HOA4 & HOQ prac !

*TextBox1=weight
2=height
3=end
4=start

*button 1 = bmi
2 = sum
3 = reset bmi
4 = reset sum

namespace esetydtyrdrt
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Button1_Click(object sender, EventArgs e)
        {
            if (TextBox1.Text == "" || TextBox2.Text == "")
            {
                MessageBox.Show("Error: Do not leave anything blank.");
            }

            else if (TextBox1.Text != "" && TextBox2.Text != "")
            {
                double weight = double.Parse(TextBox1.Text);
                double height = double.Parse(TextBox2.Text);

                double BMI = (weight * 0.45) / ((height * 0.025) * (height * 0.025));

                Label3.Text = "BMI: " + BMI;
            }


        }

        private void Button2_Click(object sender, EventArgs e)
        {
            if (TextBox3.Text == "" || TextBox4.Text == "")
            {
                MessageBox.Show("Error: Do not leave anything blank.");
            }

            else if (TextBox3.Text != "" && TextBox4.Text != "")
            {
                double start = double.Parse(TextBox4.Text);
                double end = double.Parse(TextBox3.Text);

                if (start < end)
                {
                    double n = end - start + 1;
                    double sum = n * ((start + end) / 2);

                    Label4.Text = "The sum is " + sum;
                }
                else if (start > end)
                {
                    MessageBox.Show("Error: The start has to be less than the end.");
                }
            }
        }

        private void button3_Click(object sender, EventArgs e)
        {
            TextBox1.Text = "";
            TextBox2.Text = "";
            Label3.Text = "BMI: ";
        }

        private void button4_Click(object sender, EventArgs e)
        {
            TextBox3.Text = "";
            TextBox4.Text = "";
            Label4.Text = "Sum: ";
        }
    }