Understanding LINQ method execution order

This is my answer to an interesting question asked yesterday on StackOverflow – what is the execution order of a LINQ query such as:

    var numbers = new[] { -1, 4, 9 };

    var sumOfRoots = numbers.Where(x => x > 0)
                            .Select(x => Math.Sqrt(x))
                            .Select(x => Math.Exp(x))

A quick an easy solution is to refactor the code in order to use custom delegates for each chained method (Where, Select and Sum), which makes things easier to debug. In this case I’m just printing a simple message to the console:

    static void Main(string[] args)
        var numbers = new[] { -1, 4, 9 };

        double sum = numbers.Where(IsGreaterThanZero)
                            .Sum(x => ToNumber(x));

        Console.WriteLine($"{Environment.NewLine}Total = {sum}");


    private static double ToNumber(double number)

        return number;

    private static double ToSquareRoot(int number)
        double value =  Math.Sqrt(number);

        Console.WriteLine($"Math.Sqrt({number}): {value}");

        return value;

    private static double ToExp(double number)
        double value =  Math.Exp(number);

        Console.WriteLine($"Math.Exp({number}): {value}");

        return value;

    private static bool IsGreaterThanZero(int number)
        bool isGreater = number > 0;

        Console.WriteLine($"{Environment.NewLine}{number} > 0: {isGreater}");

        return isGreater;

The output is the following:


Visual Studio: Unable to start debugging on the web server

This happened to me today – I was getting the same error whenever I tried to debug an ASP.NET application using Visual Studio:

Unable to start debugging on the web server. Could not start ASP.NET debugging. More information may be available by starting the project without debugging.


My initial reaction was to check if there was something wrong in IIS, and I was right: the application pool used by the application I wanted to debug was stopped!


At that moment I realised that I changed my Windows password 2 or 3 hours before trying to debug the application. Given that the application pool was running under my credentials, all I had to do to fix the issue was to right-click the application pool and go to Advanced Settings > Identity and update my password 🙂