Programatically Select all text in UITextField

When a UITextField receives focus, I want the text inside of it to be automatically selected so that anything the user types overwrites it. I've tried the following, but nothing work (iOS 7.1.1):

UserNameField.EditingDidBegin += (object sender, EventArgs e) =>
{
    UserNameField.SelectAll(this);
};

UserNameField.EditingDidBegin += (object sender, EventArgs e) =>
{
    UserNameField.GetTextRange (UserNameField.BeginningOfDocument, UserNameField.EndOfDocument);
};