how to make string value accessible from one area to another in C# Silverlight App

Question

*Update: making the userIP static seemed to work. However, I learned that MainPage() is exectuting before Application_Startup(), so the InitParam values aren't immediately available. I may have to put my code somewhere else.*

I'm writing a Silverlight App and am taking in a variable in InitParams and I want that variable to be accessible in some way to other areas of my code. I'd prefer not to immediately assign the value to an element in XAML and instead use C# if possible. I have to do another step before I use the data to modify the XAML. Here's what I did so far:

In my App.xaml.cs file, I added a string userIP to the App class in hopes of accessing this value later. I then try to assign the value of the InitParams variable to the userIP string that I made above. Here's how it looks.

namespace VideoDemo2
{
    public partial class App : Application
    {
        public string userIP;
        public App()
        {
            this.Startup += this.Application_Startup;
            this.Exit += this.Application_Exit;
            this.UnhandledException += this.Application_UnhandledException;

            InitializeComponent();
        }

        private void Application_Startup(object sender, StartupEventArgs e)
        {
            this.RootVisual = new MainPage();
            this.userIP = e.InitParams["txtUserIP"];
        }
...}

The only lines I added to the code were public string userIP; and this.userIP = e.InitParams["txtUserIP"];. I'm wondering if this is the right way to go about this to make this data available later.

In my MainPage.xaml.cs file, I'm trying to reference the userIP value I specified earlier, but I can't figure out how to do so. For example, I want to create a new string and then set it to be equal to the userIP:

public MainPage()
{
    InitializeComponent();
    string myUserIP;
    myUserIP = VideoDemo2.App.userIP;
}

Then I get an error that says: Error 1 An object reference is required for the non-static field, method, or property 'VideoDemo2.App.userIP'.

I have to do something with the InitParams in App.xaml.cs because that's where the arguments are passed, but I want to make one of those parameters available to other parts of my application, without putting it in the XAML, if possible. What needs to happen so that I can "see" the value later on in the application? I'm brand new to C#, so any help would be very much appreciated.

Answer 1

The problem is that VideoDemo2.App is not an instance, but is a type.

If you want to access userIP, you need to access the actual instance of your app.

Silverlight provides a static property that exposes the current application instance:

App runningApp = (VideoDemo2.App)Application.Current;
string myUserIP = runningApp.userIP;

Or, you could make userIP a static string in App. You would remove "this" where it is set, but you could access it on the type from anywhere.

namespace VideoDemo2
{
    public partial class App : Application
    {
        public static string userIP;
        public App()
        {
            this.Startup += this.Application_Startup;
            this.Exit += this.Application_Exit;
            this.UnhandledException += this.Application_UnhandledException;

            InitializeComponent();
        }

        private void Application_Startup(object sender, StartupEventArgs e)
        {
            this.RootVisual = new MainPage();
            App.userIP = e.InitParams["txtUserIP"];
        }
...}

Answer 2

You can find your instance of the App class in the static property Application.Current. Try this from anywhere in your SL app:

((VideoDemo2.App)Application.Current).userIP;

EDIT: As an aside though, I would prefer to keep settings like this in a singleton class (its an easy pattern to implement in C#, quick search should be enough). Also for "good form" anything that's public should be a property and public properties should be pascal cased. Change the public string userIp to:

public string UserIP { get; set; }

And you'll win friends and influence people in no time ;)

EDIT: This is a good read about the lifecycle of the Application class in Silverlight http://msdn.microsoft.com/en-us/library/system.windows.application%28VS.95%29.aspx

Answer 3

Both FlySwat and James Cadd's answers were helpful, but I found that for within Silverlight, using the Application's resource dictionary works best.

In the ASPX or HTML page, use the following <param> tag within your Silverlight <object> tag:

<param name="initParams" value="txtSomeVariable=SomeValue"/>

Then in the Application_Startup method of App.xaml.cs, use a foreach loop to add the values to the resource dictionary:

    private void Application_Startup(object sender, StartupEventArgs e)
    {
        //Method 1 - Resource Dictionary
        if (e.InitParams != null)
        {
            foreach (var item in e.InitParams)
            {
                this.Resources.Add(item.Key, item.Value);
            }
        }
        this.RootVisual = new MainPage();
    }

To pull the value from the dictionary during the life of your application, simply use:

App.Current.Resources["txtSomeVariable"].ToString();

I learned about InitParams and the Application Resource Dictionary from Tim Heuer's video presentation at Silverlight.Net: Using Startup Parameters with Silverlight.

Also, I wrote a blog post describing this situation in more detail: Pass the IP Address of a User to Silverlight as a Parameter.

I hope this information helps other users who stumble across this question!