C#

event EventHandler<MyArgs>

// background thread would call a method like this one, each time it needs the user's input:

public string WaitForUser(string message) {
    // do whatever is needed to get the message displayed to the user
    ...
    // wait for user interaction
    myAutoResetEvent.WaitOne();
    // get user input 
    return someGlobalVariable;
}

// UI could have a TextBox+Button, and the button click handler would do:
    public void myButton_Click(object sender, EventArgs e) {
        // get user input
        string s=myTextBox.Text;
        someGlobalVariable=s;
        // tell background to continue 
        myAutoResetEvent.Signal();
    }