So the issue that I ran into recently is that I’m getting data in an observable object list and then getting an updated list. I need to be able to compare each object in the list to see if it has been updated or not. The problem is that I have a lot of properties in each object and I had several object lists which I needed to compare. The solution I wanted involves IComparable. This is a very simple interface that implements CompareTo method only. CompareTo receives an object and returns an integer. This interface is used for comparing and doing things like sorting in arrays but in this case all I need to check for is if the CompareTo returns 0 (equals) or anything but 0 (not equal).

Another problem is that to implement this function you have to write all the checking code yourself and if you have alot of properties to check you have to do that manually somehow. I wanted something to simply check to see if all properties in the current object and the object passed in through CompareTo function were equal. Here is some code to do that:

// Implement IComparable interface and use this as your function
#region IComparable Members
            public int CompareTo(object obj)
            {
                int return_value = 0;
 
                if (obj.GetType() == this.GetType())
                {
                    Type t = this.GetType();
 
                    foreach (PropertyInfo p in t.GetProperties())
                    {
                        Type t2 = obj.GetType();
 
                        object prop1_value = p.GetValue(this, null);
                        object prop1_name = p.Name;
 
                        foreach (PropertyInfo p2 in t2.GetProperties())
                        {
                            object prop2_value = p2.GetValue(obj, null);
                            object prop2_name = p2.Name;
 
                            if (prop1_name == prop2_name)
                            {
                                if (prop1_value != null && prop2_value != null)
                                {
                                    if (!prop1_value.Equals(prop2_value))
                                    {
                                        return_value++;
                                    }
                                }
                                break;
                            }
                        }
                    }
                }
                else
                {
                    return_value++;
                }
                return return_value;
            }
            #endregion

This uses system.reflection to iterate through the properties in both objects comparing the values using the ‘Equals’ method to validate the values are the same. If the properties are not the same the return_value is incremented and more than 0 is returned. This is pretty generic and should work with most classes you build. You may need to add in an if statement to filter out properties you do not wish to include in this comparison as this just checks all properties in the object.

I recently had the need for printing a Google maps static image map in a Silverlight application. It needed to actually be embedded in the Silverlight app to take advantage of the Silverlight 4 printing library (a topic I will touch on later). I wrote an API to make it easy to do pretty much every thing that the Google static API allows you to do. Here’s a quick demo of what the results look like in silverlight:

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One of the things I needed recently is a simple way to have a textbox that had a watermark in it. Apparantly SilverLight 4 provides it,,, and it doesn’t provide it. Check this post out. I really couldn’t help but laugh when I saw this. ‘Do not use in a Silverlight 4 application.’ and at the same time the supported version is only Silverlight 4. Aaah yes, well the only other solution is to use a watermark textbox someone else has created or to make your own. I opted to make my own. Before we get into the code here it is:

This just gives you a basic idea of what you can do. The first textbox is the user control that has watermark capabilities. If you click on the water marked textbox it will clear it out and be ready for input. If you do not type in any text and leave it blank and click on the second textbox it will replace the watermark. If you click the ‘Get Text’ button it will show blank because you do not want the watermark to count as valid text. If you click ‘Set Text’ it will programatically set the text in the textbox and you will notice that the watermark disappears.

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Reading an xml file is something that every developer has to do at some point (if not regularly). XML protocol is extremely prevalent across all technologies which makes it important to understand how to parse it. Sivlerlight exposes System.XML.Linq namespace that has all the needed classes to use Linq to query your XML to get the node/s that you need.

Always Starts with the Document

As with every XML reader you start out by creating an instance of the document and then filling it with the XML file’s data. In this case Silverlight provides ‘XDocument’ class to instantiate the document object and then the ‘Load’ method to read the data into the object. Then once we’ve loaded the object with data then we can run a traditional Linq query on the data.

Test Case

So lets run through a test case. Lets say we have a configuration file in which we want to determine what logo to display based on the url that called it. So here is our example xml:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <Logos>
    <Logo url="url1">
      <LoginWindowLogo>/namespace;path/img1.png</LoginWindowLogo>
    </Logo>
    <Logo url="url2">
      <LoginWindowLogo>/namespace;path/img2.png</LoginWindowLogo>
    </Logo>
  </Logos>
</configuration>

And here is the C# code to read it:

XDocument document = XDocument.Load("Config.xml");
 
            var images = (from e in document.Descendants("Logo")
                          where e.Attribute("url").Value == App.Current.Host.Source.Host
                          select new
                          {
                              LoginWindowLogo = e.Element("LoginWindowLogo").Value,
                          }).FirstOrDefault();
 
            if (images != null)
            {
                loginwindowlogopath = images.LoginWindowLogo;
            }

App.Current.Host.Source.Host is the property that gives you the current apps domain, the function ‘FirstOrDefault’ get the results and turn it into an object that just has the properties selected.

An advantage of using Silverlight is the fact that the entire app is cached locally. While this may take a while to download initially, after everything is downloaded the app runs very very fast and only requires you download information asynchronously from a web service. The only problem is, what if you make changes? Since the app is cached those changes are sometimes not downloaded automatically. So what to do? Lars Holm Jensen wrote a nifty little post that shows how you can determine when the last time the silverlight package was was deployed by reading the datetime stamp on the ‘xap’ file. Just in case you didn’t click on the link, here’s the code:

<object id="Xaml1" data="data:application/x-silverlight-2," type="application/x-silverlight-2"
width="100%" height="100%">
<%
string orgSourceValue = @"ClientBin/SilverlightApp.xap";
string param;
 
if (System.Diagnostics.Debugger.IsAttached)
param = "<param name=\"source\" value=\"" + orgSourceValue + "\" />";
else
{
string xappath = HttpContext.Current.Server.MapPath(@"") + @"\" + orgSourceValue;
 
DateTime xapCreationDate = System.IO.File.GetLastWriteTime(xappath);
param = "<param name=\"source\" value=\"" + orgSourceValue + "?ignore="
+ xapCreationDate.ToString() + "\" />";
}
 
Response.Write(param);
%>
 
<param name="onError" value="onSilverlightError" />

The ‘if’ statement checks to see if your debugging. If you are, then grab the latest/greatest. If not, then it’s assumed that you are in production mode and then it checks to see if you have the latest and greatest. If you don’t, then download the latest version. Thanks Lars!

One very important task you have as a developer is to keep track of errors. Errors you ask? ‘Errors’? ‘Not me, I write bug free code!’. Ok Mr. Programming God, but for the rest of us mortals, we have to do something to keep track of all the human errors that get embedded into our code. There are many different ways to store these errors, some people do it in flat files, others in databases. We won’t be talking about where to store these errors, just how to get them from silverlight.

The problem

Web applications have always presented a particular problem when it comes to catching errors and logging them in a central place. First, with Javascript it is difficult to know when an error has occurred and hasn’t been caught already and if so, it’s difficult to do anything after that as many javascript errors will break the app entirely and not run anymore code.

The Solution… A Solution

Silverlight attempts to simplify this process by having a central place to catch any unhandled exceptions and then do something with them. You can catch them in the App.xaml.css file. Here is an example:

private void Application_UnhandledException(object sender, ApplicationUnhandledExceptionEventArgs e)
    {
      //Ignores logging if your debugging
      if (!System.Diagnostics.Debugger.IsAttached)
      {
        /*TODO: Some method to save the 
           the error to a file or database by using
           e.ExceptionObject.Message, e.ExceptionObject.StackTrace
        */
        e.Handled = true;
        ChildWindow errorWin = new ErrorWindow("Error", "An Error has been logged"));
        errorWin.Show();
      }
    }

Does this mean that we don’t need to do any try/catches? No, we still need to handle errors and do something with them, but in the event that we miss an error, it will be caught here and logged so that we can more easily debug the released code.

Quick and Easy

A quick and easy way to spawn threads and update the UI from those threads is the following:

new Thread(() =>
               {
 
                   try
                   {
                       Thread.Sleep(3000);
                   }
                   finally
                   {
                       this.Dispatcher.BeginInvoke(() =>
                       {
                           label1.Content = "testing within a thread";
                       });
                   }
               }).Start();

This code starts a new thread and runs it asynchronously to the main thread. The key here is the fact that there is no way to affect the main thread (that the UI is running on) directly from a spawned thread so you have to reference the parent (dispatcher) and call the ‘BeginInvoke’ method and pass it an anonymous function that runs on the same thread that the UI is running on.

Passing Parameters

Sometimes you need to get information in the function you are running. You can do this by passing in parameters into the anonymous function. For example:

new Thread((a) =>
               {
                   try
                   {
                       Thread.Sleep(3000);
                   }
                   finally
                   {
                       this.Dispatcher.BeginInvoke(() =>
                       {
                           label1.Content = (string)a;
                       });
                   }
               }).Start("test");

You have to remember to cast your parameter as it always comes in as an object and requires ‘un-boxing’.

Running external Functions

You can also start a thread on an external void method:

// The code starting the thread
Thread thr = new Thread(new ParameterizedThreadStart(this.test));
            thr.Name = "testthread";
            thr.Start("1");
 
 
       //The void method (object only parameters)
        public void test(object a)
        {
            while (runthread)
            {
                Thread.Sleep(3000);
                this.Dispatcher.BeginInvoke(() =>
                {
                    label1.Content += (string)a;
                });
            }
        }

Notice that in the external method you can only have generic ‘object’ as the parameter and must then cast the data into the appropriate type afterwards.