Самый легкий путь к контрольным числам

You can do this:

var isOnlyNumbers = str.All(s =>
    {
        double i;
        return double.TryParse(s, out i);
    });

How about using regular expressions?

 using System.Text.RegularExpressions;
 ...
 bool isNum= Regex.IsMatch(strToMatch,"^\\d+(\\.\\d+)?$");

TryParse

Try this:

string[] str = new string[] { "23", "25", "Ho" };
double trouble;
if (str.All(number => Double.TryParse(number, out trouble)))
{
    // do stuff
}

Using the fact that a string is also an array of chars, you could do something like this:

str.All(s => s.All(c => Char.IsDigit(c)));

Or without linq...

bool allNumbers = true;
foreach(string str in myArray)
{
   int nr;
   if(!Int32.TryParse(str, out nr))
   {
      allNumbers = false;
      break;
   }
}