C# 等待 lambda 回调完成

Question

摘要

我目前正在尝试使用 MySQL 数据库来执行 select 语句并以回调的方式将其返回到匿名语句 lambda。这就是这个特定的(大部分未记录的)库处理请求的方式。我还需要等待这个过程完成。

我目前正在尝试使用async方法，但是任务的完成似乎被断言得太早(即await taskName;在回调之前被绕过已完成，因此返回的 Dictionary 为空)。

我尝试使用完成标志方法，其中使用 bool 标志来表示回调是否已完成，并在返回任务之前在 while 循环中使用 Task.Yield() 。

下面是来自两个不同类的两个函数。第一个来自数据库类，第二个来自实用程序类(从中调用数据库类)。

代码

/// <summary>
/// Asynchronously executes a select MySQL statement and returns a dictionary of rows selected.
/// </summary>
/// <param name="statement"></param>
/// <returns></returns>
public async Task<Dictionary<int, object>> ExecuteSelectAsync (string statement)
{
    // Init dictionary of rows and counter for each row
    Dictionary<int, object> responseData = new Dictionary<int, object>();
    int i = 0;

    bool complete = false;

    DatabaseLibrary.execute(
        statement,
        new Action<dynamic> (s =>
        {
            // Take the data returned as 's' and populate the 'responseData' dictionary.
            Utility.LogDebug("Database", "Executed select statement with " + numberOfRows.ToString() + " rows");
        })
    );

    Utility.LogDebug("Database", "Returning select execution response"); // By this point, the lambda expression hasn't been executed.
    return responseData; // This is empty at time of return.
}

/// <summary>
/// Checks the supplied data against the database to validate.
/// </summary>
/// <param name="data"></param>
/// <returns></returns>
public static async Task<bool> ValidateData(string data)
{
    Database database = new Database();

    Task<Dictionary<int, object>> selectTask = database.ExecuteSelectAsync("SELECT fieldname FROM tablename WHERE data='" + data + "'"); // Excuse statement forming, this is just to test
    await selectTask;

    try
    {
        Dictionary<string, object> firstRow = (Dictionary<string, object>)selectTask.Result[0];

        if ((int)firstRow["fieldname"] == 0) return false; // data not valid, return false
        else return true; // data valid, return true
    }
    catch (Exception e)
    {
        LogException("Utility", e);
        LogDebug("Utility", "Database class returned empty result set");
        return false; // Empty result, presume false
    }            
}

我知道这段代码是有效的，因为在显示Returning select执行响应控制台输出后不久，第二行读取Executed select statements with x rows是输出。 这里的主要问题是存在竞争条件。如何确保数据在处理之前正确填充？

Answer 1

您需要一种方法让数据库回调向您的代码发出信号，表明它已被调用，并且可以恢复执行。最简单的方法是使用 TaskCompletionSource 。它看起来像:

public async Task<Dictionary<int, object>> ExecuteSelectAsync (string statement)
{
    // declare the TaskCompletionSource that will hold the database results
    var tcs = new TaskCompletionSource<Dictionary<int, object>>();

    DatabaseLibrary.execute(
        statement,
        new Action<dynamic> (s =>
        {
            // Take the data returned as 's' and populate the 'responseData' dictionary.
            Utility.LogDebug("Database", "Executed select statement with " + numberOfRows.ToString() + " rows");

            var data = new Dictionary<int, object>();
            // build your dictionary here

            // the work is now complete; set the data on the TaskCompletionSource
            tcs.SetResult(data);
        })
    );

    // wait for the response data to be created
    var responseData = await tcs.Task;

    Utility.LogDebug("Database", "Returning select execution response"); 
    return responseData;

    // if you don't need the logging, you could delete the three lines above and
    // just 'return tcs.Task;' (or move the logging into the callback)
}