Instant alert not number input

string invalidChars = ""; // List of invalid characters
        private void txtYear_TextChanged(object sender, EventArgs e)
        {

            Control ctr = (Control)sender;

            // Show error alert
            if (ctr.Text.Trim().Length > 0 && !char.IsDigit(ctr.Text, ctr.Text.Length - 1)) //Last character is not a digit
            {
                this.errYear.SetError(txtYear, "Invalid character");
                // Add invalid character to the list
                if (!invalidChars.Contains(ctr.Text[ctr.Text.Length - 1]))
                    invalidChars += ctr.Text[ctr.Text.Length - 1];
            }

            // Remove the character from list if it was deleted from the textbox
            if (invalidChars.Length > 0)
                if (!ctr.Text.Contains(invalidChars[invalidChars.Length-1]))
                    invalidChars = invalidChars.Remove(invalidChars.Length - 1);


            txtResult.Text = invalidChars;

            // Remove error alert
            if (invalidChars.Length == 0)
                this.errYear.Clear();
        }