Enter only letters(caps and small both) as input in to text box.

How to enter only letters(caps and small both) as input in to text box.

public static bool IsDigit(string tcString)
{
    //get the first character in the string
    char c = tcString[0];
    return Char.IsDigit(c);
}

private void button1_Click(object sender, EventArgs e)
{
    char[] letters = new char[50];
    int len = textBox1.Text.Length;

    for (int j = 0; j < len; j++)
    {
        letters[j] = Convert.ToChar(textBox1.Text.Substring(j, 1));
    }

    for (int i = 0; i < len; i++)
    {
        bool result = IsDigit(letters[i].ToString());
        if (result == true)
        {
            MessageBox.Show(letters[i] + "is a digit");
        }
    }
}