Implicit vs Explicit - Modern C# Standards and Conventions

This article is part of a series describing Modern C# Standards and Conventions. A list of the other articles in this series can be found below.

Contents

If you feel there is something else that should be included here, please drop a comment below, or better yet send me a PR.

What does it mean?

In short, an implicit declaration is one in which the type is not defined, instead it is implied (or inferred) by the declaration. For example:

var data = GetResults();

In contrast, an explicit declaration defines the type as part of the declaration. For example:

List<string> data = GetResults();

I often come across people that believe the implicit declaration to be in some way related to dynamics. This is completely wrong and should be beaten out of said people. If you do not know what dynamics are, don’t worry - they do not relate to this article.

Pros and Cons

To be able to compare the two, we should take a look at the pros and cons of each approach, and hopefully in doing so allow you to conclude that implicit declarations are better.

Readability

I believe this to be the key proponent of using explicit declarations. That is, as per the example above, it is more obvious what the type of data is. The only real counter to this is that IDE’s nowadays are superb at providing and displaying this information. However, let’s leave this as a pro for explicit whilst we examine the benefits of implicit declarations.

Refactoring

There are three key points when it comes to refactoring. Of course, we all follow the Open/Close Principle, but every now and then the real world slaps us in the face and forces us to change the signature of existing code. Let’s start by expanding upon the example above:

public void Implicit()
{
  var data = GetData();
  foreach (var item in data)
    Console.WriteLine(item);
}

public void Explicit()
{
  List<string> data = GetData();
  foreach (string item in data)
    Console.WriteLine(item);
}

Here we see that both methods fetch some data from some unknown GetData method, and then output the contents to the Console. As per the readability section above, it is clear in the Explicit code that data is of type List<string>. However, this leads to the assumption that GetData returns List<string> which may not be the case, leading us to our first point.

Casting

In the example above, if I change the return type of GetData to something that inherits from List<string>, the code will continue to work in both the implicit and explicit cases. But, in the explicit case I have changed the behaviour, because it is casting the return type.

Cascading

What if the GetData method were changed to return an IEnumerable<int>? The person making this change may be forced to then cascade this change to the Explicit method too. By using var in the implicit method, I maximise compatibility with unimportant changes in lower level components.

Consider the case in which you are not the author of the GetData method, instead it is brought in by NuGet or reference to a third party assembly. Updating versions in the Explicit case could appear to be a nightmare with potentially thousands of annoying cast exceptions, in turn leading to skipping the update.. leading to future updates being skipped? What if that skipped update would have patched a critical security issue? Ok, too many whatifs and assumptions…

Manual Verification

Some might argue that being forced to make changes in the higher level components is a good thing because it forces the developer to manually check the code. But in reality, developers will just change the explicit type by hand (or worse, explicitly cast the return type), without consideration to the code utilising those variables.

The only time they’ll look at the consumer code, is when the compiler forces them to through compilation errors. Personally, I follow TDD and have trust that my tests will notice breaking changes; manually re-typing or casting my variables is just a waste of time.

Next

The next article in this series is Comments.

Written on August 4, 2015