d.cs

using System;
using System.Data;
using System.Windows.Forms;

namespace Calculator {
    public partial class Calculator : Form {

        public Calculator() {
            InitializeComponent();
            KeyPreview = true;
        }

        private void addToDisplay(string number) {
            if (display.Text == "Enter formula" || display.Text == "Too long formula" || display.Text == "Unexpected Error") {
                display.Text = number;
            } else if (display.Text.Length < 10) {
                display.Text += number;
            }
            if (display.Text.Length == 10) {
                display.Text = "Too long formula";
            }
        }

        private void calculate() {
            string fromDisplay = display.Text;
            try {
                DataTable dt = new DataTable();
                var v = dt.Compute(fromDisplay, "");
                double result = double.Parse(v.ToString());
                result = Math.Round(result, 3);
                display.Text = result.ToString().Replace(",", ".");
            } catch {
                display.Text = "Unexpected Error";
            }
        }

        private void numberClick(object sender, EventArgs e) {
            addToDisplay(((Button)sender).Text);
        }

        private void calcClick(object sender, EventArgs e) {
            calculate();
        }

        private void clearClick(object sender, EventArgs e) {
            display.Text = "Enter formula";
        }

        private void Calculator_KeyDown(object sender, KeyEventArgs e) {
            switch (e.KeyCode) {
                case Keys.Enter:
                    calculate();
                    break;
                case Keys.Back:
                    display.Text = "Enter formula";
                    break;
            }
        }

        private void Calculator_KeyPress(object sender, KeyPressEventArgs e) {
            if (char.IsDigit(e.KeyChar))
                addToDisplay($"{e.KeyChar}");
            else switch (e.KeyChar) {
                case '*':
                case '/':
                case '+':
                case '-':
                case '.':
                    addToDisplay($"{e.KeyChar}");
                    break;
                case ',':
                    addToDisplay(".");
                    break;
            }
        }
        
    }
}