Claro, la puedes poner donde quieras, p.ej
Código PHP:
private sub button1_click(byval sender as object, byval e as eventargs) handles button1.click
dim a as decimal = 1 + 5.8
Me.textbox1.Attributes.Add("onkeypress", "mifuncion(this," & Convert.tostring(a).Replace(",",".") & ");")
end sub