You don't say what version of Visual COBOL for .NET you're using. With recent dialects, it would be something like the following:
method-id. limparTextBoxes.
procedure division using by value controles as type Control.ControlCollection.
declare ctrl as type Control
perform varying ctrl through controles
if ctrl instance of type TextBox
declare tb as type TextBox = ctrl as type TextBox
set tb::Text to string::Empty
end-if
end-perform
end method limparTextBoxes.
I haven't tried that. I didn't use all the most recent syntax enhancements, since I don't know which ones your version of Visual COBOL supports.