IsInteger · Software Engineering Wiki

Returns true if the given string can be parsed into an integer, false otherwise.

Return the result of calling Double.TryParse() with NymberStyles.Integer for the given num string.
Use Double.TryParse() to allow handling of values larger than Int64.

using System.Globalization;

public static partial class _30s 
{
  public static bool IsInteger(string num) 
  {
    Double _ = 0.0;
    return Double.TryParse(num, NumberStyles.Integer, NumberFormatInfo.CurrentInfo, out _);
  }
}

_30s.IsInteger("2"); // true
_30s.IsInteger("3.1"); // false