DigitsOnly.cs

public class DigitsOnly
{
      public static string Get(string text)
      {
          string result = string.Empty;
          if (!string.IsNullOrWhiteSpace(text))
          {
              var sb = new StringBuilder(text.Length);
              for (int i = 0; i < text.Length; i++)
                  if (text[i] >= '0' && text[i] <= '9')
                      sb.Append(text[i]);
              result = sb.ToString();
          }
          return result;
      }
}