Advantages of using const instead of variables inside methods

Question

Whenever I have local variables in a method, ReSharper suggests to convert them to constants:

// instead of this:
var s = \"some string\";
var flags = Bindin         


        

Answer 1

The compiler will throw an error if you try to assign a value to a constant, thus possibly preventing you from accidentally changing it.

Also, usually there is a small performance benefit to using constants vs. variables. This has to do with the way they are compiled to the MSIL, per this MSDN magazine Q&A:

Now, wherever myInt is referenced in the code, instead of having to do a "ldloc.0" to get the value from the variable, the MSIL just loads the constant value which is hardcoded into the MSIL. As such, there's usually a small performance and memory advantage to using constants. However, in order to use them you must have the value of the variable at compile time, and any references to this constant at compile time, even if they're in a different assembly, will have this substitution made.

Constants are certainly a useful tool if you know the value at compile time. If you don't, but want to ensure that your variable is set only once, you can use the readonly keyword in C# (which maps to initonly in MSIL) to indicate that the value of the variable can only be set in the constructor; after that, it's an error to change it. This is often used when a field helps to determine the identity of a class, and is often set equal to a constructor parameter.

Answer 2

Constants in C# provide a named location in memory to store a data value. It means that the value of the variable will be known in compile time and will be stored in a single place.

When you declare it, it is kind of 'hardcoded' in the Microsoft Intermediate Language (MSIL).

Although a little, it can improve the performance of your code. If I'm declaring a variable, and I can make it a const, I always do it. Not only because it can improve performance, but also because that's the idea of constants. Otherwise, why do they exist?

Reflector can be really useful in situations like this one. Try declaring a variable and then make it a constant, and see what code is generated in IL. Then all you need to do is see the difference in the instructions, and see what those instructions mean.