I’ve run into a few string/constants myths repeatedly and I thought I’d try to disprove them. Myth 1: constants use less memory. Myth 2: inline strings are always bad. Myth 3: String.Empty doesn’t create an object while “” does.
I’m just running a bunch of different pieces of code and dumping memory statistics before and after to measure. The trimmed output of sos’s !dumpheap -stat command are included after each line.
Thanks to Pale Musings for the sos memory leaks article.
Contents
Steps to reproduce:
- Create console app.
- Go to project properties, enable unmanaged debugging.
- In main add (changing the first line each time)
string value = "span";
Console.WriteLine(value);
Console.ReadKey(true);
- Put breakpoints on lines 1 and 2.
- Run app with debugger (F5).
- In the immediate window type
.load sos
- In the immediate window type
!dumpheap -stat
- Run to next breakpoint (F5).
- In the immediate window type
!dumpheap -stat
- Compare output from dumpheap.
- Change line 1 and repeat.
Simple inline string
code: string value = "span";
before:
MT Count TotalSize Class Name
...
790f9244 2038 129792 System.String
Total 2085 objects
after:
MT Count TotalSize Class Name
...
790f9244 2038 129792 System.String
Total 2085 objects
Enum to lower case string
code: string value = HtmlTextWriterTag.Span.ToString().ToLower();
before:
MT Count TotalSize Class Name
...
790f9244 4825 264628 System.String
Total 4875 objects
after:
Count TotalSize Class Name
...
790f9244 4932 267760 System.String
Total 5177 objects
Second enum to string
I thought the above might be unfair. Maybe I was counting memory usage from loading the System.Web assembly or the HtmlTextWriterTag for the first time.
code: span.ToString().ToLower();
before:
MT Count TotalSize Class Name
...
790f9244 4825 264628 System.String
Total 4875 objects
after:
MT Count TotalSize Class Name
790f9244 4932 267760 System.String
Total 5177 objects
Constant defined in the same class
code: string value = span; (span is a constant of the class)
before:
MT Count TotalSize Class Name
...
790f9244 2038 129792 System.String
Total 2085 objects
after:
MT Count TotalSize Class Name
...
790f9244 2038 129792 System.String
Total 2085 objects
Using the same string repeatedly inline
code: string value = "span";
string value2 = "span";
string value3 = "span";
string value4 = "span";
string value5 = "span";
before:
MT Count TotalSize Class Name
...
790f9244 2038 129792 System.String
Total 2085 objects
after:
MT Count TotalSize Class Name
...
790f9244 2038 129792 System.String
Total 2085 objects
Two different inline strings
code: string value = "span a";
string value2 = "span b";
before:
MT Count TotalSize Class Name
...
790f9244 2039 129828 System.String
Total 2086 objects
after:
MT Count TotalSize Class Name
...
790f9244 2039 129828 System.String
Total 2086 objects
Appending constants with +
code: string value = span + a;
string value2 = span + b; (span, a, b are constants)
before:
MT Count TotalSize Class Name
...
790f9244 2039 129828 System.String
Total 2086 objects
after:
MT Count TotalSize Class Name
...
790f9244 2039 129828 System.String
Total 2086 objects
Quote quote (“”)
code: string value = "";
before:
MT Count TotalSize Class Name
790f9244 2038 129784 System.String
Total 2085 objects
after:
MT Count TotalSize Class Name
...
790f9244 2038 129784 System.String
Total 2085 objects
String.Empty
code: string value = String.Empty
before:
MT Count TotalSize Class Name
...
790f9244 2037 129764 System.String
Total 2082 objects
after:
MT Count TotalSize Class Name
...
790f9244 2037 129764 System.String
Total 2082 objects
Conclusion
So: using inline strings is no better than constants as far as memory or performance are concerned. Enums are worse than constants or inline. String.Empty did save me one object instance and a whole 20 bytes.
For readability and maintainability, constants can help. Personally I find code where everything is a constant even if it is only used once annoying, but that’s just me :)
There’s probably other pages on the net that go into exactly why this all is. At the moment I’m too lazy to go searching for them. Go ahead and open the code in reflector to see what it’s getting compiled to if you want.
The actual difference in any of these is probably so small you’ll never notice it, so go with whatever is easiest and most readable for you.
