使用异步.NET 4.0返回DataTable [英] Return DataTable using async .net 4.0

问题描述

我有一个返回数据表的方法.我认为使用.net 4.0可以使逻辑异步并返回数据.但是此代码返回空的Datatable对象.任何想法这段代码有什么问题.

I have a method that returns a datatable. I thought using .net 4.0 I could just async logic and return data. But this code returns null Datatable object. Any ideas what is wrong with this code.

public DataTable GetData(string sql, string connectionName)
{
    DataTable dt = (DataTable)GetDataAsync(sql, connectionName).AsyncState;
    return dt;
}

private async Task<DataTable> GetDataAsync(string sql, string connectionName)
{
    return await TaskEx.Run(() => { return FillData(sql, connectionName); });
}

private DataTable FillData(string sql, string connectionName)
{
    SqlConnection conn = _connections.Where(w => w.ConnectionName == connectionName).Single().Connection;
    SqlDataAdapter adp = new SqlDataAdapter(sql, conn);
    DataSet ds = new DataSet();

    adp.Fill(ds);

    return ds.Tables[0];
}

推荐答案

首先，您不能在.NET 4或C#4中使用async/await.这是C#5中的一项新功能.它安装在.NET 4之上，但是这些CTP中确实存在一些错误-不要使用它们.您应该使用.NET 4.5的完整发行版，其中包括C#5编译器. (所有这些都在Visual Studio 2012中.)

Firstly, you can't use async / await with .NET 4 or C# 4. It's a new feature in C# 5. There were CTPs which installed on top of .NET 4, but there are definite bugs in those CTPs - don't use them. You should use the full release version of .NET 4.5, which includes the C# 5 compiler. (All this is in Visual Studio 2012.)

其次，如Cuong Le所示，您使用了错误的任务属性. Result 属性是您获得Task<T>.

Secondly, you're using the wrong property of the task, as Cuong Le showed. The Result property is how you get at the result of a Task<T>.

第三，在更改为使用Result属性后，您将无法获取该表-使其毫无意义.这个:

Thirdly, after making the change to use the Result property, you'd be blocking for the table to be fetched - making it pointless. This:

public DataTable GetData(string sql, string connectionName)
{
    DataTable dt = (DataTable)GetDataAsync(sql, connectionName).Result;
    return dt;
}

...在很大程度上等同于:

... is largely equivalent to:

public DataTable GetData(string sql, string connectionName)
{
    return FillData(sql, connectionName);
}

如果您要开始执行任务并立即等待，您最好同步地调用该方法.

If you're going to start a task and immediately wait on it, you might as well just call the method synchronously.

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