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This article was written by José M. Aguilar in Spanish on his excellent blog Variable Not Found, and was translated, edited and republished here by Timm Martin (and Google Translator) with permission from Mr. Aguilar.

Following are 13 tips on how to comment your source code so that it is easier to understand and maintain over time.

1. Comment each level

Comment each code block, using a uniform approach for each level.  For example:

• For each class, include a brief description, author and date of last modification
• For each method, include a description of its purpose, functions, parameters and results

Adopting comment standards is important when working with a team.  Of course, it is acceptable and even advisable to use comment conventions and tools (such as XML in C# or Javadoc for Java) to facilitate this task.

2. Use paragraph comments

Break code blocks into multiple "paragraphs" that each perform a single task, then add a comment at the beginning of each block to instruct the reader on what is about to happen.

// Check that all data records
// are correct
foreach (Record record in records)
{
    if (rec.checkStatus()==Status.OK)
    {
        . . .
    }
}
// Now we begin to perform
// transactions
Context ctx = new ApplicationContext();
ctx.BeginTransaction();
. . .

3. Align comments in consecutive lines

For multiple lines of code with trailing comments, align the comments so they will be easy to read.

const MAX_ITEMS = 10; // maximum number of packets
const MASK = 0x1F;    // mask bit TCP

Some developers use tabs to align comments, while others use spaces.  Because tab stops can vary among editors and IDEs, the best approach is to use spaces.

4. Don't insult the reader's intelligence

Avoid obvious comments such as:

if (a == 5)      // if a equals 5
    counter = 0; // set the counter to zero

This wastes your time writing needless comments and distracts the reader with details that can be easily deduced from the code.

5. Be polite

Avoid rude comments like, "Notice the stupid user has entered a negative number," or "This fixes the side effect produced by the pathetically inept implementation of the initial developer."  Such comments do not reflect well upon their author, and you never know who may read these comments in the future: your boss, a customer, or the pathetically inept developer you just insulted.

6. Get to the point

Don't write more in comments than is needed to convey the idea.  Avoid ASCII art, jokes, poetry and hyperverbosity.  In short, keep the comments simple and direct.

7. Use a consistent style

Some people believe that comments should be written so that non-programmers can understand them.  Others believe that comments should be directed at developers only.  In any event, as stated in Successful Strategies for Commenting Code, what matters is that comments are consistent and always targeted to the same audience.  Personally, I doubt many non-developers will be reading code, so comments should target other developers.

8. Use special tags for internal use

When working on code as a team, adopt a consistent set of tags to communicate among programmers.  For example, many teams use a "TODO:" tag to indicate a section of code that requires additional work:

int Estimate(int x, int y)
{
    // TODO: implement the calculations
    return 0;
}

Tag comments don't explain code; rather they seek attention or deliver a message.  But if you use this technique, remember to follow up and actually do what the message is asking.

9. Comment code while writing it

Add comments while you write code and it's fresh in your memory.  If you leave comments until the end, it will take you twice as long, if you do it at all.  "I have no time to comment," "I'm in a hurry," and "The project is delayed" are all simply excuses to avoid documenting your code.  Some developers believe you should write comments before code as a way to plan out your ultimate solution.  For example:

public void ProcessOrder() 
{
    // Make sure the products are available
    // Check that the customer is valid
    // Send the order to the store
    // Generate bill
}

10. Write comments as if they were for you (in fact, they are)

When it comes to commenting code, think not only about the developers who will maintain your code in the future, but also think about yourself.  In the words of the great Phil Haack:

"As soon as a line of code is laid on the screen, you’re in maintenance mode on that piece of code."

As a result, we ourselves will be the first beneficiaries (or victims) of our good (or bad) comments.

11. Update comments when you update the code

There is no point in commenting correctly on code if the comments are not changed with the code.  Both code and comments must move in parallel, otherwise the comments will actually make life more difficult for developers who maintain your code.  Pay special attention to refactoring tools that automatically update code but leave comments unchanged and hence obsolete in the same instant.

12. The golden rule of comments: readable code

One of the basic principles for many developers: Let your code speak for itself.  Although one suspects this movement is led by programmers who do not like to write comments, it is true that self-explanatory code can go a long way toward making code that's easier to understand and can even render comments unnecessary.  For example, the code in my Fluid Interfaces article shows how clear self-explanatory code can be:

Calculator calc = new Calculator();
calc.Set(0);
calc.Add(10);
calc.Multiply(2);
calc.Subtract(4);
Console.WriteLine( "Result: {0}", calc.Get() );

In this example, comments are not needed and would likely violate tip #4.  To facilitate readable code, you might consider using proper names (as described in the classic Ottinger's Rules), ensure correct indentation, and adopt coding style guides.  Failure to comply with this tip may result in comments that seem to apologize for bad code.

13. Share these tips with your colleagues

Although tip #10 shows how we ourselves benefit immediately from good comments, these tips will benefit all developers, especially in the context of team working together.  Therefore, feel free to share these commenting tips with your colleagues to create code that is easier to understand and maintain.

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