Unit testing custom StyleCop rules using Typemock Isolator

Saturday, July 03, 2010

I’ve never was a big fan of “coding standard” – Although I always thought that the same style should be kept throughout a project or even the entire company’s code base - the idea of forcing developers to write the same code based on a document nobody ever read seemed just wrong.

Fast forward a few years and suddenly I’m responsible that my team’s code will be written according to the coding standard of the company.

At first I thought it shouldn’t be too hard – everybody knows the coding standard – and boy was I wrong. The coding standard document was copied from a previous document and even the developers that did read it couldn’t remember all of it’s 20+ pages of rules and ideas.

It was clear that I needed help – preferably in a form of some tool that would process my team’s code. After looking a bit I’ve found that tool – named StyleCop.

What is StyleCop

StyleCop is a free static code analysis tool from Microsoft that checks C# code for conformance to StyleCop's recommended coding styles and a subset of Microsoft's .NET Framework Design Guidelines. StyleCop analyzes the source code, allowing it to enforce a different set of rules from FxCop. The rules are classified into the following categories:

  • Documentation
  • Layout
  • Maintainability
  • Naming
  • Ordering
  • Readability
  • Spacing

StyleCop includes both GUI and command line versions of the tool. It is possible to create new rules to be used.

StyleCop was re-released as an open source project in April of 2010, at http://stylecop.codeplex.com.

[From Wikipedia]

On the process of implementing custom StyleCop rules

Back to the problem at hand – although my company has a coding standard it’s not exactly similar to Microsoft’s so I needed to develop some custom rules luckily this topic was already covered by my fellow bloggers:

A custom Rule would look something like:

public class NamingRules : SourceAnalyzer
public override void AnalyzeDocument(CodeDocument document)
var csdocument = (CsDocument)document;

if (csdocument.RootElement != null && !csdocument.RootElement.Generated)
csdocument.WalkDocument(VisitElement, null, null);

private bool VisitElement(CsElement element, CsElement parentElement, object context)
if (element.Generated)
return true;

if(element.ElementType == ElementType.Class && !(element.Parent is Class))
var csClass = (Class)element;

var fileName = csClass.Document.SourceCode.Name;
var fileNameWithoutExtension = string.Format("class {0}", Path.GetFileNameWithoutExtension(fileName));
var className = csClass.GetShortName();

if(fileNameWithoutExtension.Equals(className) == false)
AddViolation(element, element.LineNumber, "FileNameMustMatchClassName");

return true;

In case you were wondering the code above checks that a class resides in a file with the same name.


But there is a problem with writing style rules – they look trivial at first but tend to accumulate corer cases as you progress. My solution was to find a way to test the custom rules I’ve written so that I won’t accidently break during my work.

The added value of using unit tests is that I didn’t need to manually test my new rules – a process that consists from the following steps:

  1. Implement a new style rule

  2. Compile the custom rule assembly

  3. Copy the assembly to StyleCop’s folder

  4. Open a new instance of Visual Studio

  5. Write code to test the new rule

  6. Run the new rule

  7. More often than not – find a bug. close visual studio and go to step #1

Instead I got the following:

  1. Write failing test

  2. Run test

  3. Implement a new style rule

  4. Run the test again

  5. If test still fail go to step #1

Better, Simpler, Faster

Writing unit tests for my custom rules

I’ve wanted to be able to parse an actual file and analyze it using StyleCop and my new rules – using some Reflector magic I was able to discover how StyleCop worked and I was able to write the following “helper” method:

   1:  public static CodeDocument ParseDocument(string codeFileName, string projectFileName) 

   2:  {

   3:     var parser = new CsParser();

   4:     var configuration = new Configuration(null);

   5:     var project = new CodeProject(projectFileName.GetHashCode(), projectFileName, configuration);

   6:     var sourceCode = new CodeFile(codeFileName, project, parser);


   8:     CodeDocument document = new DummyCodeDocument(sourceCode);


  10:     parser.ParseFile(sourceCode, 0, ref document);


  12:     return document;

  13:  }

The method receives a file name and a project name and creates StyleCop’s representation of that file.


  • Lines 3–6: Creation of StyleCop’s types I needed to parse the code file.

  • Line 8: Due to some design fluke I needed a CodeDocument to pass to the parser. Unfortunately CodeDocument is an abstract class so I just inherited it in a dummy class I’ve created. No need to implement anything because this instance will be replaced after the new command.

  • Line 10: parse the source file – and that’s it.

Armed with a method that enable me to parse code files I was now able to test my new rule – almost, I’ve needed to fake a call to AddViolation and verify it got called and for that I’ve used Typemock Isolator:

   1:  [TestMethod]

   2:  [DeploymentItem(@"..\..\..\.StyleCop.Rules.Tests.TestClasses\FileNameMustMatchClassNameRule.cs")]

   3:  public void AnalyzeDocument_FileNameDoesNotMustMatchClassNameRule_AddViolationCalledWithCorrectRule() 

   4:  {

   5:     const string projectFileName = @"StyleCop.Rules.Tests.TestClasses.csproj";

   6:     const string codeFileName = @"FileNameMustMatchClassNameRule.cs";


   8:     var document = TestHelpers.ParseDocument(codeFileName, projectFileName);


  10:     var namingRules = new NamingRules();


  12:     Isolate.WhenCalled(() => namingRules.AddViolation(null, 0, string.Empty)).IgnoreCall();


  14:     namingRules.AnalyzeDocument(document);


  16:     Isolate.Verify.WasCalledWithArguments(() => namingRules.AddViolation(null, 0, string.Empty))

  17:        .Matching(objects => {

  18:           var ruleName = objects[2].ToString();

  19:           return ruleName.Equals("FileNameMustMatchClassNameRule");

  20:        });

  21:  }


  • Line 2: I’ve created a new project that contains my test classes. I’ve used MSTest Deploy to make sure the file I want will be copies to the place the tests are run.

  • Lines 5–6: I don’t really need to explain that one – right?

  • Line 8: invoking the helper method

  • Line 10: Create a new instance of the class that holds the custom rules

  • Line 12: Using Isolator to make sure that AddViolation does not get called. In order to call it I would have needed to initialize a lot more of StyleCop’s environment and instead I’ve used this simple line

  • Line 14: Call my method under test

  • Lines 16-19: Using Isolator to test  that AddViolation was called and that the correct string was passes.


That’s enough code for now. Using this method I was able to write tests to all of the custom rules I’ve implemented.

As for my opinion about the need for an official coding style - it changed, after fixing a few (thousands) lines – the code actually look better and more importantly it’s more readable.


  1. Can you post the code for DummyCodeDocument? I followed this and was unable to get my code to run correctly.

  2. Can you post the code for DummyCodeDocument

  3. It's been a while and I couldn't find it, the company where I wrote this code went bankrupt.
    However the DummyCodeDocument is just an empty class implementing CodeDocument:
    "CodeDocument is an abstract class so I just inherited it in a dummy class I’ve created. No need to implement anything because this instance will be replaced after the new command."

  4. what namespace you used to access "Isolate.WhenCalled" I am getting error "The name 'Isolate' does not exist in the current context" in function "AnalyzeDocument_FileNameDoesNotMustMatchClassNameRule_AddViolationCalledWithCorrectRule"

  5. That's Typemock Isolator - a .NET mocking framework I've been using


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