.NET Framework Task Parallel Library (TPL) Flowing execution context with AsyncLocal
Example
When you need to pass some data from the parent task to its children tasks, so it logically flows with the execution, use AsyncLocal class:
void Main()
{
AsyncLocal<string> user = new AsyncLocal<string>();
user.Value = "initial user";
// this does not affect other tasks - values are local relative to the branches of execution flow
Task.Run(() => user.Value = "user from another task");
var task1 = Task.Run(() =>
{
Console.WriteLine(user.Value); // outputs "initial user"
Task.Run(() =>
{
// outputs "initial user" - value has flown from main method to this task without being changed
Console.WriteLine(user.Value);
}).Wait();
user.Value = "user from task1";
Task.Run(() =>
{
// outputs "user from task1" - value has flown from main method to task1
// than value was changed and flown to this task.
Console.WriteLine(user.Value);
}).Wait();
});
task1.Wait();
// ouputs "initial user" - changes do not propagate back upstream the execution flow
Console.WriteLine(user.Value);
}
Note: As can be seen from the example above AsynLocal.Value has copy on read semantic, but if you flow some reference type and change its properties you will affect other tasks. Hence, best practice with AsyncLocal is to use value types or immutable types.
