C#：使用 CancellationToken 取消 MySqlCommand 给出 NULLReferenceException

Question

我正在尝试使用 CancellationToken 取消 MySqlCommand。未请求取消时查询成功执行。

public async Task<int> ExecuteNonQueryAsync(string connectionString, string query, 
       CancellationToken cancellationToken)
{
    int affectedRowsCount = 0;
    await Task.Run(() =>
    {
        using (MySqlConnection connection = new MySqlConnection(connectionString))
        {
            using (MySqlCommand command = new MySqlCommand())
            {
                connection.Open();
                command.Connection = connection;
                cancellationToken.Register(() => command.Cancel());

                command.CommandText = query;
                command.CommandTimeout = 0;

                affectedRowsCount = command.ExecuteNonQuery();
                connection.Close();
             }
         }
     });

     return affectedRowsCount;
}

但是当请求取消时它会产生 NullReferenceException。无法弄清楚什么是 NULL。

我通过

调用上面的方法

deletedRowsInLastIteration = await 
    mySqlHelperService.ExecuteNonQueryAsync(
       connectionString,
       query, 
       cancellationToken);

如果我尝试

cancellationToken.ThrowIfCancellationRequested();

在调用ExecuteNonQueryAsync()方法之前，它起作用了。但是MySqlCommand的取消不起作用。

这是堆栈跟踪

System.NullReferenceException HResult=0x80004003 Message=Object reference not set to an instance of an object. Source=MySql.Data
StackTrace: at MySql.Data.MySqlClient.MySqlConnection.CancelQuery(Int32 timeout)
at MySql.Data.MySqlClient.MySqlCommand.Cancel() at ProjectName.Common.MySqlHelperService.<>c__DisplayClass1_1.b__1() in C:\Users\username\source\repos\ProjectName\Applications\ProjectName.Common\MySqlHelperService.cs:line 55 at System.Threading.CancellationToken.ActionToActionObjShunt(Object obj) at System.Threading.CancellationCallbackInfo.ExecutionContextCallback(Object obj) at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) at System.Threading.CancellationCallbackInfo.ExecuteCallback() at System.Threading.CancellationTokenSource.CancellationCallbackCoreWork(CancellationCallbackCoreWorkArguments args) at System.Threading.CancellationTokenSource.ExecuteCallbackHandlers(Boolean throwOnFirstException)

Answer 1

你是如何创建你的取消令牌的，他的价值是多少？

这里还有一个解决方案，如何使用取消令牌取消 sql 命令

private CancellationTokenSource cts;
private async void TestSqlServerCancelSprocExecution()
{
cts = new CancellationTokenSource();
try
{
    await Task.Run(() =>
    {
        using (SqlConnection conn = new SqlConnection("connStr"))
        {
            conn.InfoMessage += conn_InfoMessage;
            conn.FireInfoMessageEventOnUserErrors = true;
            conn.Open();

            var cmd = conn.CreateCommand();
            cts.Token.Register(() => cmd.Cancel());
            cmd.CommandType = CommandType.StoredProcedure;
            cmd.CommandText = "dbo.[CancelSprocTest]";
            cmd.ExecuteNonQuery();
        }
   });
}
catch (SqlException)
{
    // sproc was cancelled
}

}

The code above is from this question, which had kinda the same problem, that the cancellation token won't cancel the sql command.

Answer 2

您不应使用 Task.Run 将同步方法转换为异步方法。充其量，这会浪费一个线程来等待一些 IO 操作完成。

MySqlCommand 有一个接受取消标记的 ExecuteNonQueryAsync 方法。 MySqlConnection 本身有一个 OpenAsync 方法。您应该将代码更改为：

public async Task<int> ExecuteNonQueryAsync(string connectionString, string query, 
       CancellationToken cancellationToken)
{
    using (MySqlConnection connection = new MySqlConnection(connectionString))
    {
        using (MySqlCommand command = new MySqlCommand(query,connection))
        {
            await connection.OpenAsync();
            command.CommandTimeout = 0;

            var affectedRowsCount = await command.ExecuteNonQuery(cancellationToken);
         }
    }

    return affectedRowsCount;
}

C#：使用 CancellationToken 取消 MySqlCommand 给出 NULLReferenceException

C# : Cancelling MySqlCommand using CancellationToken giving NULLReferenceException

.net

c#

mysql

mysqlcommand