Extract Numbers From String

private string ExtractNumbers(string Expression)
{
string result = null;
char Letter;

for (int i = 0; i < Expression.Length; i++)
{
Letter = Convert.ToChar(Expression.Substring(i, 1));

if (Char.IsNumber(Letter))
{
result += Letter.ToString();
}
}

// MessageBox.Show(result);
return result;
}