Exceptions

The asynchronous method should always throw exceptions that are assigned to the returning task. However, the usage errors, such as passing null parameters to the asynchronous method, should be properly handled.

Let's suppose we want to generate several documents dynamically based on a predefined templates list, where each template populates the placeholders with dynamic values and writes it on the filesystem. We assume that this operation will take a sufficient amount of time to generate a document for each template. Here is a code snippet showing how the exceptions can be handled:

static void Main(string[] args) 
{ 
  List<Template> templates = GetTemplates(); 
  IEnumerable<Task> asyncDocs = from template in templates select 
  GenerateDocumentAsync(template); 
  try 
  { 
    Task.WaitAll(asyncDocs.ToArray()); 
 
  }catch(Exception ex) 
  { 
    Console.WriteLine(ex); 
  } 
  Console.Read(); 
} 
 
private static async Task<int> GenerateDocumentAsync(Template template) 
{ 
  //To automate long running operation 
  Thread.Sleep(3000); 
  //Throwing exception intentionally 
  throw new Exception(); 
}

In the preceding code, we have a GenerateDocumentAsync method that performs a long running operation, such as reading the template from the database, populating placeholders, and writing a document to the filesystem. To automate this process, we used Thread.Sleep to sleep the thread for three seconds and then throw an exception that will be propagated to the calling method. The Main method loops the templates list and calls the GenerateDocumentAsync method for each template. Each GenerateDocumentAsync method returns a task. When calling an asynchronous method, the exception is actually hidden until the Wait, WaitAll, WhenAll, and other methods are called. In the preceding example, the exception will be thrown once the Task.WaitAll method is called, and will log the exception on the console.