c# - C#：使用 CancellationToken 取消 MySqlCommand，给出 NULLReferenceException

Question

我试图取消 aMySqlCommand使用 a CancellationToken。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。

我通过调用上述方法

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

如果我尝试

cancellationToken.ThrowIfCancellationRequested();

在调用该ExecuteNonQueryAsync()方法之前，它可以工作。但是 MySqlCommand 的取消不起作用。

这是堆栈跟踪

System.NullReferenceException HResult=0x80004003 Message=对象引用未设置为对象的实例。Source=MySql.Data
StackTrace：在 MySql.Data.MySqlClient.MySqlConnection.CancelQuery（Int32 超时）
在 C:\Users\username\source\repos\ProjectName\Applications\ProjectName.Common\MySqlHelperService.cs 中的 ProjectName.Common.MySqlHelperService.<>c__DisplayClass1_1.b__1() 中的 MySql.Data.MySqlClient.MySqlCommand.Cancel()：第 55 行在 System.Threading.CancellationToken.ActionToActionObjShunt(Object obj) 在 System.Threading.CancellationCallbackInfo.ExecutionContextCallback(Object obj) 在 System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx) 在 System. Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx) 在 System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) 在 System.Threading.CancellationCallbackInfo.ExecuteCallback() 在 System.Threading.CancellationTokenSource.CancellationCallbackCoreWork(CancellationCallbackCoreWorkArguments args) 在 System.Threading.CancellationTokenSource.ExecuteCallbackHandlers(Boolean throwOnFirstException)

score 3 · Accepted Answer

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

MySqlCommand有一个ExecuteNonQueryAsync接受取消令牌的方法。MySqlConnection本身就有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;
}

score 0 · Accepted Answer

How are you creating your cancellation Token and what is his value?

Also here is a solution how to cancel a sql command with a cancellation token

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.

c# - C#：使用 CancellationToken 取消 MySqlCommand，给出 NULLReferenceException

2 回答 2

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