Buttons have a number of different states, and you can set the button text once for all states or assign different values for each state.

To set the ‘default’ text for a button, call SetTitle for UIControlState.Normal and that text will be used for all states:

buttonRect = UIButton.FromType(UIButtonType.RoundedRect);
buttonRect.SetTitle ("Click me", UIControlState.Normal);

Alternatively you can make each state different, as this image shows:

The code for these three states is:

buttonRect = UIButton.FromType(UIButtonType.RoundedRect);
buttonRect.SetTitle ("Click me", UIControlState.Normal);
buttonRect.SetTitle ("Clicking me", UIControlState.Highlighted);
buttonRect.SetTitle ("Disabled", UIControlState.Disabled);
buttonRect.SetTitleColor (UIColor.LightGray, UIControlState.Disabled);

You can also set the different states using the Xamarin Designer for iOS by choosing each one from the drop-down-list of the properties panel.