Monday, January 31, 2011

Using Except() Method in LINQ

There is a method that I rarely use in LINQ called "Except" - but it comes handy in my latest coding. So I thought I'd share how I use it. This method is for an IEnumerable and has multiple signatures: one that takes a second IEnumerable to exclude and the other one takes an IEnumerable and a comparer. You can read the spec in full here.

Here is an example:

int[] oneToTen = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
int[] oneToTenEven = { 2, 4, 6, 8, 10 };
int[] oneToTenOdd;

// do some exclusion here
oneToTenOdd = oneToTen.Except(oneToTenEven);

// so that oneToTenOdd will have { 1, 3, 5, 7, 9 };
OK - that looks simple. But what about when the IEnumerable is of type T or some other complex type? This is where the second signature comes in handy. In this scenario, we will have to make our own comparer class to specify how we want both list items to be compared against each other.

Imagine this hypothetical situation where you are at a dealership and want to separate the cars that have been washed and ones that have not. Cars may have the same make, model, color, type, but each has different VIN number.

IEnumerable<Car> allCars = GetAllCars();
IEnumerable<Car> carAlreadyWashed = GetCarAlreadyWashed();
IEnumerable<Car> carNotWashed;

// do some exclusion here
carNotWashed = allCars.Except(carAlreadyWashed);

The above code which will normally work for simple comparison, won't work because the run-time will have no idea that it has to compare based on VIN number. We have to tell it to use that field to do comparison.

public class CarComparer : IEqualityComparer<Car> {
    public bool Equals(Car x, Car y) {
        //Check whether the compared objects reference the same data.
        if (Object.ReferenceEquals(x, y)) return true;

        //Check whether any of the compared objects is null.
        if (Object.ReferenceEquals(x, null) || Object.ReferenceEquals(y, null))
            return false;

         //Check whether the Car' properties are equal.
        return x.VIN == y.VIN;

    // If Equals() returns true for a pair of objects
    // then GetHashCode() must return the same value for these objects.
    public int GetHashCode(Car car) {
        //Check whether the object is null
        if (Object.ReferenceEquals(plan, null)) return 0;
        //Get hash code for the VIN field.
        int hashCarVIN = car.VIN.GetHashCode();
        return hashCarVIN;
Now then we can modify our code to be such as this:

IEnumerable<Car> allCars = GetAllCars();
IEnumerable<Car> carAlreadyWashed = GetCarAlreadyWashed();
IEnumerable<Car> carNotWashed;

// do some exclusion here
carNotWashed = allCars.Except(carAlreadyWashed, new CarComparer());

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Friday, January 7, 2011

Microsoft Certification exams

My employer (AWH - encourages us (the employees) to take MS certifications. Not only this will benefit the company in maintaining our status as a Gold Partner, but also will benefit us as far as knowledge, new technology, etc. Plus the certifications are attached to the individual, so if you change jobs, those certifications certainly belong in your resume and can be leveraging points.

AWH will pay for the tests, including first failure. So if I take a test and fail, and then retake the test and pass, AWH will cover both exams. So I have to pass before the reimbursement can come through.

Anyway, so last November, I saw a promotion on Prometric's website for a 2/3/4/5 exam packets with discounted price and FREE retakes for each exams in the packet. With that good deal in mind, I decided that I will set a goal in this area: getting a MCPD - Microsoft Certified Professional Developer. This certification requires 4 exams:

  • TS 70-515 - ASP.NET 4.0, Web Forms, MVC, IIS
  • TS 70-516 - ADO.NET, LINQ, EF
  • TS 70-513 - WCF
  • TS 70-519 - Web application solution/architect
I took the first test early in December and passed it quite easily. Since ASP.NET is what I use daily, most of the questions in the test are familiar to me and I can answer them correctly.

I scheduled to take the ADO.NET test on Dec 23rd, but when I came to the facility, it was closed. I was not notified of any closing and Prometric also was not aware of the closing of the local facility (New Horizons). Prometric was able to reschedule me for Dec 27th at a different facility. I failed my ADO.NET test - probably by 2 or 3 questions, it was so close. Although I use ADO.NET stuff pretty much daily, but there are some things in there that are pretty new to me and I have not used them in production quality code - like Entity Framework. Those questions stumped me and resulting in a failing grade. Thank God for the free retake!

So I studied more EF, practice making some projects with with etc and retake the test on December 29th. I passed.

So next up is WCF - scheduled for Jan 12th. Hopefully by end of Feb 2011, I will have become an MCPD.

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