Parameter naming: filename or fileName?

'filename' assumes that this word describes a singular object like 'cow' or 'chair'
'fileName' assumes that this is a complex object, that there is an object called file and that this object describes the name of that file.

Two philosophical approaches, take your pick.

It is acceptable English to write "filename" or "file name". When you translate that into coding, capitalizing the "n" or not capitalizing the "n" can go either way (assuming camelCase or PascalCase).

By the way, you did make a grammatical error in the question--ironically, in the very sentence in which you were expressing your hope that there were no grammatical errors. You said, "I just hope there's no grammar errors in this post!" But "errors" is plural, therefore the "is" of "there's" represents a subject-verb disagreement.

* I just hope there are no grammatical errors in this post!

As far as I am concerned,

thisIsMuchMoreReadable than readingthis.

Lower camel case is recommended for fields and parameters.

Example 1:

fileName // for fields, parameters, etc.
FileName // for properties, class names, etc.

Generally, fileName is used and NOT filename; you can verify that by reading source code of open source stuff created by Microsoft, such as Enterprise Library.

Reasons:

1. The main point behind this is that names are more readable in this case.
2. Also this approach adds consistency when several parameters (fields, variables..) are used in the same method (class..) and the with same prefix "file", as demonstrated below:
3. ...there are a few other reasons, but they are more subjective.

Example 2:

fileName, fileSize... // instead of filename AND filesize

See also:

• Naming Conventions at Wikipedia
• General Naming Conventions at MSDN

For a full set of naming convention rules, I recommend checking this book:

• Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries
  (2nd Edition) by Krzysztof, published on Nov, 2008
  (personally we don't use 100% recomendations from this book, but in overall there are pretty good guidelines)

And also check some stuff at IDesign.net