using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace Kontrola_vstupu
{
    public partial class Form1 : Form
    {
        Kontrola kontroluj;
   
          public Form1()
        {
            kontroluj = new Kontrola();
            InitializeComponent();
        }

        private void comboBox1_TextUpdate(object sender, EventArgs e)
        {

            comboBox1.Text = kontroluj.KonrolaCisla(comboBox1.Text);
           
        }     
    }
}
  

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Kontrola_vstupu
{
    class Kontrola
    {

        public string KonrolaCisla(string cislo)
        {
            float a;
           
            if (!float.TryParse(cislo, out a))
            {
 
 
                for (int i = 0; i < cislo.Length; i++)
                {

                    if (!float.TryParse(cislo.Substring(0, i + 1), out a))
                    {
                        cislo = cislo.Remove(i, 1);
                        i--;
                       
                    }
                }

            }
           
            return cislo;
        }

    }

}