List<T> as 'out' parameter causes an error. Why?

Question

In this code:

public bool SomeMethod(out List<Task> tasks)
{
    var task = Task.Factory.StartNew(() => Process.Start(info));
    tasks.Add(task);
}

I get an error, "Use of unassigned out parameter 'tasks'". Why?

In an MSDN example there's just use of out parameter

class OutExample
{
    static void Method(out int i)
    {
        i = 44;
    }

    static void Main()
    {
        int value;
        Method(out value);
        // value is now 44
    }
}

Is it because of List<T>?

Answer 1

You have to initialize the out parameter in the method body (that is create a new List<Task> instance and assign it to the out parameter):

public bool SomeMethod(out List<Task> tasks) {
  var task = Task.Factory.StartNew(() => Process.Start(info);
  tasks = new List<Task>() { task };
  ...
}

I'm using the collection initializer syntax to add the task to the list, but you could call the Add method instead if you prefer.

You should call the method like this:

List<Task> tasks;
SomeMethod(out tasks);
var newTask = tasks[0]; // Access the task just created.

C# 7.0 has introduced new simpler syntax where you declare the variable in the call to the function with the out parameter:

SomeMethod(out var tasks);
var newTask = tasks[0]; // Access the task just created.

As a List<T> is passed by reference you can get rid of the out parameter. You then have to create the list before calling the method:

public bool SomeMethod(List<Task> tasks) {
  var task = Task.Factory.StartNew(() => Process.Start(info);
  tasks.Add(task);
  ...
}

And call it like this:

var tasks = new List<Task>();
SomeMethod(tasks);
var newTask = tasks[0]; // Access the task just created.

In general it is good practice to avoid out parameters because they can be confusing.