问题
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 throwsOperationCancelledException
?
Thank you!
回答1:
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.
回答2:
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 :)
回答3:
try using a SqlConnection
and catch the exception on cancel
var sqlConn = db.Database.Connection as SqlConnection;
sqlConn.Open();
_cmd = new SqlCommand(textCommand, sqlConn);
_cmd.ExecuteNonQuery();
and cancel the SqlCommand
_cmd.Cancel();
回答4:
I was using one SqlConnection
for multiple threads. And then when I changed it so that each Thread
created it's own SqlConnection
the error disappeared.
来源:https://stackoverflow.com/questions/25540793/cancellationtoken-with-async-dapper-methods