No need to create a new string:
bool IsAllUpper(string input)
{
for (int i = 0; i < input.Length; i++)
{
if (!Char.IsUpper(input[i]))
return false;
}
return true;
}
Edit: If you want to skip non-alphabetic characters (The OP’s original implementation does not, but his/her comments indicate that they might want to) :
bool IsAllUpper(string input)
{
for (int i = 0; i < input.Length; i++)
{
if (Char.IsLetter(input[i]) && !Char.IsUpper(input[i]))
return false;
}
return true;
}