รบกวรช่วยหน่อยค่ะ เรื่องโค้ด VB.NET การคำนวณเกรดเฉลี่ย

 if (comboBox1.Text == "A")
            { textBox1.Text = "4"; }
            if (comboBox1.Text == "B")
            { textBox1.Text = "3"; }
            if (comboBox1.Text == "C")
            { textBox1.Text = "2"; }
            if (comboBox1.Text == "D")
            { textBox1.Text = "1"; }

 private void med55555555555555555555555555() //เม็ดทอด
        {
            var ABC="";
            if (comboBox1.Text == "A")
            { ABC = "4"; }
            if (comboBox1.Text == "B")
            { ABC = "3"; }
            if (comboBox1.Text == "C")
            { ABC = "2"; }
            if (comboBox1.Text == "D")
            { ABC = "1"; }

            textBox1.Text = ABC; // จะ กี่ textBox ก็ไม่หวั่น จับ = abc หมด
        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e) // อีเว้น

        {


            med55555555555555555555555555(); เอา เม็ดทอดไปวางตาที่ต้องการ
        }

 Dim GradeValue As Double

        Select Case ComboBox1.Text
            Case "A"
                GradeValue = 4
            Case "B+"
                GradeValue = 3.5
            Case "B"
                GradeValue = 3
            Case "C+"
                GradeValue = 2.5
            Case "C"
                GradeValue = 2
            Case "D+"
                GradeValue = 1.5
            Case "D"
                GradeValue = 1
            Case "F"
                GradeValue = 0
        End Select