Sum of Odd and Even number in multiple textbox.

Arrays make programming easier.

using System;
using System.Drawing;
using System.Windows.Forms;

public partial class Form1 : Form
{
    private readonly TextBox[] textBoxes;
    private readonly Label[] labels;

    public Form1() {
        InitializeComponent();
        textBoxes = new TextBox[] { textBox1, textBox2, textBox3, textBox4 };
        labels = new Label[] { label1, label2, label3, label4};
        foreach (TextBox textBox in textBoxes) {
            textBox.TextChanged += TextBox_TextChanged;
        }
    }

    private void TextBox_TextChanged(object sender, EventArgs e) {
        var textBox = (TextBox)sender;
        var label = labels[Array.IndexOf(textBoxes, textBox)];
        if (textBox.Text == "") {
            textBox.BackColor = Color.Red;
            DialogResult result = MessageBox.Show("This fild is empty", "Warning", MessageBoxButtons.OK, MessageBoxIcon.Warning);
            if (result == DialogResult.OK) {
                textBox.BackColor = Color.White;
                this.Refresh();
            }
        } else {
            int a = Convert.ToInt32(textBox.Text);
            if (a % 2 == 0) {
                label.ForeColor = Color.Violet;
                label.Text = "EVEN";
            } else {
                label.ForeColor = Color.Blue;
                label.Text = "ODD";
            }
        }

        int even = 0, odd = 0;
        foreach (var box in textBoxes) {
            int.TryParse(box.Text, out int value);
            if (value % 2 == 0) {
                even += value;
            } else {
                odd += value;
            }
        }
        lblEven.Text = even.ToString();
        lblOdd.Text = odd.ToString();
    }
}