Issue about width and height measurement of textbox

Question

Error:  Watermarktextbox does not contain a definition for measure and no extension method measure accepting a first argument of type watermarktextbox could be found(are you missing a using directive or an assembly reference?)

 

 

 MyCode:

 

txtbox_Commnet.measure(new Size(double.positiveInfinity,500));

return new size (txtbox_Commnet.DesiredSize.Width,txtbox_Commnet.DesiredSize.Height);