Work with asynchronous tasks that return values

Question

How to work correctly with a method that takes an asynchronous task as an argument, for example Task<NpgsqlConnection> ? Suppose I have an asynchronous method:

  public async Task<NpgsqlConnection> CreateConnectionAsync() { var connection = new NpgsqlConnection(this.ConnectionString); await connection.OpenAsync(); return connection; }

And I need to get the NpgsqlConnection object in this method, which creates a command for the connection.

  public NpgsqlCommand NpgCommandCreator(NpgsqlConnection connection, string commandtext) { using (var connector = connection) { NpgsqlCommand COMMAND = connector.CreateCommand(); COMMAND.CommandText = commandtext; return COMMAND; } }

The problem is that NpgsqlConnection and Task<NpgsqlConnection> not the same thing, how can I modify NpgCommandCreator that it can work with the object returned from CreateConnectionAsync ?

Accepted Answer · 2017-08-18T21:46:28

Since your CreateConnectionAsync method returns Task<NpgsqlConnection> you can not upgrade anything, but rather wait for the asynchronous operation to complete using the await operator, and then transfer the resulting value to the NpgCommandCreator method.

if await is applied to the result of a method call that returns Task<TResult> , then the type of await expression is TResult

For example:

 public async void DoWorkAsync() { // Дожидаемся выполнения асинхронной операции. // после ее завершения в переменной будет объект типа NpgsqlConnection. var connection = await CreateConnectionAsync(); var myCommandText = "SELECT Name FROM Users"; // Передаем данный объект в метод. var command = NpgCommandCreator(connection, myCommandText); }

In this case, the connection is made asynchronously, and the request is executed synchronously.
It is possible that the execution of the request also needs to be done asynchronously.
@TylerDurden but where did you see the query execution here?
First a connection is created, then a command is created that will be executed within the specified connection.
When transferring the connection 'NpgCommandCreator', I assumed that the request was fulfilled, I am wrong (premature judgment)

Andrei S. Andrei S. 439 2 7 · Answer 2 · 2017-08-19T08:55:58

When creating methods using the async / await model, the entire call chain should be created using the async / await model. In other words, async / await should be cross-cutting . for example

 async Task<int> FirstMethod() { await Task.Delay(100); return 1; } async Task<int> SecondMethod() { await Task.Delay(100); return 2; } async Task<int> UseBothMethods() { var first = await FirstMethod();//Текущий поток высвобождается и может заняться чем-то еще, пока FirstMethod() не вернет значение var second = await SecondMethod();Текущий поток высвобождается и может заняться чем-то еще, пока SecondMethod() не вернет значение return first+second; }

If the UseBothMethods method is created without using async / await, then we will have to wait for the FirstMethod and SecondMethod execution, thereby suspending (freezing) the execution flow.

 Task<int> UseBothMethods() { var first = FirstMethod().Result; // ждем 100 мс пока выполниться метод. Поток заморожен и ничего не делает var second = SecondMethod().Result;// ждем 100 мс пока выполниться метод. Поток заморожен и ничего не делает return first+second; }

If async / await is not done end-to-end, it will disrupt the use of the pattern and will deprive us of the benefits of asynchronous waiting for the execution of "long" methods, which is to release the thread to do any work.
Now I will explain how all this relates to the current issue. The answer is, if you create async / await end-to-end, as it should be, then questions on the type of what the author had, simply will not arise.

Work with asynchronous tasks that return values

2 answers 2

More articles: