一篇好文,成功或未成功在客户端使用IE下载Active…

By Conrad Herrmann

25 July 1997

So you’ve just build a nifty new ActiveX control or ActiveForm, and you’re eager to test it out in the field. You type in the information into the Delphi’s Web Deployment options dialog and select the Web Deploy command. Then you go to your coworker’s machine—one on which neither your control nor Delphi have ever been installed—and you go to the test web page Borland’s deployment Wizard generates. After a second or two, your heart jumps as you see the magic words appear:

A bit more crunching, and the screen looks like this:

You’ve just been given the Big Red X—an indication that the Internet Explorer was unable to download or create your control. What happened, and how can you fix it?

Any number of errors could have occurred, but Internet Explorer suppresses all the error messages making it frustratingly difficult to diagnose failures. This is a usability feature.

The idea is that the Internet Explorer shouldn’t pop up error messages that the user isn’t capable of dealing with anyway, and most end-users can’t tell the difference between an Authenticode security violation and the Start button. So, your control’s users will be happy and ignorant—who cares if you get a bald patch prematurely?

This paper presents an overview of the code download process Internet Explorer uses to retrieve and install your control, and presents a diagnostic schema for analyzing what’s wrong in your installation.

In order to create your control in the web browser, Internet Explorer must hav received an HTML file containing an <OBJECT> tag. This tag contains parameters that dictate the first steps in the download process.

For example:

The #version part of the tag is optional—if missing, it means "any version will do." The version tag must not appear if the client is the Netscape Navigator with the NCompass ActiveX plugin.

On the web server, there will be a file called MyControl.htm (many servers are case sensitive) , which has been stamped with a version tag (presumably 1.0.1.0), and which can create an ActiveX control with the specified ClassID. The first thing Internet Explorer does is to determine if the specified ClassID is already installed on the client machine. If it is, IE will check to see if the installed version matches the requested version. If the local version matches the version requested in the HTML, IE will simply use the local copy to create the control.

If the version number requested by the HTML page is newer than the one currently installed, or the ClassID is not yet installed, IE knows it must download and install the codebase before it can create the control.

Downloading and Installing the codebase involves three basic steps:

1. Copy the code base onto the local machine’s Windows/OCCache directory or a subdirectory of it.
2. Check the code signature of the codebase, and
3. Install the codebase.

If this process completes successfully, the machine should have a correctly registered OCX library from which the control can be created.

Copying the codebase

The first step, copying the control to the local machine, will be done by attempting to retrieve the file by the mechanism that the codebase URI specifies. If the URI says use HTTP(the default), then IE will use HTTP to download the control. You can also specify FTP, or FILE URIs.

This step can fail in a nuber of ways—if the remote server can’t be found, if the local disk is out of disk space, if the codebase name is spelled incorrectly (remember, many servers are case-sensitive), etc.

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