[ClassInterface(ClassInterfaceType.None)]
[Guid("86332C4E-0BDE-46EC-94C5-0A946C33C682")]
[TypeLibType(TypeLibTypeFlags.FCanCreate)]
public class MyComObjectClass : IMyComObject, MyComObject
{
    public MyComObjectClass();

    [DispId(1)]
    public virtual string Echo(string message);
}

and the sample class that uses it:
public class MyComObjectClassProxy
{
    public string Echo(string message)
    {
        IMyComObject itf = null;
        try
        {
            itf = new MyComObjectClass();
            return itf.Echo(message);
        }
        finally
        {
            if (itf != null)
            {
                Marshal.ReleaseComObject(itf);
            }
        }
    }
}
When I try to call the arranged constructor I am getting an error "Cannot create instances of type requiring managed activation", the unit test code:
[TestMethod]
public void TestMethod1()
{
    var mock = Mock.Create<MyComObjectClass>();
    Mock.Arrange(() => new MyComObjectClass()).Returns(mock);

    var sut = new MyComObjectClassProxy();
    var actual = sut.Echo("Message");

    Assert.AreEqual(string.Empty, actual);
}