Advanced Topics in .NET

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Advanced Topics
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The simple solution to this problem is to disable client access during the few seconds that you need to replace all the files in the application. However, you can never be sure that you won t stop the server while a user is in the middle of a download. Besides, not all users are willing to switch to a new version without being warned in advance, espe cially if the new version has a slightly different user interface or behavior; therefore, you need another, more flexible solution to this problem. An easy way to solve the versioning problem relies on a small application on the client whose purpose is to read an XML file on the server that keeps track of the most recent versions of the application. Having this launcher program on the client partially defies the usefulness of no-touch deployment, but greatly simplifies your job and opens up a few interesting options. To illustrate, let s suppose that you have both version 1.0 and 1.1 of the HttpFormDemo.exe sample application. You should deploy these versions in two distinct subdirectories under the main application directory on the server for example, in Ver10 and Ver11. The XML file containing versioning information can be as simple as this one:
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< xml version="1.0 encoding="utf-8 > <configuration> <versioning latestVersion="1.1"> <version number="1.0"> <url>Ver10/HttpFormDemo10.exe</url> </version> <version number="1.1"> <url>Ver11/HttpFormDemo11.exe</url> </version> </versioning> </configuration>
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You should name this file versioninfo.xml and deploy it in the virtual directory. The complete directory structure on the server now looks like this:
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C:\MyApplication (accessible from clients as http://yourservername/HttpDemo/) versioninfo.xml Ver10 (folder) HttpFormDemo10.exe Ver11 (folder) HttpFormDemo11.exe
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The launcher program that clients must run is a Windows Forms application that doesn t display any window. (You can t use a console application because its empty console window would disorient users.) It connects to the specified Web site, reads the versioninfo.xml configuration file, reads the URL for the most recent version, and exe cutes that assembly. Here s the complete source code for a simple launcher:
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Imports System.Net Imports System.IO Imports System.Xml
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34:
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Module Module1 Const BASEURL As String = http://localhost/HttpDemo/" Sub Main() Request the versioninfo.xml file. Dim versionUrl As String = BASEURL & versioninfo.xml" Dim webReq As WebRequest = WebRequest.Create(versionUrl) Dim webRes As WebResponse = webReq.GetResponse() Read its contents as XML text. Dim xmlDoc As New XmlDocument xmlDoc.Load(webRes.GetResponseStream) Get the latest version number, using an XPath query.
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Dim node As XmlNode = xmlDoc.SelectSingleNode( //configuration/versioning )
Dim latestVersion As String = node.Attributes( latestVersion ).Value
Read information about the latest version, using an XPath query.
Dim xpathExpr As String = //configuration/versioning/version[@number= _
& latestVersion & ]/url" Dim node2 As XmlNode = xmlDoc.SelectSingleNode(xpathExpr) Dim url As String = CType(node2, XmlElement).InnerText Convert to absolute path if needed. If Not url.StartsWith( / ) Then url = BASEURL & url Run the assembly from there. Dim appDom As AppDomain = AppDomain.CreateDomain( newappdom ) appDom.ExecuteAssembly(url, Nothing, Nothing) End Sub End Module
Except for the WebRequest and WebResponse objects, which I ll cover later in this chapter, this is all familiar stuff that I won t explain in more detail. (You might want to review the XPath search expressions in 23.) Thanks to this launcher program, you can now prepare a new set of application files and store them in a separate direc tory, and then replace only the versioninfo.xml file with a new version. Replacing this XML file can be considered an atomic operation, and clients will never download files from mixed versions. (You don t even need to temporarily stop the Web server while you update these files.) You can improve this mechanism in many ways. For example, if you believe it s safe to do so, you can pass an evidence object to the ExecuteAssembly method to load the assembly as if it were executing from a local disk (which would make it fully trusted). In another improvement, the launcher might take the value of BASEURL from the com mand line so that you can reuse the same launcher with all the Windows Applications you run from an HTTP address. The launcher can also keep track of the version num ber used in the last version. If a newer version is available, the launcher can inform the end user and ask whether the newer version should be used. As I mentioned previously, the launcher must be installed on a local drive and run from there. This requirement stems from the implicit permission demand that fires when the launcher creates another AppDomain. This permission is necessary only when loading
Part VII:
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