How can i control event with 'if else' on Gtk#2.0

I am new in C#. I work C# with monodevelop on linux. I created GUI and added label. I could control text of Label by button(When I click button, button's event changed text of label ). But I don't want use button. I want to do that:

for example:

a=5

if(a==5)
{
  label.text="Hello";
} 
else 
{
  label.text="world";
}

I searched but I couldn't find anything this. Thanks your time.

Best regards

Faruk