Question

I am trying to unit test an action filter I wrote. I want to mock the HttpClientCertificate but when I use MOQ I get exception. HttpClientCertificate doesnt have a public default constructor.

code:

//Stub HttpClientCertificate </br>
var certMock = new Mock<HttpClientCertificate>();
HttpClientCertificate clientCertificate = certMock.Object;
requestMock.Setup(b => b.ClientCertificate).Returns(clientCertificate);
certMock.Setup(b => b.Certificate).Returns(new Byte[] { });

Answer 1

This is the most awkward case of creating unit testable systems in .NET. I invariable end up adding a layer of abstraction over the component that I can’t mock. Normally this is required for classes with inaccessible constructors (like this case), non-virtual methods or extension methods.

Here is the pattern I use (which I think is Adapter pattern) and is similar to what MVC team has done with all the RequestBase/ResponseBase classes to make them unit testable.

//Here is the original HttpClientCertificate class
//Not actual class, rather generated from metadata in Visual Studio

public class HttpClientCertificate : NameValueCollection {
    public byte[] BinaryIssuer { get; }
    public int CertEncoding { get; }
    //other methods
    //...
}

public class HttpClientCertificateBase {
    private HttpClientCertificate m_cert;

    public HttpClientCertificateBase(HttpClientCertificate cert) {
       m_cert = cert;
    }
    public virtual byte[] BinaryIssuer { get{return m_cert.BinaryIssuer;} }
    public virtual int CertEncoding { get{return m_cert.CertEncoding;} }
    //other methods
    //...
}

public class TestClass {
  [TestMethod]
  public void Test() {
      //we can pass null as constructor argument, since the mocked class will never use it and mock methods will be called instead
      var certMock = new Mock<HttpClientCertificate>(null);
      certMock.Setup(cert=>cert.BinaryIssuer).Returns(new byte[1]);
  }
}

In your code that uses HttpClientCertificate you instead use HttpClientCertificateBase, which you can instantiate like this – new HttpClientCertificateBase(httpClientCertificateInstance). This way you are creating a test surface for you to plug in mock objects.