Advantages of using const instead of variables ins

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 = BindingFlags.Public | BindingFlags.Instance;

// ReSharper suggest to use this:
const string s = "some string";
const BindingFlags flags = BindingFlags.Public | BindingFlags.Instance;

Given that these are really constant values (and not variables) I understand that ReSharper suggest to change them to const.

But apart from that, is there any other advantage when using const (e.g. better performance) which justifies using const BindingFlags instead of the handy and readable var keyword?

BTW: I just found a similar question here: Resharper always suggesting me to make const string instead of string, but I think it is more about fields of a class where my question is about local variable/consts.

Answer 1

A const value is also 'shared' between all instances of an object. It could result in lower memory usage as well.

As an example:

public class NonStatic
{
    int one = 1;
    int two = 2;
    int three = 3;
    int four = 4;
    int five = 5;
    int six = 6;
    int seven = 7;
    int eight = 8;
    int nine = 9;
    int ten = 10;        
}

public class Static
{
    static int one = 1;
    static int two = 2;
    static int three = 3;
    static int four = 4;
    static int five = 5;
    static int six = 6;
    static int seven = 7;
    static int eight = 8;
    static int nine = 9;
    static int ten = 10;
}

Memory consumption is tricky in .Net and I won't pretend to understand the finer details of it, but if you instantiate a list with a million 'Static' it is likely to use considerably less memory than if you do not.

    static void Main(string[] args)
    {
        var maxSize = 1000000;
        var items = new List<NonStatic>();
        //var items = new List<Static>();

        for (var i=0;i<maxSize;i++)
        {
            items.Add(new NonStatic());
            //items.Add(new Static());
        }

        Console.WriteLine(System.Diagnostics.Process.GetCurrentProcess().WorkingSet64);
        Console.Read();
    }

When using 'NonStatic' the working set is 69,398,528 compared to only 32,423,936 when using static.

Answer 2

The const keyword tells the compiler that it can be fully evaluated at compile time. There is a performance & memory advantage to this, but it is small.

Answer 3

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.

Answer 4

As per my understanding Const values do not exist at run time - i.e. in form of a variable stored in some memory location - they are embeded in MSIL code at compile time . And hence would have an impact on performance. More over run-time would not be required to perform any house keeping (conversion checks / garbage collection etc) on them as well, where as variables require these checks.

Answer 5

Besides the small performance improvement, when you declare a constant you are explicitly enforcing two rules on yourself and other developers who will use your code

1. I have to initialize it with a value right now i can't to do it any place else.
2. I cannot change its value anywhere.

In code its all about readability and communication.

Answer 6

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.