Question

我正在玩任务，而且我不知道如何在抛出异常时取消任务。我正在尝试TaskCompletionSourcecancel，但它无法正常工作。

当抛出异常时，有没有办法完全停止任务？

public async Task Migrate()
{
    var tcs = new TaskCompletionSource<bool>();
    if (IsDataMigratorItemParameteresCorrect())
    {
        try
        {
            using (SqlConnection srcconnection = this.SourceConnection)
            {
                await srcconnection.OpenAsync();
                using (SqlCommand srccmd = CreateMigrateTaskCommand(this.SourceForHash, srcconnection))
                {
                    using (SqlDataReader srcreader = await srccmd.ExecuteReaderAsync(CommandBehavior.SequentialAccess))
                    {
                        using (SqlConnection destconnection = this.DestinationConnection)
                        {
                           await destconnection.OpenAsync();

                           await CreateStageTable(destconnection, srcreader);
                           await BulkCopy(destconnection, srcreader);
                           await Merge(destconnection, srcreader);
                           await DropTable(destconnection); 
                        }
                    }
                }

            }
        }

        catch (Exception ex)
        {
            Console.WriteLine(DestinationCommand + " canceled\nException : " + ex.Message);
            tcs.SetCanceled();
        }
    }
}

private SqlCommand CreateMigrateTaskCommand(string commandQuery, SqlConnection connection)
{
    SqlCommand scmd = new SqlCommand(commandQuery, connection);
    return scmd;
}

private async Task Merge(SqlConnection conn, SqlDataReader reader)
{
    using (SqlCommand cmdMerge = conn.CreateCommand())
    {
        string mergeQuery = SqlGenerator.BuildMergeQuery(reader, DestinationCommand, this.SourceUniqueKey, this.DestinationUniqueKey);

        cmdMerge.CommandText = mergeQuery;
        cmdMerge.CommandType = CommandType.Text;
        cmdMerge.CommandTimeout = 0;
        await cmdMerge.ExecuteNonQueryAsync();
        Console.WriteLine("{0} - MERGE START\n", DestinationCommand);
    }
    Console.WriteLine("{0} - MERGE SUCCESS!\n", DestinationCommand);
}

private async Task BulkCopy(SqlConnection conn, SqlDataReader reader)
{
    using (SqlBulkCopy bcp = new SqlBulkCopy(conn.ConnectionString, SqlBulkCopyOptions.TableLock)) //table lock
    {
        bcp.SqlRowsCopied += bcp_SqlRowsCopied;
        bcp.BatchSize = this.BatchSize;
        bcp.EnableStreaming = true;
        bcp.BulkCopyTimeout = 0;
        bcp.NotifyAfter = 1000;

        string destTable = this.DestinationCommand + "_stage";
        bcp.DestinationTableName = destTable;

        await bcp.WriteToServerAsync(reader);
    }
}

private async Task CreateStageTable(SqlConnection conn, SqlDataReader reader)
{
    using (SqlCommand cmdCreateTable = conn.CreateCommand())
    {
        try
        {
            string createTableQuery = SqlGenerator.BuildCreateTableQuery(reader, this.DestinationCommand);

            cmdCreateTable.CommandText = createTableQuery;
            cmdCreateTable.CommandType = CommandType.Text;

            await cmdCreateTable.ExecuteNonQueryAsync();
            Console.WriteLine(this.DestinationCommand + " created!");
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.Message);
        }
    }
}

private async Task DropTable(SqlConnection conn)
{
    using (SqlCommand cmdCreateTable = conn.CreateCommand())
    {
        try
        {
            string tableName = DestinationCommand + "_stage";
            var builder = new SqlCommandBuilder();
            string escapedTableName = builder.QuoteIdentifier(tableName);
            cmdCreateTable.CommandText = "drop table " + escapedTableName;
            Console.WriteLine(escapedTableName + " created!");
        }
        catch (Exception ex)
        {
            Console.WriteLine(DestinationCommand + " canceled\nException : " + ex.Message);
            tcs.SetCanceled();
        }
    }
}

Answer 1

cancellation分为两部分：请求取消（TaskCompletionSource.SetCanceled）并回复该取消请求。

当发生异常时，您的代码正在调用SetCanceled，如果您希望在一个异常时取消其他操作，则可以。

但是，代码中没有任何内容正在侦听取消令牌。它应该传递给支持CancellationToken的API，或者定期检查（CancellationToken.ThrowIfCancellationRequested），或者附加一个取消回调（CancellationToken.Register）。否则，它只是一个设置的标志，没有人检查它。

在等待中取消任务

1 个答案: