Trigger control's event programmatically

Button controls have a PerformClick() method that you can call.

button1.PerformClick();

The .NET framework uses a pattern where for every event X there is a method protected void OnX(EventArgs e) {} that raises event X. See this Msdn article. To raise an event from outside the declaring class you will have to derive the class and add a public wrapper method. In the case of Button it would look like this:

class MyButton : System.Windows.Forms.Button
{

    public void ProgrammaticClick(EventArgs e)
    {
        base.OnClick(e);
    }

}

You can just call the event handler function directly and specify null for the sender and EventArgs.Empty for the arguments.

void ButtonClicked(object sender, EventArgs e)
{
    // do stuff
}

// Somewhere else in your code:
button1.Click += new EventHandler(ButtonClicked);

// call the event handler directly:
ButtonClicked(button1, EventArgs.Empty);

Or, rather, you'd move the logic out of the ButtonClicked event into its own function, and then your event handler and the other code you have would in turn call the new function.

void StuffThatHappensOnButtonClick()
{
    // do stuff
}

void ButtonClicked(object sender, EventArgs e)
{
    StuffThatHappensOnButtonClick();
}

// Somewhere else in your code:
button1.Click += new EventHandler(ButtonClicked);

// Simulate the button click:
StuffThatHappensOnButtonClick();

The latter method has the advantage of letting you separate your business and UI logic. You really should never have any business logic in your control event handlers.