如何在Lambda中使用async/await [英] how to use async/await inside a lambda

问题描述

如何在lambda表达式中正确使用async/await关键字?这是代码:

How can I use the async/await keywords correctly in a lambda expression ? here is the code :

public async Task<IHttpActionResult> GetUsers() {

        var query = await _db.Users.ToListAsync();

        var users = query.Select(async u =>  new
        {
            FirstName = u.FirstName,
            LastName = u.LastName,
            IsGeek = await _userManager.IsInRoleAsync(u.Id, "Geek")
        });

        return Ok(users);
}

如您所见，此代码在webapi控制器中运行，它可以编译而没有任何错误，问题在于它需要在某个地方添加 extra (await)，因为此操作永远不会恢复.

As you can see this code is running inside a webapi controller, it compiles without any error, the problem is it needs an extra await somewhere because this action never retuns.

请注意，_db和_usermanager是应用程序的DbContext和UserManager.

Notice that _db and _usermanager are the DbContext and the UserManagerfor the application.

谢谢.

更新:

此等效代码永远不会失败(但它不是优美的:():

This equivalent code never fails (but it's not ellegant :( ):

var query = await _db.Users.ToListAsync();
var users = new List<object>();
foreach (var u in query)
{
     bool IsGeek = await _userManager.IsInRoleAsync(u.Id, "IsGeek");
     users.Add( new {
         FirstName = u.FirstName,
         LastName = u.LastName,
         IsGeek= IsGeek
      });
 }
return Ok(users);

推荐答案

考虑您的类型.

var query = await _db.Users.ToListAsync();

query是用户列表.

var users = query.Select(async u =>  new
{
    FirstName = u.FirstName,
    LastName = u.LastName,
    IsGeek = await _userManager.IsInRoleAsync(u.Id, "Geek")
});

当您使用async lambda Select时，结果是任务序列.因此，要(异步)等待所有这些任务完成，请使用Task.WhenAll:

When you Select with an async lambda, the result is a sequence of tasks. So, to (asynchronously) wait for all those tasks to complete, use Task.WhenAll:

var result = await Task.WhenAll(users);
return Ok(result);

这篇关于如何在Lambda中使用async/await的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！