Question

I have a Visual Studio 2008 C# .NET 3.5 project that I am implementing unit tests for, but I’ve run in to a problem. My code references a 3rd party assembly that implements objects with internal constructors.

For example:

// in 3rd party assembly
public class Bar
{
    // internal constructor
    internal Bar();
    public int Id { get; }
    public string Name { get; }
    public Foo Foo { get; }
}

public class Foo
{
    // internal constructor
    internal Foo();
    public Collection<Bar> GetBars();
}

One method of mine that I would like to unit test is this:

// in my assembly
public static Bar FindByName(this Collection<Foo> c, string name)
{
    // search through the Foos to find the first bar with the matching name
}

And test it like this:

void TestMethod()
{
    Collection<Foo> foo_pool = new Collection<Foo>()
    {
        new Foo() { /*..*/ } // Error! ctor is inaccessible
    };

    Bar b = foo_pool.FindByName("some_name");
    assert_equal (b.Name, "some_name");
}

But, I can’t create objects of type Foo or type Bar. So, how can I unit test my method?

Thanks

Answer 1

For unit tests, you can use the PrivateObject class (namespace Microsoft.VisualStudio.TestTools.UnitTesting) to create objects with private constructors and even test private methods.

http://msdn.microsoft.com/en-us/library/microsoft.visualstudio.testtools.unittesting.privateobject(v=vs.90).aspx

http://www.gangleri.net/2007/11/15/PrivateObjects.aspx

Here’s an example:

[TestMethod]
public void TestMethod2()
{
    // Arrange
    var po = new PrivateObject(typeof(MyObject));
    var obj = (MyObject)po.Target;
    // Act
    var result = obj.Calculate(2);
    // Assert
    Assert.AreEqual(3, resul);
}

public class MyObject
{
    internal MyObject()
    {

    }

    public int Calculate(int a)
    {
        return 1 + a;
    }
}

It uses reflection in the same way as Jim’s suggestion, but PrivateObject class encapsulates all the work to create the instance with private constructors.