Update data Decimal DataType Column in SQL Server in C#

Hi,

i am using the below code to enter decimal value and calculate total on leave event even for the blank textBox.

I want to update the same in SqlServer for DataType Decimal for the same TextBox, For the blank textBox It is not updating shows error and when I input even 0 and same gets updated.

private void General_Leave(object sender, EventArgs e)
        {
            CalculateTotalColor();
        }
        private void CalculateTotalColor()
        {
            decimal num = 0, num1 = 0, num2 = 0, num3 = 0, num4 = 0, num5 = 0, num6 = 0, num7 = 0;
            

            if (textBox5.Text.Trim() != string.Empty)
            {
                num = Convert.ToDecimal(textBox5.Text);
            }
            if (textBox6.Text.Trim() != string.Empty)
            {
                num1 = Convert.ToDecimal(textBox6.Text);
            }
            if (textBox8.Text.Trim() != string.Empty)
            {
                num2 = Convert.ToDecimal(textBox8.Text);
            }
            if (textBox10.Text.Trim() != string.Empty)
            {
                num3 = Convert.ToDecimal(textBox10.Text);
            }
            if (textBox12.Text.Trim() != string.Empty)
            {
                num4 = Convert.ToDecimal(textBox12.Text);
            }
            if (textBox14.Text.Trim() != string.Empty)
            {
                num5 = Convert.ToDecimal(textBox14.Text);
            }
            if (textBox16.Text.Trim() != string.Empty)
            {
                num6 = Convert.ToDecimal(textBox16.Text);
            }
            if (textBox19.Text.Trim() != string.Empty)
            {
                num7 = Convert.ToDecimal(textBox19.Text);
            }

            textBox200.Text = (num + num1 + num2 + num3 + num4 + num5 + num6 + num7).ToString();

private void update_Click(object sender, EventArgs e)
        {
            SqlConnection con = new SqlConnection("server=IT-SHRAWAN;database = mba; trusted_connection = true;");
            con.Open();
            SqlCommand cmd = new SqlCommand("UPDATE OrderEntry set Colour1Size1Qty1= '" + (float)Convert.ToDecimal(textBox5.Text) + "',Colour2Size1Qty1= '" + (float)Convert.ToDecimal(textBox34.Text) + "',Colour3Size1Qty1='" + (float)Convert.ToDecimal(textBox53.Text) + "',Colour4Size1Qty1='" + (float)Convert.ToDecimal(textBox72.Text) + "',Colour5Size1Qty1='" + (float)Convert.ToDecimal(textBox91.Text) + "',Colour6Size1Qty1='" + (float)Convert.ToDecimal(textBox110.Text) + "',Colour7Size1Qty1='" + (float)Convert.ToDecimal(textBox129.Text) + "',Colour8Size1Qty1='" + (float)Convert.ToDecimal(textBox148.Text) + "' where PIN='" + textBox208.Text + "'", con);
           
                    int x = cmd.ExecuteNonQuery();
                    con.Close();
                    MessageBox.Show("Your Data is successfully saved");
                
               
        }