StubTests.cs

[TestMethod]
public void Ctor_PassingOutput_ShouldCallWrite()
{
    var invoked = false;
         
    var stub = new StubIOutput()
    {
        WriteLogEntry = (entry) =>
            {
                invoked = true;
            }
    };

    var l = new LogEntry("Some Message", stub);
    Assert.IsTrue(invoked, "Write of output should be invoked");
}

[TestMethod]
public void Ctor_PassingOutput_ShouldSetPropertiesBeforeCallingIt()
{
    string passedMessage = null;
    var expected = "Log test!";

    var stub = new StubIOutput()
    {
        WriteLogEntry = (entry) =>
        {
            passedMessage = entry.Message;
        }
    };

    var l = new LogEntry(expected, stub);
    Assert.AreEqual(expected, passedMessage, 
        "Message property should be setted before calling output");
}