bmforce

List<string> list = new List<string> { "sdfsd", "dfgdf", "ssss" };
bool result;
result = list.TrueForAll(s => s.All(c => Char.IsLetter(c)));
Console.WriteLine(result); //true

list = new List<string> { "sdf5", "dfgdf", "ssss" };
result = list.TrueForAll(s => s.All(c => Char.IsLetter(c))); 
Console.WriteLine(result); //false