如何取消Ado.Net异步调用？

Question

I have a function to query database

private async Task<IEnumerable<T>)> RunQuery<T>(string sql)
{
    using (var conn = new SqlConnection(_connStr))
    using (var cmd = new SqlCommand(sql, conn))
    {
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.CommandTimeout = 300;
        await conn.OpenAsync().ConfigureAwait(false);
        var reader = await cmd.ExecuteReaderAsync().ConfigureAwait(false);
        // ... yield return the result from the reader

Some queries take a long time to run and I need to enable it to be cancelable (so it will release the database locks, etc). How to implement it?

Answer 1

As mentioned in the comments on the question, use the overload that takes a CancellationToken , as you can then trigger cancellation of the request via the CancellationTokenSource used to generate the token.

Simulating a long running task by dropping the following code into a Windows Forms app, shows this at work:

CancellationTokenSource _cts = new CancellationTokenSource();
bool _started = false;

private async void button1_Click(object sender, EventArgs e)
{
    if (!_started)
    {
        _started = true;
        using (var conn = new SqlConnection(_connStr))
        using (var cmd = new SqlCommand("WAITFOR DELAY '00:00:30'", conn))
        {
            cmd.CommandType = CommandType.Text;
            cmd.CommandTimeout = 300;
            await conn.OpenAsync().ConfigureAwait(false);
            var reader = await cmd.ExecuteReaderAsync(_cts.Token).ConfigureAwait(false);

            MessageBox.Show("Done");
        }
    }
    else
    {
        _cts.Cancel();
    }
}

Clicking the button on the form once triggers the request to the database and a second time requests that it be cancelled. This does cause an exception to be thrown which then needs to be handled appropriately: