When string.ToLower() is Evil

Did you know how evil string.ToLower() can sometimes be?

Let me explain...

Very often I see code similar to this:

void DoBadAction (string val)
{
if (val.ToLower() == "someValue")
{ //do something
}
}

The code above can lead up to 4 times in performance loss when doing string comparison operations.

Best method to do such kind of case insensitive comparison is using string.Equals(...) method.

void DoGoodAction(string val)
{
if (val.Equals("someValue", StringComparison.OrdinalIgnoreCase))
{ //do something
}
}

Why is it so? The reason lies in the string type peculiarity - it is immutable.

Since it is an immutable - string.ToLower() will always return new string instance. Thus generating extra instance of string on every ToLower() call.

Detailed information about string.Equals with StringComparison enumeration can be found here.
Other performance related tips and tricks can be found here.