Detouring a constructor with Microsoft Fakes #dev #coding #tdd

I wanted to ensure every SmtpClient instance that was created by a library was also disposed of, to do this I first needed to Shim SmtpClient using Microsoft Fakes:

  1. Add Fakes Assembly for System.
  2. Edit Fakes/System.fakes
    Add

    <ShimGeneration>
      <Add TypeName="SmtpClient"/>
    </ShimGeneration>
  3. Save the file and close
  4. Rebuild, open the System.4.0.0.0.Fakes reference and under System.Net.Mail.Fakes you should see ShimSmtpClient.

My idea was to count up every time the constructor was called and to count down every time one was disposed, to do this I created a counter:

int instancesNotDispose = 0;

Incremented it in the Constructor Shim, that is wrap the constructor and do some more things rather than completely detouring the constructor:

ShimSmtpClient.Constructor =
    x =>
    {
        // Execute outside of Shims otherwise you'll be in StackOverflowException territory
        ShimsContext.ExecuteWithoutShims(
        () =>
        {
            // Use reflection to get the default constructor and invoke it, the magic here is the instance is stashed into x!
            var constructor = typeof(SmtpClient).GetConstructor(new Type[0]);
            constructor.Invoke(x, new Object[0]);
        });

        instancesNotDispose++;
    };

Then decremented it in the Dispose Shim:

ShimSmtpClient.AllInstances.Dispose =
    x =>
    {
        // Execute outside of Shims otherwise you'll be in StackOverflowException territory
        ShimsContext.ExecuteWithoutShims(x.Dispose);

        instancesNotDispose--;
    };

Then I can call my library a bunch of times to do whatever it does and sends some e-mails, then I can assert they have all be disposed:

Assert.AreEqual<int>(0, instancesNotDispose, String.Format("Doh! {0} were not disposed", instancesNotDispose));

Assuming there aren’t other SmtpClient constructors that do not call the default constructor then this code should work fine, if there are other constructors called that do not call the default constructor then this Assert will also help find those. There are other ways, for example, stashing each instance in a List and ensuring each is in both the Constructor List and the Dispose List, but this counting approach was enough for my needs for now.

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