Question

I start some task like this

Task.Factory.StartNew(SomeMethod, TaskCreationOptions.LongRunning);

void SomeMethod()
{

}

Is there is a way to return data when task is completed?

like

List<User> SomeMethod()
{
  List<User> = List<User>();
   ...
   return userList;
}

Answer 1

Yes like this.
By using the generic overload of Task.Factory.StartNew<TResult>(Func<TResult> function) your task defines a return type. Then with task.Result you can access your result.

Task<List<User>> task = Task.Factory.StartNew<List<User>>(SomeMethod, TaskCreationOptions.LongRunning);

RunSomeOtherMethod();         // We can do other work in parallel...

List<User> result = task.Result;  // Wait for task to finish and fetch result.

---

List<User> SomeMethod()
{
  List<User> = List<User>();
   ...
   return userList;
}

---

If you would like to pass one or more parameters to your function use a lambda to start your function:

Task<List<User>> task = Task.Factory.StartNew<List<User>>(
    () => SomeMethod("inputString"), //Starting the method using a lambda so you can pass in a parameter.
    TaskCreationOptions.LongRunning
);

RunSomeOtherMethod();         // We can do other work in parallel...

List<User> result = task.Result;  // Wait for task to finish and fetch result.

---

List<User> SomeMethod(string input)
{
  List<User> = List<User>();
   ...
   return userList;
}

For more information about threading and Task Parallelism I would like to refer you to the online ebook: Threading in C# by Joseph Albahari