side-effect

MyBusinessLogic-1.cs

public event Action<string> OperationFinishedNotification; 

public string AwesomeBusinessOperation()
{
    var firstPart = TakesALongTimeToProcess("Tons");
    var secondPart = TakesALongTimeToProcess(firstPart + " of ");
    var result = TakesALongTimeToProcess(secondPart + "money");

    var eventToFire = OperationFinishedNotification;
    if(null!=eventToFire)
    {
        OperationFinishedNotification(result);
    }
    return result;           
} 

private string TakesALongTimeToProcess(string word)
{
    // This operation takes a while
    Thread.Sleep(1000);
    return word;
}

MyBusinessLogic-2.cs

public event Action<string> OperationFinishedNotification;

public async Task<string> AwesomeBusinessOperationAsync()
{
    var firstPart =await TakesALongTimeToProcess("Tons");
    var secondPart = await TakesALongTimeToProcess(firstPart + " of ");
    var result = await TakesALongTimeToProcess(secondPart + "money");

    var eventToFire = OperationFinishedNotification;
    if(null!=eventToFire)
    {
        OperationFinishedNotification(result);
    }
    return result;           
}

private Task<string> TakesALongTimeToProcess(string word)
{
    // Remember, this is just a simulation
    // Usually you would use some other async API here
    return TaskEx.Run(() =>
    {
        // This operation takes a while
        Thread.Sleep(1000);
        return word;

    });
}

MyBusinessLogicTest-1.cs

[Test]
public void ExpectEventToBeFired(){
    var toTest = new MyBusinessLogic();
    var eventFiredExpected = "";
    toTest.OperationFinishedNotification 
        += eventArgument =>
                    {
                        eventFiredExpected = eventArgument;
                    };
    var result = toTest.AwesomeBusinessOperation();
    Assert.AreEqual("Tons of money",eventFiredExpected);
}

MyBusinessLogicTest-2.cs

[Test]
public void ExpectEventToBeFired(){
    var toTest = new MyBusinessLogic();
    var eventFiredExpected = "";
    toTest.OperationFinishedNotification 
        += eventArgument =>
                    {
                        eventFiredExpected = eventArgument;
                    };
    var result = toTest.AwesomeBusinessOperationAsync();
    // just wait for the asynchronous operation
    result.Wait();
    Assert.AreEqual("Tons of money",eventFiredExpected);
}