c# only letters

bool result = input.All(Char.IsLetter);

bool result = input.All(Char.IsLetterOrDigit);

bool result = input.All(c=>Char.IsLetterOrDigit(c) || c=='_');

// Instantiate random number generator.  
private readonly Random _random = new Random();  
  
// Generates a random number within a range.      
public int RandomNumber(int min, int max)  
{  
  return _random.Next(min, max);  
}


if (str.All(char.IsDigit)) {
  // String only contains numbers
}