private void Calculate_Click(object sender, RoutedEventArgs e) { int a, b; float fA, fB; a = int.Parse(textBoxA.Text); b = int.Parse(textBoxB.Text); fA = float.Parse(textBoxfA.Text); fB = float.Parse(textBoxfB.Text); try { ClassLibrary1.Class1 externalDLL = new ClassLibrary1.Class1(); Substract.Sub substractDLL = new Substract.Sub(); textBoxCalc.Text = externalDLL.sum(a, b).ToString(); textBoxCalcSubstract.Text = substractDLL.substract(fA, fB).ToString(); } catch (Exception ex) { MessageBox.Show(ex.ToString()); } }