Introduction

Whether a parameter should be named filename or fileName is not really a question about English grammar. It is a question about consistency with the naming convention of the language, framework, and codebase you are working in. The better name is usually the one that matches the surrounding style rather than the one that looks best in isolation.

Match the Language Convention First

Different language ecosystems expect different identifier styles. In many camelCase languages, fileName is the natural form for a parameter.

public void openFile(String fileName) {
    System.out.println(fileName);
}

In languages that prefer snake_case, the analogous choice would be file_name, not fileName.

def open_file(file_name: str) -> None:
    print(file_name)

So the first rule is simple: do what the language and local style guide already expect.

filename Is Not Automatically Wrong

Some teams and APIs prefer filename as a single lowercase word, especially when it reads naturally and the codebase already uses compound words that way.

void open_file(const char *filename) {
    printf("%s\n", filename);
}

This can be perfectly readable, especially in older C-style codebases where shorter lowercase parameter names are common.

The problem is not that filename is inherently bad. The problem is mixing it unpredictably with fileName, file_name, and other variants in the same project.

Prioritize Consistency Over Micro-Preference

If a codebase already uses customerId, userName, and filePath, then fileName is the more consistent choice.

If the codebase uses filename, filepath, and username, then introducing fileName just because it seems more modern creates noise rather than clarity.

Naming consistency matters because readers build expectations from repetition. A codebase that follows one pattern is easier to scan than one where every developer chooses compound-word style ad hoc.

Consider API Surface and Readability Together

Parameter names also appear in documentation, stack traces, and code completion. That means readability should still matter.

For example, this is more readable than a vague abbreviation:

public void SaveDocument(string fileName, string destinationFolder) {
    Console.WriteLine(fileName);
}

Even if the style choice is subtle, the bigger win is choosing descriptive words over short ambiguous ones.

The real question is often not filename versus fileName, but whether the name communicates the right concept at all. If the parameter is actually a full path, path or filePath may be better than either option.

Follow Project and Tooling Rules

If the team uses analyzers, linters, or stylecop-like tooling, follow that rule even if you would personally choose a different spelling. Shared convention is more valuable than individual preference in collaborative code.

That is especially true for public APIs, where inconsistent naming becomes part of the long-term maintenance burden.

Common Pitfalls

The biggest mistake is treating this as a universal naming truth instead of a style-context decision. Another is focusing on this micro-choice while ignoring the larger problem of unclear or inconsistent naming across the rest of the codebase.

Developers also often forget that compound names should reflect what the parameter really is. If the value is a path, URL, or key, filename versus fileName is already the wrong debate.

Finally, do not rename parameters across a mature codebase casually unless the change improves consistency globally. One isolated rename can make the overall style less consistent, not more.

Summary

• The better choice is the one that matches the language and project naming convention.
• 'fileName fits camelCase codebases; filename can still be fine in codebases that already use that style.'
• Consistency is more valuable than personal preference on one parameter.
• Prefer names that describe the real meaning of the value, not just its spelling shape.
• If the team has a style guide or linter rule, follow it.