cancellation token on Dapper

.net async-await c# cancellation-token dapper

Question

I'm using Dapper 1.31 from Nuget. I have this very simple code snippet,

string connString = "";
string query = "";
int val = 0;
CancellationTokenSource tokenSource = new CancellationTokenSource();
using (IDbConnection conn = new SqlConnection(connString))
{
    conn.Open();
    val = (await conn.QueryAsync<int>(query, tokenSource.Token)).FirstOrDefault();
}

When I press F12 on QueryAsync, it points me to

public static Task<IEnumerable<T>> QueryAsync<T>
     (
        this IDbConnection cnn, 
        string sql, 
        dynamic param = null, 
        IDbTransaction transaction = null, 
        int? commandTimeout = null, 
        CommandType? commandType = null
     );

There is no CancellationToken on its signature.

Questions:

  • Why is the snippet completely buildable assuming that there is no compiler error on the whole solution?
  • Forgive me as I cannot test if calling tokenSource.Cancel() would really cancel the method because I don't know how to generate long running sql query. Will the .Cancel() really cancels the method and throws OperationCancelledException?

Thank you!

Accepted Answer

You are passing the cancellation token as the parameter object; that won't work.

The first async methods in dapper did not expose a cancellation token; when I tried to add them as an optional parameter (as a separate overload, to avoid breaking existing assemblies), things got very confused with "ambiguous method" compilation problems. Consequently, I had to expose this via a separate API; enter CommandDefinition:

val = (await conn.QueryAsync<int>(
    new CommandDefinition(query, cancellationToken: tokenSource.Token)
).FirstOrDefault();

This then passes the cancellation-token down the chain to all the expected places; it is the job of the ADO.NET provider to actually use it, but; it seems to work in most cases. Note that it can result in a SqlException rather than an OperationCancelledException if the operation is in progress; this again is down to the ADO.NET provider, but makes a lot of sense: you could have interrupted something important; it surfaces as a critical connection issue.

As for the questions:

Why is the snippet completely buildable assuming that there is no compiler error on the whole solution?

Because... it is valid C#, even if it doesn't do what you expect.

Forgive me as I cannot test if calling tokenSource.Cancel() would really cancel the method because I don't know how to generate long running sql query. Will the .Cancel() really cancels the method and throws OperationCancelledException?

ADO.NET provider-specific, but yes it usually works. As an example of "how to generate long running sql query"; the waitfor delay command on SQL server is somewhat useful here, and is what I use in the integration tests.


Popular Answer

You can fix SqlMapper.cs in Dapper lib by adding these lines:

    internal IDbCommand SetupCommand(IDbConnection cnn, Action<IDbCommand, object> paramReader)
    {
        var cmd = cnn.CreateCommand();

#if ASYNC
        // We will cancel our IDbCommand
        CancellationToken.Register(() => cmd.Cancel());
#endif

Rebuild your own Dapper lib and enjoy :)



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