Avoid boolean parameters in method parameters

*Updated* 12/07/2010: Response from Uncle Bob added at the end

Uncle Bob has a coding rule: Avoid boolean parameters in method parameters

He preaches that up and down the street, and I couldn't agree more with that!

The reason:

  • We all know that 1 method should do only 1 thing (Single Responsibility Principle  for methods and classes)
  • If your method has a boolean parameter it is highly possible that your method is doing 2 things


See the following code examples

        private string GetAuctionStatus(long auctionID, bool includeClientDetails)

if (includeClientDetails)

-- snip snip snip -

Bad Figure: A method with a boolean parameter --> Create 2 methods!


So that should look like

        private string GetAuctionStatus(long auctionID)

-- snip snip snip -

        private string GetAuctionStatusWithDetails(long auctionID)

-- snip snip snip -

Good Figure: 2 methods that do different things

This is how Uncle Bob would like to see it


What about the below Uncle Bob?


With C# Named Parameters we can call the method like this

   logHelper.Debug(GetAuctionStatus(auction.Key, includeClientDetails:true));


@UncleBob: Are you happy with that?


I am quite happy.
I still suggest to create 2 different methods, when the 1 method does 2 different things.
If its just passing parameters to lower methods, lets use the named parameters

Only 1 problem: Others on the team can still call the method like this

    logHelper.Debug(GetAuctionStatus(auction.Key, true));


Response from Uncle Bob on Twitter:

. @peitor Named parameters good. Boolean arguments to functions bad. Boolean arguments to constructors mostly bad.

  • Named parameters good
  • Boolean arguments to functions bad
  • Boolean arguments to constructors mostly bad


gogox@yahoo.com said...

Uncle Bob - What if you write WCF - do you want to have thousands of methods accompanied with data/message contracts or you want to have single method with "rich" filter ?

Imagine "GetMessage" service operation on Exchange server - imagine that it has 30 filter criterias to include or not include certain message "properties" in response like "sender email", "date created", "date sent", "date received" and so on - should Uncle Bob create numerous methods like "GetMessageWithSenderEmail", "GetMessageWithSenderEmailAndDateCreated", "GetMessageWithDateReceived" etc... ?

The answer is simply "No". So the real decision must be taken in accordance of current context.
So if we talk about service operations, the best approach to me is methods with one parameter - something like this:

GetUsersResponse GetUsers(GetUsersRequest request)

Later, if you want to add another filter criteria for GetUsers (say UserRegionID), just add public int UserRegionID { get; set; }. But not another method "GetUsersByRegionID" with accompanying GetUsersByRegionRequest message contract...

Best Regards.

Peter Gfader said...

Hi Gogox

Good points.

The discussion raised more from: Is it good/bad to use boolean parameters in method parameters.

Since a XXXRequest object is not a bool it is fine...
But I am curious about the code that handles all the different possibilities of the filter parameters.
Maybe you can post that somewhere?

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Ryutaku Zaki said...

Thank you so much! I was using boolean parameters a lot. But I always felt weird doing that. It is really true: it affects the single responsability principle.

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