等待在可枚举上使用

Question

我有一段代码，我想将 using 语句应用于 commands 可枚举中的每个命令。这个的 C# 语法是什么？

await using var transaction = await conn.BeginTransactionAsync(cancel);
IEnumerable<DbCommand> commands = BuildSnowflakeCommands(conn, tenantId);

var commandTasks = new List<Task>();
foreach (var command in commands)
{
    command.CommandTimeout = commandTimeout;
    command.Transaction = transaction;
    commandTasks.Add(command.ExecuteNonQueryAsync(cancel));
}

try
{
    await Task.WhenAll(commandTasks);
}
catch (SnowflakeDbException)
{
    await transaction.RollbackAsync(cancel);
    return;
}

await transaction.CommitAsync(cancel);

编辑：Rider / ReSharper 似乎认为这两者是等价的，或者至少我得到了将 for 转换为 foreach 的提示（这显然是 错误的):

for (var i = 0; i < commands.Count; i++)
{
    await using var command = commands[i];
    command.CommandTimeout = commandTimeout;
    command.Transaction = transaction;
    commandTasks.Add(command.ExecuteNonQueryAsync(cancel));
}

和

foreach (var command in commands)
{
    command.CommandTimeout = commandTimeout;
    command.Transaction = transaction;
    commandTasks.Add(command.ExecuteNonQueryAsync(cancel));
}

编辑 2：经过一些讨论和一些有用的回答，这就是我要处理的内容：

var transaction = await conn.BeginTransactionAsync(cancel);
var commands = BuildSnowflakeCommands(conn, tenantId);

var commandTasks = commands.Select(async command =>
{
    await using (command)
    {
        command.CommandTimeout = commandTimeout;
        command.Transaction = transaction;
        await command.ExecuteNonQueryAsync(cancel);
    }
});

try
{
    await Task.WhenAll(commandTasks);
    await transaction.CommitAsync(cancel);
}
catch (SnowflakeDbException)
{
    await transaction.RollbackAsync(cancel);
}
finally
{
    await transaction.DisposeAsync();
}

Answer 1

您不能单独使用语言语法调用 Dispose 或 DisposeAsync 一组 IDisposable 或 IAsyncDisposable。

您可以迭代每一个并调用适当的方法。在重新枚举之前，我会将所有命令缓存为数组或只读集合。

我会避免按照您的 JetBrains 工具的建议在您的循环中进行处理。该命令的寿命必须不止于此。

就我个人而言，我会这样做：

var commands = BuildSnowflakeCommands(conn, tenantId).ToArray();
var commandTasks = new List<Task>(commands.Length);

foreach (var command in commands)
{
    command.CommandTimeout = commandTimeout;
    command.Transaction = transaction;
    commandTasks.Add(command.ExecuteNonQueryAsync(cancel));
}

/// later...
foreach (var command in commands)
{
    command.Dispose();
}

Answer 2

您可以使用 LINQ：

var commandTasks = commands.Select(async command =>
{
    using (command)
    {
        command.CommandTimeout = commandTimeout;
        command.Transaction = transaction;
        await command.ExecuteNonQueryAsync(cancel);
    }
});

该命令将在退出作用域后立即处理。

完整代码：

await using var transaction = await conn.BeginTransactionAsync(cancel);
IEnumerable<DbCommand> commands = BuildSnowflakeCommands(conn, tenantId);

var commandTasks = commands.Select(async command =>
{
    using (command)
    {
        command.CommandTimeout = commandTimeout;
        command.Transaction = transaction;
        await command.ExecuteNonQueryAsync(cancel);
    }
});

try
{
    await Task.WhenAll(commandTasks);
}
catch (SnowflakeDbException)
{
    await transaction.RollbackAsync(cancel);
    return;
}

await transaction.CommitAsync(cancel);

绝对不要使用for循环示例； await 将导致每个命令连续发生，因为在启动下一个查询之前必须等待每个查询的完成。

等待在可枚举上使用

Await using on enumerable

c#

using

async-await