only numbers in C#

OnlyNumbers.cs

class OnlyNumbers
{
    private string value;

    public OnlyNumbers(string value)
    {
        this.value = value;
    }

    public override string ToString()
    {
        string onlyNumber = "";
        foreach (char s in this.value)
        {
            if (Char.IsDigit(s))
            {
                onlyNumber += s;
            }
        }
        return onlyNumber;
    }
}