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Best Practice: Declare Variables as Constants

In the bad old days, I only used constants as a way to assign names to values. Things like this, for example:

Private Const DaysInWeek As Integer = 7

However, these days most of my variables are holding objects; Some of my other variables are holding things like collections or delegates. That changes what we mean by "constant."

When a variable is declared as a constant, it doesn't mean that the object it's holding is immutable. You can, for example, still change values in the properties of an object held in a constant or add items to a collection held in a constant. It just means that you can't replace the object (or collection) that was assigned when the variable was initialized.

Especially in TypeScript (where many of my variables are holding functions), I'm switching to declaring my variables as constants. A subset of these variables are fields (class-level variables) that are set precisely once, in an object's constructor, and I'm taking the time to mark as those read only also.

If I want an actual immutable object I'll declare the object's properties as read only or use one of .NET's read only collections.

In part, I'm doing this because I'm thinking more in terms of value objects (an important concept in Domain Driven Design). However, there are other benefits. I'm told, for example, it makes life easier for the compiler. But it's also because this practice sends a clear message to the next developer (either using my code or modifying it) about how I think that the object should be used.

Posted by Peter Vogel on 02/12/2018


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