Question

I am creating an email software which send email to some accounts. I want to append text every time new email is sent or failed. But textbox shows me report after sending all emails. If the toList is very large like 30+ emails the app screen goes white and after sending all emails GUI comes back with the updated OutPutTextBox. Here is the code inside the SendButton_Click method

foreach (String to in toList)
{
    bool hasSent = SendMail(from, "password", to, SubjectTextBox.Text, BodyTextBox.Text);

    if (hasSent)
    {
        OutPutTextBox.appendText("Sent to: " + to);
    }
    else
    {
        OutPutTextBox.appendText("Failed to: " + to);
    }
}

Answer 1

What you actually want to do is invoke SendMail asynchronously. There are several ways to do this in .NET 4.0. I recommend starting a Task object in your foreach loop, and scheduling a task continuation for each one to the UI thread:

string subject = SubjectTextBox.Text;
string body = BodyTextBox.Text;
var ui = TaskScheduler.FromCurrentSynchronizationContext();
List<Task> mails = new List<Task>();
foreach (string to in toList)
{
  string target = to;
  var t = Task.Factory.StartNew(() => SendMail(from, "password", target, subject, body))
      .ContinueWith(task =>
      {
        if (task.Result)
        {
          OutPutTextBox.appendText("Sent to: " + to); 
        }
        else
        {
          OutPutTextBox.appendText("Failed to: " + to); 
        }
      }, ui);
  mails.Add(t);
}

Task.ContinueWhenAll(mails.ToArray(), _ => { /* do something */ });

(syntax might be slightly off; I didn’t compile it).