Running several EntityFramework database queries i

问题:

I am trying to run 3 db queries in parallel but im not sure that I am doing it correctly.

I have made 3 functions which each make a query to the db.

    private static async Task<string> getAccountCodeAsync(string deviceId)
    {
        long deviceIdLong = long.Parse(deviceId);
        using (var db = new NetworksEntities())
        {
            return db.Devices.Where(x => x.DeviceId == deviceIdLong).Select(x => x.AccountCode).FirstOrDefault();
        }
    }

    private static async Task<string> getDeviceTypeAsync(string deviceId)
    {
        long deviceIdLong = long.Parse(deviceId);
        using (var db = new NetworksEntities())
        {
            return db.Devices.Where(x => x.DeviceId == deviceIdLong).Select(x => x.DeviceType).FirstOrDefault().DeviceType;
        }
    }

    private static async Task<string> getUserNameAsync(string userId)
    {
        int userIdInt;
        Int32.TryParse(userId, out userIdInt);
        using (var db = new NetworksEntities())
        {
            return db.Users.Where(x => x.UserId == userIdInt).Select(x => x.Email).FirstOrDefault();
        }
    }   

I then in my code run the three tasks:

    var TaskAccountCode = await getAccountCodeAsync(deviceId);
    var TaskDeviceType = await getDeviceTypeAsync(deviceId);
    var TaskUsername = await getUserNameAsync(userId);
    await Task.WhenAll();   
    //use the results from my 3 tasks to make a new insert into the db.

Is what I am doing actually running my 3 db queries in parallel?

EDIT:

    private static async Task<string> getAccountCodeAsync(string deviceId)
    {
        long deviceIdLong = long.Parse(deviceId);
        using (var db = new NetworksEntities())
        {               
            return db.Devices.Where(x => x.DeviceId == deviceIdLong).Select(x => x.AccountCode).FirstOrDefaultAsync();
        }
    }

回答1:

You will have to change the last part of the code to make it run in parallel:

var taskAccountCode = getAccountCodeAsync(deviceId);
var taskDeviceType = getDeviceTypeAsync(deviceId);
var taskUsername = getUserNameAsync(userId);
await Task.WhenAll(taskAccountCode, taskDeviceType, taskUsername);
var accountCode = taskAccountCode.Result;
var deviceType = taskDeviceType.Result;
var username  = taskUsername.Result;

Notice that there is only one await. In your original code you await every task one after the other making them run in sequence instead of in parallel.

Also, the methods getAccountCodeAsync etc. are not really async methods (you should get a compiler warning about this). However, Entity Framework 6 has support for async operations and to use that you should replace FirstOrDefault with FirstOrDefaultAsync. For each parallel operation you will have to use a separate context, and that is exactly what you are doing.

回答2:

No they run one after eachother if you call them like that using await foreach method, it will run first method, then the second one...etc. And the last part await Task.WhenAll(), you did not supplied the tasks to wait for completion.

To run them in parallel you have to do it like this:

var TaskAccountCode = getAccountCodeAsync(deviceId);
var TaskDeviceType = getDeviceTypeAsync(deviceId);
var TaskUsername = getUserNameAsync(userId);
await Task.WhenAll(TaskAccountCode, TaskDeviceType,TaskUsername);