Inside .Net Constants

By definition, constants are fixed numbers or values which represent the universal facts. When comes to programming language, it represents the same analogy- symbol with “Never-Changing Value”. In .Net Framework, constant(const) represent a symbol which contain a fixed/never changing value. Defining a constant in C# – 

           public const string Bullet = “\u2022”; 

We can use any primitive types supported in .Net Framework to define constant.   When we define a constant, the symbol is stored in the module’s metadata in compile time and compiler gets its value and copies it to the emitted IL code generated. In short, const value is directly embedded inside the code.           

Some facts about .Net const:

1.       Value of the const is decided in compile time. 
2.       We can only define const for primitive type.
3.       const are saved in the module’s metadata which then extracted and emitted in the IL code generated during compile time.
4.       Since there is no memory address is allocated for the constant – we can’t pass a const by reference. Therefore, it does not have good cross-module versioning usability. I will come back to this point.
5.       const symbol’s value never changes. That’s why, const symbols are associated with the .Net Type rather than instances.

Design consideration while using const: 

const should only be used when it is known that it’s value will never change, as I mentioned in the definition– universal fact. J For example – I defined here a const that represented the Unicode of the Character Bullet which kind of an Universal fact and will never change. Now, if we define const symbol that changes [I think which needs a code reviewJ], the problem that we might face is Cross-Module versioning. Let’s check out the following code which resides in a .dll –   public class ConnectionStrings
public const string UserDbConnectionString = “…”;

And we are using
ConnectionStrings class by adding reference of the .dll in our application. As we already know, the const symbol’s value  just emitted in the IL code and UserDbConnectionString const actually don’t need any Memory allocation, which makes the .dll file not being loaded at runtime. The value of UserDbConnectionString is already copied from the module’s metadata to our application’s generated IL. We can simply delete the .dll file and it should work accordingly.

There is no problem in so far. However, due to some reason, the value of the UserDbConnectionString get changed [ May be due to the deployment issue.]. Now we build the dll. How come the change is not reflected in our application’s code ? J Dude, we need to re-build our application as well. L 

To state the whole Versioning things in short – we need to keep into consideration while we define a constant symbol in our module whether the symbol will be referenced across the module. If the const symbol does not need any cross-module reference – then we should go for const otherwise, we need to change the implementation, For the above mentioned example I would change the const to readonly –   public class ConnectionStrings
      public readonly string UserDbConnectionString = “…”;

Lunch breakJ. I will try to post on readonly later on.


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