In the article Introduction to Unit Testing a recipe was identified that could test any class in a system except for the edge. The recipe depends on the test subject knowing of other types through abstractions. The abstraction allows for indirection to be used to insert a test double. This methodology of testing is known as interaction based testing.

Interaction Based Recipe:

  1. Does the test subject interact through abstractions? If not perform safe refactorings till compliant.
  2. Write doubles for each abstract type the test subject interacts with. Ignore if using nMock or the like.
  3. Create Test Method.
    1. Create any parameters need for creation of the test subject.
    2. Create instance of test subject.
    3. Create all doubles the test subject method will interact with that were not passed in the creation of the test subject.
    4. Set return values for expected interactions.
    5. If interactions occur through the test subjects public properties and or fields populate with the correct doubles.
    6. Call the test subject method passing doubles and or other values.
    7. Assert on expected interactions with doubles and or test subject public properties and fields.

The edge of a system has external dependencies and most types exposed in third party libraries are not abstract. This prevents test double substitution. The Warehouse class in the previous article was dependent on a database. This tight coupling to a database means that we can not isolate Warehouse. Remember that a unit test exercises just one class. This article will explore the use of the Adaptor Pattern and code generation to shrink the edge of your system and expand the creamy center. The creamy center being the sweet spot where types are easily isolated for unit testing.

Figure 1

Test Subject

To keep the examples simple and easy for you to get working on your machine the file system will be used instead of a database. The test subject is going to be very simple in the article: I want the focus to stay on how to apply indirection. It uses a FileInfo object to create a text file and write one line, “Hello World!”.

public void UseIt(FileInfo file)
    using (StreamWriter Writer = file.CreateText())
        Writer.WriteLine("Hello World!");

Think about what this test subject will be like to test. You should be listing the responsibilities of the test subject in your head.

  1. Create a File specified by the file parameter.
  2. Write one line of the text “Hello World!\n”.
  3. Close the File.

If the file exists then #1 must have been satisfied. If we can open the file to read the contents then #3 must have been satisfied. We would need to compare the contents to what we expect for #2, “Hello Word!\n”.

Testing With the Real Deal

Below is a simple test using the file system.

public void TestUseItWithTheFileSystem()
    FileConsumer TestSubject = new FileConsumer();
    if (!Directory.Exists(@"C:\Temp"))
    if (File.Exists(@"C:\Temp\TestFile.txt"))
    FileInfo TestFile = new FileInfo(@"C:\Temp\TestFile.txt");
    String FileName = @"C:\Temp\TestFile.txt";
    String Contents;
    using (TextReader Reader = File.OpenText(FileName))
        Contents = Reader.ReadToEnd();
    Assert.AreEqual(string.Format("Hello World!{0}", System.Environment.NewLine), Contents);

First we make an instance of the test subject. The test subject will be creating a file in the C:\Temp directory. We can’t count on that directory existing so the test must check to see if it exists and if not create it. If the file that the test subject creates exists it needs to be deleted. We don’t want a previously successful test run to mask a problem. I don’t want to get crazy with making my test code super robust but there are already potiential problems with concurrency. We are hoping that there is no other process on the system vieing for these resources. Next we create the FileInfo object and pass it to the test subject’s method UseIt. The first assertion is on the existence of the file. The second implied assertion is in opening the file to read. To assert on the contents we read the file into a string variable and compare. Don’t forget the new line character.

Finding the Knobs and Dials


It is Always Indirectionreveal hidden content

If you knew of a way to test that was easier for you and was not brittle (e.g. resource contention issues) I bet you would use it. Well here is the key: indirection, same as last time. To make things better we need to introduce some indirection. But where and why? It is needed in front of the file system. Specificaly the FileInfo type, the StreamWriter type is abstract. It is needed to allow substitution of test doubles1). This is the control that we can exert to shrink the edge and expand the creamy center.

Figure 2

See the tight coupling to the FileInfo type, test subject is on the edge.

Figure 3

Wrappers and Test Doubles

How can indirection be introduced when not all the file system classes are derivatives and I don’t own the file system code to refactor it. Use the Adapter Pattern2) to introduce the indirection. Create a new interface that mimics the public interface of the type in question, FileInfo for this example. Create a new class, a wrapper, that implements the mimicking interface. It will need to hold an instance of the wrappie and pass all calls to it.

Figure 4

Will this indirection the test subject will need to alter a bit. Notice only the type of the parameter file was changed from FileInfo to IWrapperFileInfo.

public void UseIt(IWrapperFileInfo file)
    using (StreamWriter Writer = file.CreateText())
        Writer.WriteLine("Hello World!");

Now the test can adjust to use Recording Test Stubs3).

public void TestUseItWithRecorders()
    FileConsumer TestSubject = new FileConsumer();
    RecorderIWrapperFileInfo FileRecorder = new RecorderIWrapperFileInfo("A Bogas Bunch of Junk!");
    RecorderStreamWriter Streamrecorder = new RecorderStreamWriter(new MemoryStream());
    FileRecorder.Recordings.CreateTextRecording.ReturnValue = Streamrecorder;
    Assert.AreEqual("Hello World!", Streamrecorder.Recordings.WriteLineStringRecording.PassedStringvalue);

Notice that it is even a few lines shorter than the test against the file system types. This certainly introduces the needed indirection and will make writing the unit test easier but three new types were just created. Overall this does not look like an easier solution yet. This is nothing new. Michael C. Feathers documented this technique as Skin and Wrap the API4) giving the following advice:

Skin and Wrap the API is good in these circumstances:

  • The API is relatively small.
  • You want to completely separate out dependencies on a third-party library.
  • You don’t have tests, and you can’t write them because you can’t test through the API.

Wrapping It Up With Automation

If you have a tool to create those types for you the effort required on your part drops dramatically and we arrive at the stated goal. It is easier for you to introduce indirection and use test doubles than test against the real deal. If you are working with .NET then I have just the tool for you. If not then you can put one together. Doubler is a code generation tool for unit testing. One of the code generators that it offers is for creating a wrapper/interface. The wrapper, interface, and test stub recorder used in this article were all created with Doubler. Lets take a look at the out put; first the interface.

public interface IWrapperFileInfo
      // Methods
      StreamWriter AppendText();
      FileInfo CopyTo(string destFileName);
      FileInfo CopyTo(string destFileName, bool overwrite);
      FileStream Create();
      ObjRef CreateObjRef(Type requestedType);
      StreamWriter CreateText();
      void Delete();
      bool Equals(object obj);
      int GetHashCode();
      object GetLifetimeService();
      void GetObjectData(SerializationInfo info, StreamingContext context);
      object InitializeLifetimeService();
      void MoveTo(string destFileName);
      FileStream Open(FileMode mode);
      FileStream Open(FileMode mode, FileAccess access);
      FileStream Open(FileMode mode, FileAccess access, FileShare share);
      FileStream OpenRead();
      StreamReader OpenText();
      FileStream OpenWrite();
      void Refresh();
      string ToString();

      // Properties
      FileAttributes Attributes { get; set; }
      DateTime CreationTime { get; set; }
      DateTime CreationTimeUtc { get; set; }
      DirectoryInfo Directory { get; }
      string DirectoryName { get; }
      bool Exists { get; }
      string Extension { get; }
      string FullName { get; }
      DateTime LastAccessTime { get; set; }
      DateTime LastAccessTimeUtc { get; set; }
      DateTime LastWriteTime { get; set; }
      DateTime LastWriteTimeUtc { get; set; }
      long Length { get; }
      string Name { get; }


Most of the Way But Not all The Wayreveal hidden content

Notice that it is not just the interface defined by FileInfo but everything that is added to, overriden, and overloaded in the inheritance chain after Object.

The wrapper is really pretty simple. The event forwarding is the most comlplex thing is does. Thou there are not events on FileInfo so I wont show you how that works. Below is a snippet showing how the wrappie instance is created, stored, and called on.

public class WrapperFileInfo : IWrapperFileInfo
    private System.IO.FileInfo _wrappedSubject;
    public WrapperFileInfo(string fileName)
        this.WrappedSubject = new FileInfo(fileName);
    private System.IO.FileInfo WrappedSubject
            return this._wrappedSubject;
            this._wrappedSubject = value;
    public string Name
            return this.WrappedSubject.Name;
    public long Length
            return this.WrappedSubject.Length;

Don’t for get we generated two test doubles as well. One against the IWrapperFileInfo and one against StreamWriter. All the files can be reviewed below, the solution is avalible for download too.


Solution Zip

FileConsumerreveal hidden content

TestConsumerreveal hidden content

IWrapperFileInforeveal hidden content

WrapperFileInforeveal hidden content

RecorderIWrapperFileInforeveal hidden content

RecorderStreamWriterreveal hidden content

shrinking_the_edge_and_expanding_the_creamy_center.txt · Last modified: 2006/10/14 21:13 by jflowers
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