RollCalculator/Form1.cs

using System;
using System.Windows.Forms;

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

        private void btnBerechnen_Click(object sender, EventArgs e)
        {
            try
            {
                double L1 = double.Parse(txtL1.Text);
                double D1 = double.Parse(txtD1.Text);
                double H1 = double.Parse(txtH1.Text);
                double L2 = double.Parse(txtL2.Text);
                double H2 = double.Parse(txtH2.Text);

                double A1 = D1 * D1 - H1 * H1;
                double A2 = L2 * A1 / L1;

                double D2 = Math.Sqrt(A2 + H2 * H2);

                lblErgebnis.Text = $"New outer diameter: {D2:F2} cm";
            }
            catch (Exception ex)
            {
                MessageBox.Show("Error during calculation: " + ex.Message);
            }
        }
    }
}