VisualComponents/VisualComponentsLib/MyTextBox.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using System.Windows.Forms;
using VisualComponentsLib.CustomException;

namespace VisualComponentsLib
{
    public partial class MyTextBox : UserControl
    {
        public double? TextBoxValue
        {
            get
            {
                if (checkBox.Checked)
                {
                    return null;
                }

                if (!CheckValue())
                {
                    return null;
                }

                return Convert.ToDouble(textBox.Text);
            }

            set
            {
                checkBox.Checked = !value.HasValue;

                textBox.Text = value?.ToString();
            }
        }

        private string? ErrorMessage
        {
            get;

            set;
        }

        public MyTextBox()
        {
            InitializeComponent();
        }

        private EventHandler _textChanged;

        public new event EventHandler? TextChanged
        {
            add => _textChanged += value;
            remove => _textChanged -= value;
        }

        private void CheckBox_CheckedChanged_1(object sender, EventArgs e)
        {
            if (textBox.ReadOnly == true)
            {
                textBox.ReadOnly = false;
            }
            else
            {
                textBox.ReadOnly = true;
            }
        }

        private void TextBox_TextChanged(object sender, EventArgs e)
        {
            _textChanged?.Invoke(sender, e);
        }

        private bool CheckValue()
        {
            if (textBox.Text == null && !checkBox.Checked)
            {
                ErrorMessage = "Недопустимое значение null";

                return false;
            }

            if (!textBox.Text.Contains('.') ||
                    textBox.Text.IndexOf('.') == textBox.Text.Length - 1 ||
                    textBox.Text.Length <= 2)
            {
                ErrorMessage = "Некорректное double значение";

                return false;
            }

            return true;
        }
    }
}