Best practices: C# Extension methods namespace and promoting extension methods

We put them all in their own namespace Company.Common.Extensions. That way, if you have any of our extension methods, you have them all. Plus, at least at my shop, we don't have to worry about our developers not knowing about extension methods. I have the opposite worry, extension method overload! :)

The problem here is not the naming of the namespace, it's the lack of documentation and education of your developers.

Put them in whatever namespace makes sense, write a wiki article documenting all your extension methods, then send an email to your developers with a link to the wiki article.

This is not a namespace problem it is a communication problem.

If these methods are useful you need to communicate this to the developers and, conversely, act on the feedback from them (with appropriate levels of judgement).

Placing anything into the System namespace is a recipe for disaster and confusion later. The only times you ever want to do this is to 'back port' functionality into older frameworks and then you probably shouldn't do it yourself but should use something like LinqBridge to do it.

Be wary of the desire to throw all extensions into one namespace unless they really are widely useful together. Some developers may find the wood lost for the trees if they are bombarded with everything and the kitchen sink via intellisense.

Keeping the namespace the company name is sensible in general to avoid confusion.