A Textbox Validation (Only Doubles) Needs C# Gurus :-)

Thank you very much. Tonight I will try this...

Better handling

For blocking you can do this

So you would like to block all the other letters?

Exactly... User can enter E letter but not other letters...

I am trying to make a unit converter software. But input must be double precision. Now I am trying your
=============================
Regex reg = new Regex("^[+-]?([0-9]*\\.?[0-9]+|[0-9]+\\.?[0-9]*)([eE][+-]?[0-9]+)?$");

 if (reg.IsMatch(textBox1.Text))
{
}
else
{
MessageBox.Show("Invalid value");
}
==============================
code. It seems OK but I need develeop the contrent of those codes. Thanks for all..

Thank you very much. I am writing a windows form software and trying to find a place to paste your codes :-)
MY CODES ARE HERE:

using System;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Double DoubleKatsayi1 = 1;
            Double DoubleKatsayi2 = 1;

            if (listBox1.SelectedIndex == -1)
            {
                listBox1.SelectedIndex = 0;
            }

            if (listBox2.SelectedIndex == -1)
            {
                listBox2.SelectedIndex = 0;
            }
            Double DoubleSayi = Double.Parse(textBox1.Text);

            switch (listBox1.SelectedItem.ToString())
            {
                case "meter": DoubleKatsayi1 = 1; break;
                case "decimeter": DoubleKatsayi1 = 0.1; break;
                case "centimeter": DoubleKatsayi1 = 0.01; break;
                case "milimeter": DoubleKatsayi1 = 0.001; break;
            }

            switch (listBox2.SelectedItem.ToString())
            {
                case "meter": DoubleKatsayi2 = 1; break;
                case "decimeter": DoubleKatsayi2 = 0.1; break;
                case "centimeter": DoubleKatsayi2 = 0.01; break;
                case "milimeter": DoubleKatsayi2 = 0.001; break;
            }
            Double DoubleSonuc = DoubleSayi * (DoubleKatsayi1 / DoubleKatsayi2);
            textBox2.Text = DoubleSayi.ToString() + " " + listBox1.SelectedItem.ToString() + " = " + DoubleSonuc.ToString() + " " + listBox2.SelectedItem.ToString();

        }

        private void button2_Click(object sender, EventArgs e)
        {
            int index1 = listBox1.SelectedIndex;
            int index2 = listBox2.SelectedIndex;
          
            listBox1.SelectedIndex = index2;
            listBox2.SelectedIndex = index1;
        }


     }
}

If you would like to validate using text box with regular expression validation you can do like this

Sample code

Can you help me about the syntax? I am newbee..

Try this REGEX