Actually, it turns out you can redirect URI requests by registering custom prefixes in Silverlight 3 and later, here's a sample I've worked up that should get you started. You will notice two buttons, one that downloads the map files into isolated storage, and the other that will load the shape files from isolated storage using the ShapeFileReader:

The Xaml:

 <Grid x:Name="LayoutRoot">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition />
        </Grid.RowDefinitions>

        <Button Grid.Row="0" 
                x:Name="download" 
                Click="download_Click" 
                Content="Download Map" />

        <Button Grid.Row="1" 
                x:Name="loadFromIso" 
                Click="loadFromIso_Click" 
                Content="Load from ISO" />

        <igMap:XamWebMap Grid.Row="2"
                         x:Name="theMap">
            <igMap:XamWebMap.Layers>
                <igMap:MapLayer x:Name="layer1" />
            </igMap:XamWebMap.Layers>
        </igMap:XamWebMap>
    </Grid>

The code behind:

public partial class MainPage : UserControl
    {
        public MainPage()
        {
            InitializeComponent();

            WebRequest.RegisterPrefix("iso",
                new IsoStorageRequestCreator());
        }

        private void download_Click(object sender, RoutedEventArgs e)
        {
            wc.OpenReadCompleted += 
                new OpenReadCompletedEventHandler(wc_OpenReadCompleted);
            wc.OpenReadAsync(new Uri("ShapeFiles/usa_st.shp", 
                UriKind.RelativeOrAbsolute), "usa_st.shp");
        }

        private WebClient wc = new WebClient();

        void wc_OpenReadCompleted(object sender, OpenReadCompletedEventArgs e)
        {
            using (IsolatedStorageFile isf = 
                IsolatedStorageFile.GetUserStoreForApplication())
            {
                using (IsolatedStorageFileStream isfs =
                    new IsolatedStorageFileStream(e.UserState.ToString(),
                        FileMode.Create, isf))
                {
                    using (StreamWriter sw = new StreamWriter(isfs))
                    {
                        byte[] buffer = new byte[4096];
                        int bytesRead;

                        while ((bytesRead = 
                            e.Result.Read(buffer, 0, buffer.Length)) != 0)
                        {
                            isfs.Write(buffer, 0, bytesRead);
                        }
                    }
                }
            }

            if (e.UserState.ToString().EndsWith("shp"))
            {
                wc.OpenReadAsync(new Uri("ShapeFiles/usa_st.dbf", 
                    UriKind.RelativeOrAbsolute), "usa_st.dbf");
            }
        }

        private void loadFromIso_Click(object sender, RoutedEventArgs e)
        {
            ShapeFileReader reader = new ShapeFileReader();
            reader.Uri = "iso://usa_st";

            theMap.Layers[0].Reader = reader;
            theMap.Layers[0].Imported += 
                new MapLayerImportEventHandler(MainPage_Imported);
            theMap.Layers[0].ImportAsync();
        }

        void MainPage_Imported(object sender, MapLayerImportEventArgs e)
        {
            theMap.WindowFit();
        }
    }

The important line here being:

WebRequest.RegisterPrefix("iso",
                new IsoStorageRequestCreator());

Which will register the iso:// prefix with our custom request creator which is defined as such:

    public class IsoStorageRequestCreator
        : IWebRequestCreate
    {
        #region IWebRequestCreate Members

        public WebRequest Create(Uri uri)
        {
            return new IsoStorageRequest(uri);
        }

        #endregion
    }

Which this will make sure that for the iso scheme the WebClient will create IsoStorageRequests.
The IsoStorageRequests will, in turn, create IsoStorageResponses, which will return streams for
the appropriate files in Isolated storage.

 public class IsoStorageRequest : WebRequest
    {
        public override Uri RequestUri
        {
            get
            {
                return requestUri;
            }
        }
        private readonly Uri requestUri;

        public IsoStorageRequest(Uri uri)
        {
            this.requestUri = uri;
        }

        public override void Abort()
        {
            throw new NotImplementedException();
        }

        public override IAsyncResult BeginGetRequestStream(
            AsyncCallback callback, object state)
        {
            throw new NotImplementedException();
        }

        public override IAsyncResult BeginGetResponse(
            AsyncCallback callback, object state)
        {
            IsoStorageAsyncResult result = 
                new IsoStorageAsyncResult() { AsyncState = state };
            callback(result);
            return result;
        }

        public override string ContentType
        {
            get
            {
                throw new NotImplementedException();
            }
            set
            {
                throw new NotImplementedException();
            }
        }

        public override Stream EndGetRequestStream(
            IAsyncResult asyncResult)
        {
            throw new NotImplementedException();
        }

        public override WebResponse EndGetResponse(
            IAsyncResult asyncResult)
        {
            return new IsoStorageResponse(requestUri);
        }

        public override WebHeaderCollection Headers
        {
            get
            {
                throw new NotImplementedException();
            }
            set
            {
                throw new NotImplementedException();
            }
        }

        public override string Method
        {
            get
            {
                throw new NotImplementedException();
            }
            set
            {
                throw new NotImplementedException();
            }
        }
    }

    public class IsoStorageResponse : WebResponse
    {
        private Uri uri;
        private long contentLength = 0;

        public IsoStorageResponse(Uri uri)
        {
            this.uri = uri;
        }

        public override long ContentLength
        {
            get
            {
                return contentLength;
            }
        }

        public override string ContentType
        {
            get
            {
                return @"application/octet-stream";
            }
        }

        public override Stream GetResponseStream()
        {
            using (IsolatedStorageFile isf =
                IsolatedStorageFile.GetUserStoreForApplication())
            {
                string file = uri.AbsoluteUri.Replace("iso://", "");
                if (file.EndsWith("/"))
                {
                    file = file.Substring(0, file.Length - 1);
                }

                using (IsolatedStorageFileStream isfs =
                    new IsolatedStorageFileStream(
                        file, FileMode.Open, isf))
                {
                    contentLength = isfs.Length;

                    byte[] buffer = new byte[4096];
                    int bytesRead;

                    MemoryStream mem = new MemoryStream();

                    while ((bytesRead = 
                        isfs.Read(buffer, 0, buffer.Length)) != 0)
                    {
                        mem.Write(buffer, 0, bytesRead);
                    }

                    mem.Seek(0, SeekOrigin.Begin);
                    return mem;
                }
            }
        }

        public override void Close()
        {
            throw new NotImplementedException();
        }

        public override Uri ResponseUri
        {
            get { return uri; }
        }
    }

    public class IsoStorageAsyncResult
        : IAsyncResult
    {
        #region IAsyncResult Members

        private object _asyncState;

        public object AsyncState
        {
            get
            {
                return _asyncState;
            }
            set
            {
                _asyncState = value;
            }
        }

        public System.Threading.WaitHandle AsyncWaitHandle
        {
            get
            {
                throw new NotImplementedException();
            }
        }

        public bool CompletedSynchronously
        {
            get { throw new NotImplementedException(); }
        }

        public bool IsCompleted
        {
            get { return true; }
        }

        #endregion
    }

Let me know if you have any questions.

-Graham

Hi

I'm not able to use your sourcecode. I add to my WebRequest prefix iso but I never enter in

1.     public WebRequest Create(Uri uri)   
2.     {   
3.         return new IsoStorageRequest(uri);   
4.     }  

Do you have any idea?

Thnaks

First of all, are your trying this with the map or are you trying to use it with something else in Silverlight? Unfortunately many Silverlight components like the Image control seem like they make their web requests with lower level functionality that is not possible to redirect with the above logic. Since we request the shapefiles using a higher level API than, say, the image control, the above redirect functions to insert the custom request handler when you use the iso prefix.

If you are trying this with the map, make certain you are adding the prefix before the map would be initialized. You may even want to add it to the application initialization.