write like this i hope it will work
TextBox5.Text =c.addition(a, b).ToString
in function also make change
Public Function addition(ByVal c1 As complexnumber, ByVal c2 As complexnumber) As complexnumber
Dim result As New complexnumber(c1.Real + c2.Real, c1.Imaginary + c2.Imaginary)
addition= result
End Function