Developing For .NET

I’m still going strong on the ASP.NET MVC development, and I wanted to experiment with HtmlHelper.Select rather than hard coding specific values into my View. To that end, I am building a supporting class that will retain static properties (which I’m sure will be cached once I figure out how to do that). Some of these properties are lists of information, like product types and prices, that could be fed from a database table. Others may be static lists with the potential to become database driven, like state codes and names. Ultimately, where they come from does not matter, but in my coding today I came across something interesting.

To begin this process, I decided to populate a static Dictionary<string, string> with a list of codes and values. This is a small list, so for initial testing I just wanted to hard code the values. It has become a habit for me to use Object Initializers whenever possible, so naturally this seemed a perfect fit. But let me ask you: have you ever tried this? I have to admit that at first I was a little stumped. I first tried to build a method tree (a concept that really doesn’t exist in C#):

BusinessTypes = new Dictionary<string, string>()
.Add("APP", "Appraisal")
.Add("GOV", "Government")
.Add("INS", "Insurance")
.Add("LAW", "Legal")
.Add("LEN", "Lending")
.Add("REL", "Real Estate")
.Add("RES", "Research")
.Add("OTH", "Other");

Even though I didn’t really expect this to work, I thought it would be interesting to see what happened. [This could be a good candidate for a future language enhancement.] Naturally it failed, but in an unexpected fashion. The first call to .Add passed the compiler, but the second call to .Add threw the following error:

Operator ‘.’ cannot be applied to operand of type ‘void’

I also tried adding the method call inside the Initializer block, but since you can’t put method calls inside of Initializer blocks I pretty much knew that wouldn’t work either.

So I looked at it some more and asked myself “what is the Dictionary looking for?” My answer was “a KeyValuePair”, so I tried that next:

BusinessTypes = new Dictionary()
{
    new payday us fast cash loan,fast cash payday loancash till payday loan,cash loan payday,advance cash cash loan payday paydayloan oneclickcash paydaycash fast loan paydaycash advance loan illinois,5 advance cash illinois loanfull pay video pokerbest video pokerinternet casino craps,internet casino,internet casino gambling gamefree online casino gamblingduces wild video pokercasino link online suggestno deposit bonus online casino,free online gambling casino bonus,online casino bonusplay blackjack online freefree video poker gamescraps free online play,casino craps free gambling online,free online crapsfree online casino,online casino,top online casinoplaying blackjackplay free casinofree casino cash bonus,casino cash,free cash casinofree video poker gamereal money backgammonplay roulette onlinefree casino moneytriple play video pokerfree roulette,free roulette download,play free rouletteroulette gambling,best gambling online roulette,online roulette gamblingplay black jack online freeonline gambling casino,gambling casino online bonus,free online casino gamblingcasino download,free online casino download,download online casino gamemicrogaming casino bonusbaccarat card game,baccarat game,baccarat the internet casino gamecasino no deposit bonus code,online casino sign up bonus,casino bonusvideo poker for winnersvideo poker strategy,free video poker,video pokerkeno gameplay slots,play for fun online slots,free slots play for freecraps rulesplay free slots,free slots,free online slots gameinternational online casinovideo poker machineshand held video pokercraps gambling strategy,craps gambling game,gambling crapsbest casino bonus888 black jackfree online slots no download,no download free slots game,free slots no downloadblack jack downloadplay casino game onlinetournament backgammon888 poker,888 poker info,888 poker tournamentbest online casino bonus KeyValuePair<string, string>("APP", "Appraisal"),
    new KeyValuePair<string, string>("GOV", "Government"),
    new KeyValuePair<string, string>("INS", "Insurance"),
    new KeyValuePair<string, string>("LAW", "Legal"),
    new KeyValuePair<string, string>("LEN", "Lending"),
    new KeyValuePair<string, string>("REL", "Real Estate"),
    new KeyValuePair<string, string>("RES", "Research"),
    new KeyValuePair<string, string>("OTH", "Other")
};

That also threw an error:

No overload for method ‘Add’ takes ‘1’ arguments

But the error pointed me back to my original idea: somehow I needed to use Add(), but how? And then it dawned on me: what the message is really telling me is that I was already using Add(), I just didn’t realize it! Since we use curly braces to denote Initializers, I decided to try that within the context of the parent Initializer to send parmameters to the Add method:

BusinessTypes = new Dictionary<string, string>()
{
    {"APP", "Appraisal"},
    {"GOV", "Government"},
    {"INS", "Insurance"},
    {"LAW", "Legal"},
    {"LEN", "Lending"},
    {"REL", "Real Estate"},
    {"RES", "Research"},
    {"OTH", "Other"}
};

This compiles fine and now I have successfully used an Object Initializer on a Dictionary<TKey, TValue> object. I was curious about why this works, so I hit the Google pavement and found this article on MSDN from October of 2006. In it, the author has this to say:

Our resolution to this is to refine our understanding of collection initializers a little bit. The list you provide is not a ?list of elements to add?, but a ?list of sets of arguments to Add methods?. If an entry in the list consists of multiple arguments to an Add method, these are enclosed in { curly braces }. This is actually immensely useful. For example, it allows you to Add key/value pairs to a dictionary, something we have had a number of requests for as a separate feature.

From previous text in the article, we find that any IEnumerable with a public Add method will behave this way. As such, we should be able to use this construct in almost all the generic Collection classes.

This is definitely useful, and yes I should have searched for a solution before I spent the entire 3 minutes trying to figure it out. But sometimes, I just like solving the puzzle.

I am working on a project that requires a configuration file that can be edited or even replaced by the user. Looking at the requirements, what I really need is a series of “records” of the same layout, similar to a database table. Naturally, given the current state of technology, an XML file is the obvious choice. So I decided that now would be a good time to investigate LINQ to XML.

Understanding LINQ to XML

First of all, unlike other LINQ topics, this one is not really query oriented. LINQ to XML (also known as XLINQ) is primarily about reading and writing XML files. Once read into program variables, you can use LINQ to Objects technologies to perform query functions.

First, the XLINQ classes are not part of the standard System.Linq namespace. Instead, you will need to include System.Xml.Linq. This namespace contains the XML classes that make XLINQ worthwhile, and we’ll review a few of these as we go along.

Second, this article will be a far cry from exhaustive on the subject: there are a ton of additional features and topics that I will not cover. Chief among the reasons for this is that I am far from an XML expert. I consider myself a typical developer where XML is concerned: my programs need to find and consume information stored in an XML format. I may even have occasion to update or write XML content. Otherwise, XML holds no particular glamor for me.

Reading an XML File

Most articles I have read on this topic begin with using XLINQ to create XML structures and files. My first task was to read an existing XML file, so that is where I will begin.

Here is the XML data I will be using for this article:

<LayoutItems Class="">
  <LayoutItem Name="FullName">
    <Row>2</Row>
    <Column>5</Column>
  </LayoutItem>
  <LayoutItem Name="Address1">
    <Row>3</Row>
    <Column>10</Column>
  </LayoutItem>
  <LayoutItem Name="Address2">
    <Row>4</Row>
    <Column>10</Column>
  </LayoutItem>
  <LayoutItem Name="City">
    <Row>5</Row>
    <Column>10</Column>
  </LayoutItem>
  <LayoutItem Name="State">
    <Row>5</Row>
    <Column>40</Column>
  </LayoutItem>
  <LayoutItem Name="Zip">
    <Row>5</Row>
    <Column>44</Column>
  </LayoutItem>
</LayoutItems>

This is a simple configuration file for a printing product I am writing. The root element LayoutItems has a Class attibute and contains a collection of child LayoutItem objects. Each LayoutItem has a Name attribute that references a Property name in the Class listed in the LayoutItems Class attribute. Each LayoutItem element then contains Row and Column elements. I’m pretty confident you can guess what these represent. This is a simple example, but no matter how complex your XML layout is, these same techniques will work.

First, we need to get an object that we can use to read and process our XML data. There are two that we can use: XDocument and XElement. If we use XDocument, we have to pull an XElement out of it to use our data, so an easier solution is to bypass the XDocument altogether and just use the XElement approach.

There are several ways to create a usable XElement object. This example uses XElement’s static Load method. In this case I am passing it the path of the XML file:

// Load XML from file
XElement xml = XElement.Load(XmlPath);

Dropping into debug after this happens shows us that the XElement object now contains all the XML from our file. All the nodes in an XElement are represented by other XElements. This nesting may look confusing at first, but it makes sense, like the Nodes of a TreeNode. Each subsequent element contains all the information for that element, including any other child elements. In our simple example there is only one sublevel of elements, but again the same techniques would work regardless of how deeply nested your data.

Now, in order to read through my XML data, I am going to loop through the Elements collection:

// Loop through Elements Collection
foreach (XElement child in xml.Elements())
{
    // Process child XElement
}

Each of these XElement objects will represent one LayoutItem section. The LayoutItem element has a Name attribute that I need to read, so I am going to use the XElement’s Attribute property:

// Read an Attribute
XAttribute name = child.Attribute("Name");

There is also an Attributes property that is a collection of all the XAttribute objects. If you had multiples to process or did not know the attribute names, this would be a simple enough option to use.

I want to point out that frequently the methods ask for an XName value. You will quickly find though that XName has no constructor. According to the documentation, wherever an XName is asked for, sending a string will perform an implicit conversion and create an XName object. I’m sorry, but this is really stupid: it creates unnecessary confusion in Intellisense and is not clear or intuitive. All that being said, now that you know, whenever you see “XName”, think “string”.

Now, since I have no further nesting to deal with, I’m ready to go ahead and read my child element data. The code is very similar to the Attribute code, except now we are using the “Element” property:

// Read an Element
XElement row = child.Element("Row");
XElement column = child.Element("Column");

Now that we have our Attributes and Elements, it’s time to actually use the values. To do this, we need to extract the values out of our XElement object. To accomplish this, you have two options. First, you can always use ToString() to get the string representation of the data and then manually convert to the desired type:

// using ToString() to extract data
string val = row.ToString();
float realVal = Convert.ToSingle(val);

Another option that will let you bypass that step is to use one of the built in cast operators to get the appropriate type directly:

// using a built in Cast operator
float realVal = (float)row;

There are built in cast operators for all the expected cast of characters like int, bool, DateTime, etc.

Creating and Writing XML

I used Visual Studio to create my XML file, but writing XML is pretty straightforward as well. Essentially, you need to create XElement objects and add other XElement objects to their Elements collection. Let’s create the above document in code.

The XElement constructor has several overloads. To just create an XElement, you can simply create a new one and pass it the name of the Element:

// Create a new XElement
XElement root = new XElement("LayoutItems");
// Add an Attribute
root.Add(new XAttribute("Class", ""));

Now, what would be intuitive would be able to access the Attributes and Elements collections and use their Add() methods to add content to them. Unfortunately, this is not possible because these are Read-only collections. XElement has an Add method of its own that accepts single content objects or a params[] construct. You can also use this method on the constructor, so let’s use the constructor method to create a few new XElements and add them to our root:

// Create and add more elements
XElement elm1 = new XElement("LayoutItem",
    new XAttribute("Name", "FullName"),
    new XElement("Row", 2),
    new XElement("Column", 5));
root.Add(elm1);

XElement elm2 = new XElement("LayoutItem",
    new XAttribute("Name", "Address1"),
    new XElement("Row", 3),
    new XElement("Column", 10));
root.Add(elm2);

XElement elm3 = new XElement("LayoutItem",
    new XAttribute("Name", "Address2"),
    new XElement("Row", 4),
    new XElement("Column", 10));
root.Add(elm3);

XElement elm4 = new XElement("LayoutItem",
    new XAttribute("Name", "City"),
    new XElement("Row", 4),
    new XElement("Column", 10));
root.Add(elm4);

XElement elm5 = new XElement("LayoutItem",
    new XAttribute("Name", "State"),
    new XElement("Row", 5),
    new XElement("Column", 40));
root.Add(elm5);

XElement elm6 = new XElement("LayoutItem",
    new XAttribute("Name", "Zip"),
    new XElement("Row", 5),
    new XElement("Column", 44));
root.Add(elm6);

Debug will now show that our root XElement object now contains all the properly formatted XML. To write it to the file system, you can use the standard StreamWriter options, or you could use the built in Save() method:

// Write file
root.Save("myFileName.xml");

In this example, I am just passing the string of the path of the file I want. There are several other overloads, but I’m confident you will be able to use them with no problem.

Conclusions

Like I said at the beginning, you didn’t see any query features here, because in this case the LINQ parts really happen in the XML processing. The Attributes and Elements collections implement IEnumerable<T>, so if you need to query them you can do so using LINQ to Objects techniques.

Personally, these new classes finally make XML an attractive solution. I finally feel like reading and processing XML can be pain free and straightforward. I know that I will be using a lot more XML in the future as a result.

I’ve written a couple of times before about Automatic Properties, a tiny little feature that I have really come to enjoy. Remember that an Automatic Property looks something like this:

// An Automatic Property
public string FirstName { get; set; }

I’ve also written that if you want to add any behavior to this property, then you must create a variable and complete the code just as you would have a regular property. I was coding the other day and I had to transform add some behavior to what had been an automatic property. For grins, I wanted to see what I could get away with, so I tried a couple of different approaches.

First, I really only wanted to add behavior to the Setter, so I tried leaving the Getter as is:

// Will not compile
public string FirstName
{
    get;
    set
    {
        FirstName = value.TrimEnd(new char[] { ' ' });
    }
}

This will not compile, resulting in the following error:

‘ConsoleTestBed.Person.FirstName.get’ must declare a body because it is not marked abstract, extern, or partial

OK, so no joy there, I have to fill out the Getter. But notice in the Setter block there is no error! So I fill out the Getter and it looks like this:

// This compiles
public string FirstName
{
    get { return FirstName; }
    set
    {
        FirstName = value.TrimEnd(new char[] { ' ' });
    }
}

This compiles just fine. What I’m hoping at this point is that the Compiler will recognize what I am doing (given its context) and still create the variable for me. However, when I run it, I get a StackOverflowException. What you have probably figured out, and what I was afraid of, is that by referencing the Property name within the Property, I have created a recursion problem. So, unfortunately, I was originally correct: in order to make these changes work, I have to create a variable to support the Property:

// This compile AND works
private string _firstName;
public string FirstName
{
    get { return _firstName; }
    set
    {
        _firstName = value.TrimEnd(new char[] { ' ' });
    }
}

So if your program throws a StackOverflowException on a Property, be sure that you haven’t created a Recursion issue. Maybe someday the C# compiler guys can find a way to make this work based on the Context, or maybe a Property Attribute.