Archive for October, 2013

How do I – Validate user input is numeric from Powershell?

Written by Cornelius J. van Dyk on . Posted in Blog

As the level of complexity and power of our Powershell scripts increase, it is inevitable that we reach a point where we need to interact with the user and prompt them for some information.  In this case we were building a script to automate the creation and scaling of Search so we needed to know how many servers would be configured as Crawl servers.  We all know the Read-Host cmdlet can provide us with the input, but how do we know that the user actually entered numeric only data?  The easiest way is to simply use a try/catch block and attempt to convert the input to an int value.  Here’s how we do that:

$numberOfCrawl = 0
$inputOK = $false
    [int]$numberOfCrawl = Read-Host -ForegroundColor red "Please enter the number of servers you wish to configure as Crawl servers."
    $inputOK = $true
    Write-Host -ForegroundColor red "INVALID INPUT!  Please enter a numeric value."
until ($inputOK)
Write-Host –ForegroundColor green "You chose to configure [$numberOfCrawl] Crawl servers."

We start by defining the $inputOK variable as false and then wrap the entire piece of code in a do/until loop that repeats until the user enters a valid numeric only value.  Inside of this loop, we have a try/catch block.  Inside the try, we attempt to assign the entered value from Read-Host to the $numberofCrawl variable which we cast as an int.  If the user enters a non-numeric value, the cast will fail and will be caught by the catch.  In the catch we output an error message in red text and simply allow the loop to restart from the top.  If however, the user entered a valid numeric value, the cast will succeed afterwich we set the $inputOK variable to true thus allowing the loop to end gracefully.  As a final step, we display the entered value to the user.



How do I – Sort a list of custom class objects in descending order in C#?

Written by Cornelius J. van Dyk on . Posted in Blog, How Do I...

Suppose we have a custom class defined thus:

    class CustomClass
        public string String1 { get; set; }
        public string String2 { get; set; }
        public string String3 { get; set; }
        public int Int1 { get; set; }
        public int Int2 { get; set; }

        public CustomClass()
            String1 = "";
            String2 = "";
            String3 = "";
            Int1 = 0;
            Int2 = 0;

        public int Calculate()
            return Int1 + Int2;
As we can see, the class initializes it’s variables and has a Calculate() method that simply returns the added value of the two int variables of the class. Now suppose we have a list of these class objects in our app with varying values thus:
    CustomClass MyClass = new CustomClass();
    MyClass.Int1 = 10;
    MyClass.Int2 = 5;
    CustomClass My2ndClass = new CustomClass();
    My2ndClass.Int1 = 7;
    CustomClass My3rdClass = new CustomClass();

    List lst = new List();
We now have a list containing 3 of our custom class objects. The first object in the list has an Int1 value of 10. The second has an Int1 value of 5 and the third, given the class’ initialization method, will have an Int1 value of 0. Given the order that we added the class objects to our list, the list, when looking at the Int1 field would look thus: 7, 10, 0 How do we go about sorting this list by any one of the variable fields of the custom class, say Int1 in our case? The built in support for Linq in Visual C# and the .NET Framework, actually makes this very easy. Using a Lambda expression, the task becomes a one liner thus:
  lst = lst.OrderByDescending(x => x.Int1).ToList();
We start with the OrderByDescending() method and then pass the Lambda expression to reference back to the object itself. The x => part of the expression tells C# to reference back to the calling object which in our case is lst. C# will reference back to lst as x which is where the x.Int1 then instructs the OrderByDescending method to use the Int1 field as the value by which to sort, in descending order, the x object which translates to lst. By appending the ToList() method to the back of the statement, C# will take the sorted result and generate a new List<> object which we simply assign back to the original lst variable. If we needed to sort in ascending order we would use the OrderBy() method instead thus making the statement:
 lst = lst.OrderBy(x => x.Int1).ToList();