (0xc0000005) 'Access violation'.  
The program '[6904] dotnet.exe: Program Trace' has exited with code 0 (0x0).

This behavior is 100% repeatable.

Hypothesis: Connection Pooling and Asynchronicity

As far as I can see, the most probable cause is that there's some problem with opening two connections asynchronously, but I don't immediately know what to do with that hypothesis. I've written the method as safely as I know how.

Secondarily, I checked to see if my Azure SQL Server instance had any unusual limitations on it. It's a basic instance, but I doubt that's the issue since I would expect a SQL exception (that the debugger will break on and show a normal stack trace) if that were the problem.

Environmental information

• Azure SQL Server 2016
• Windows 10
• .NET Core 2.1
• Visual Studio 2017

Conclusion

Does anyone see any problem with the way I've implemented this or have any suggestions to learn more about the error?

Try this:

private async Task<T> executeAsyncHelper<T>(Func<IDataReader, Task<T>> then)
{
    using (SqlConnection conn = new SqlConnection(connectionString))
    {
        conn.Open();
        using (SqlCommand cmd = new SqlCommand(name, conn))
        {
            parameters.Select(p => cmd.Parameters.Add(p))
                .ToImmutableList(); 
            cmd.CommandType = CommandType.StoredProcedure;
            using (IDataReader rdr = await cmd.ExecuteReaderAsync())
            {
                return await then(rdr);
            }
        }
    }
}

Note the changes to the signature, and awaiting on return.