Async/Await with Tasks.WhenAll in For 循环未并行运行

Question

tI'm doing a data migration where I have a long list of pets that I am looping through to perform the migration. In order to migrate each pet a lot of business logic has to be done. I'd like to have multiple migration tasks running at once to speed this along, but the async addition to the below seems to be operating that same as it would synchronously. I've found several stack overflow responses and blog posts about this, but for some reason this still isn't working.

I tried to keep the code below to the minimum required, but if more context is needed I can provide that.

public static async Task MigrationAsync(MyDbContext myContext)
{
    await MainMigrationAsync(myContext);

}

public static async Task MainMigrationAsync(MyDbContext myContext)
{
    var pets = myContext.Pets.ToList();

    var tasks = new List<Task>();
    for (var eachPet = 0; eachRelation < pets.Count; eachRelation++)
    {
        var task = SingleLongRunningPetMigrationAsync(myContext, pets[eachPet]);
        tasks.Add(task);
    }
    await Task.WhenAll();

    myContext.SaveChanges();
}

public static async Task SingleLongRunningPetMigrationAsync(MyDbContext myContext, Pet pet)
{
    //these need to run synchronously for each pet, but need multiple pet tasks to be running at once
    MigrationMethodOne(myContext);
    MigrationMethodTwo(myContext);
    MigrationMethodThree(myContext);
    MigrationMethodFour(myContext);
}

Answer 1

Important: Since EF core doesn't allow parallel usage of a DbContext you must provide one DbContext per "job".

And there is no await in the SingleLongRunningPetMigrationAsync method, so it should not be of type async Task , just void .

You could for instance use a " Parallel.ForEach " in your scenario (given that each call gets its own DbContext ).