Bit Field Enumerations

I learned a neat trick today which allows you to store multiple values in a single database field by linking it to an enumeration.

The programming construct behind the idea is called a Bit Field. It's simple, really. An enumeration is basically just a list of numeric values linked to text labels, and the usual form is to increase the numeric values incrementally, so ...

Public Enum ColoursEnum
Clear = 0
Red = 1
Blue = 2
Green = 3
Yellow = 4
Purple = 5
End Enum

But what if, instead of assigning your numeric values incrementally you decide to double them up each time, creating a number series which can be represeted by a bit:

Public Enum ColoursEnum
Clear = 0
Red = 1
Blue = 2
Green = 4
Yellow = 8
Purple = 16
End Enum

Looks a bit odd, no? Well whilst it does *look* odd you can do something clever with this series of values: the gap between each one means that if you wanted to, say, have an Enum with more than one value each potential value has a unique numeric identifier created by adding the values together. A numeric value of 12, in the above example, can only be made up by adding Green and Yellow. A numeric value of 19 can only be reached by adding Red, Blue and Purple. A numeric value of 2 can only be reached if the Enum is Blue and Blue alone.

So if you create a single database field to hold a single int, by making it a bitwise field you can make it hold more than one value. This has a multitude of uses: I came across it whilst trying to create a "status" column for a database object that could in fact have more than one status at once. Rather than the ugly approach of creating a boolean column for each potential status, I can have a single status column, link it to an Enum and have a bit field join to make it hold multiple values. Neat!

Be aware that in such an enumeration the "zero" value has to be unique - it can't be a part of any other combination of numbers. So you need to assign zero a value and it needs to be some sort of "resting" value indicating the enumeration is effectively empty - even if that means linking it to a text flag like "empty" or "not set".

However you may think you've spotted a flaw here: am I not going to have to introduce some slightly annoying and convoluted logic somewhere to take the value from the database, look at the enumeration and decide what set of values my field is actually holding? Thankfully the .net framework automates all of this for us.

All you have to do is add a Flags attribute to the enum:

_
Public Enum ColoursEnum
Clear = 0
Red = 1
Blue = 2
Green = 4
Yellow = 8
Purple = 16
End Enum

And .Net does all the work for you. It's slightly different in C# by the look of things: you need to use [FlagsAttribute] instead of but the end result is the same.

.Net will now treat each potential set of values from this enumeration differently. That is to say that if you loop through all the potential values of Colours above you'll get separate entries for Red, RedBlue, RedBlueGreen, RedGreen and so on.

You can also set your Enum to hold multiple values by using a bitwise "OR" operator (a pipe |):

Dim myRed as ColoursEnum = ColoursEnum.Red
Dim myBrown As ColoursEnum = ColoursEnum.Red | ColoursEnum.Purple

And in the same manner you can use a bitwise "AND" operator (an ampersand &) to isolate a single value and check it. Colours are a bad way of demonstrating this but hopefully the logic will read through.

Dim myBrown As ColoursEnum = ColoursEnum.Red | ColoursEnum.Purple

Dim myRed As ColoursEnum = myBrown & ColoursEnum.Red
'This will set MyRed equal to Red, since it matches something in the enum
Dim noColour As ColoursEnum = myBrown & ColoursEnum.Green
'This will set noColour equal to Clear since green doesn't match anything in the enum


URL Routing Niggles

I've been playing with URL re-routing in Web Forms recently. Although originally supplied as part of the MVC framework the classes can now be used in standard web forms so you can have nice, neat, copy-and-paste friendly, search engine optimised URLs in your web forms application.

The basics of how to do this are well documented, so I won't bore you with them. If you don't know how it's done, I'll let 4-Guys bore you instead.

Trying to implement it I ran into a couple of small hurdles though, which I thought I'd document for reference. It's a little embarrassing really because both of these things are quite clear if you read the documentation properly instead of skimming and getting down to trying it out.

Optional & Extensible Parameters

The first is how to make parameters optional. Let's say for example that I have a search function with two parameters: firstname and surname. I might want the url for that search function to look like this:
/people/search/(searched text for firstname)/(searched text for surname)/
Let's further assume that I have various other pages I want to map inside the people folder, so things like:
/people/summary/
/people/management/

So I define my virtual URL like this:

Dim urlPattern As String = "people/{action}/{firstname}/{surname}"

And as soon as I do that, /people/summary/ fails. It's expecting a firstname and a surname as part of the URL and doesn't recognise the address.

To solve thisWe add a dictionary to the route value, like so:

reportRoute = New Route(urlPattern, New ReportRouteHandler)
reportRoute.Defaults = New RouteValueDictionary(New With {.firstname = "", .surname = ""})

This instructs the route that the firstname and surname parameters can be replaced with a default - in this case an empty string - if they're missing. And of course this being a dictionary you can add as many key/value pairs as you need.

But there's still one more thing. What if we might want to include some additional information in that URL, or even just make sure that longer URLs fail gracefully? At the moment /people/search/matt/j/thrower/ will fail. To solve this, add an asterisk to the final part of your virtual URL:

Dim urlPattern As String = "people/{action}/{firstname}/{*surname}"

And voilà! The * indicates that the final parameter is extensible and can be of any length.

Getting Values from Parameters in Target Pages

The other thing that you'll probably want to do with this is to have a target page read the items in the url parameter collection. Right now you can access the values in {action}, {firstname}, {lastname} and so on inside route handling functions, but the target web form won't know about them.

My first approach to this was to change the parameters into querystrings and include them in the redirect part of the route handler:

Dim handlerUrl As String
Dim hand As Web.IHttpHandler
Select Case CStr(routeData.Values("action")).ToLower()
Case "search"
handlerUrl = String.Format("/people/search.aspx?firstname={0}&surname={1}", routeData.Values("firstname"), routeData.Values("surname"))
End Case

hand = DirectCast(System.Web.Compilation.BuildManager.CreateInstanceFromVirtualPath(handlerUrl, GetType(Web.UI.Page)), Web.IHttpHandler)

Which blows up in your face because the querystring isn't a valid virtual path.

The way round this is to utilise a handy collection in HttpContext called Items, which allows you to store key/value pairs of data:

Dim handlerUrl As String
Dim hand As Web.IHttpHandler
Select Case CStr(routeData.Values("action")).ToLower()
Case "search"
handlerUrl = "/people/search.aspx"
For Each urlParm In requestContext.RouteData.Values
requestContext.HttpContext.Items(urlParm.Key) = urlParm.Value
Next
End Case

hand = DirectCast(System.Web.Compilation.BuildManager.CreateInstanceFromVirtualPath(handlerUrl, GetType(Web.UI.Page)), Web.IHttpHandler)

And you can pick these up in your target page like so:

Dim firstname As String = HttpContext.Current.Items("firstname").ToString
Dim surname As String = HttpContext.Current.Items("surname").ToString


Quey across a join in nHibernate

My god, I don't think I've ever come across any framework with quite such a poor popularity-to-documentation framework as Fluent nHibernate. Everyone is using it, but no one writes down how.

Stupid nHibernate.

So, after spending ages trying to get something as simple as a rowcount, today I have spent an equally frustrating period trying to get a results set which does a logic OR operation across a joined table. Something as petty as this, in SQL terms:


SELECT * from product p
inner join sku s on s.id = p.id
WHERE p.Name like '%widget%'
OR s.SkuCode like '%wd_%'


The issue arises because even though my mappings are correct, nHibernate throws a wobbler at you if you start with Product and try and get it to recognise that it should be able to filter on a field in the joined sku table. This, for example, compiles but fails horribly even though to my mind it's the intuitive way of presenting the above query in nHibernate:


Dim results As ArrayList = session.CreateCriteria(Of DataTransferObjects.Product) _
.Add(Expression.Or( _
Expression.Like("Name", "widget"), _
Expression.Like("SkuCode", "wd_"))) _
.List()


No, what you have to do is set up a CreateAlias to the table and use that:

Dim results As ArrayList = session.CreateCriteria(Of DataTransferObjects.Product) _
.CreateAlias("Skus", "sku") _
.Add(Expression.Or( _
Expression.Like("Name", "widget"), _
Expression.Like("sku.SkuCode", "wd_"))) _
.List()


Certainly not the way I'd expect a "highly intuitive", productivity-enhancing framework to function. Not by a long shot.

I could have done the required work by creating a couple of custom data handling objects and accompanying T-SQL stored procedures in about ten minutes. To do the same thing in nHibernate has taken two days. What's that about enhancing productivity?

nHibernate Projections in VB.net

Well, this has lead me a merry dance, I can assure you. You'd think it was pretty straightforward to do something as simple as a row count in nHibernate, wouldn't you? Well, in point of fact it actually is pretty simple but it's just not well documented anywhere. Particularly perplexing is the fact that most of the examples seem to have been ripped straight from the Java hibernate implementation and I'm not entirely sure this will even port to C# and work, let alone translate to VB (I haven't tried in C#).

Anyway, just in case some poor soul has the same problem and gets stuck, this is how you do it:


Dim pResults As IList = session.CreateCriteria(GetType(dto.Product)) _
.SetProjection(Projections.RowCount()).List()


Of course "dto.Product" is a custom class I'm using that's specific to what I'm trying to do, so stick your own in there and you should be good to go.

Reading XML with XPath

There's more than one way to read an XML file in dot net, but the XPath model doesn't seem to be terribly widely documented. Which is a shame because although it's a little unintuitive, it offers some functionality that doesn't come with other tools.


using System.Xml.XPath;

XPathDocument xDoc = new XPathDocument(“filepath”);
XPathNavigator xNav = xDoc.CreateNavigator();

Xnav.MoveToRoot();
Xnav.MoveToFirstChild();

do
{
//do stuff .. like xNav.GetAttribute(""AttributeName", "");
}
while (Xnav.MoveToNext());


So, load your xml file by replacing "filepath" with the path of the - ahem - file you want to load. MoveToRoot and MoveToFirstChild do exactly what you'd expect them to, and you can obviously vary these commands depending on where you want to start reading. Then the do-while loop should pass through each node in turn, allowing you to access its attributes and such.

Writing files to disc

This is a simple enough procedure but, in honesty, I always, always forget how to do it, and when you go and look it up on the interweb it seems that the examples provided are far more complicated than they really need to be. I guess it must be that the System.IO namespace simply isn't as intuitive as perhaps it should be. So here's the stripped down, basic code you need to write a file to disc - don't forget that the namespace goes at the top of your file, while the actual operational code goes inside it.


using System.IO;

FileStream myStream = new FileStream("insert File path here", FileMode.Create);
StreamWriter myWriter = new StreamWriter(myStream);

myWriter.Write("Some Text");
myWriter.Write(Environment.NewLine);
//this creates a new line. Duh!

myWriter.Close();
myStream.Close();


So put the target path into "insert File path here" and put your text into where it says "some text". Obviously you can keep adding as much text or new lines as you need.