How does CheckAndDownloadUpdateAsync actually work behind the scenes?

Question

它会下载所有二进制文件还是仅下载一部分二进制文件，以便评估二进制文件是否为新版本？

Answer 1

In short, it evaluates the XAP, which contains all binaries, but if the version on the XAP hasn't changed it doesn't see any changes.

Here is a bit more light on that method (lifted from here but pasted for historical reasons):

Features:

• Detect network connectivity (and sometimes it fails miserably…)

• Connect to the original authorized URL that it was installed from

• Download the new XAP file and compare the current version against the downloaded version from the manifest

• Detect the current Silverlight version vs. the new version's Silverlight version

• If a failure occurs, failure exception types are provided for recovery such as “PlatformNotSupportedException”

Limitations:

• Can't interrupted the request. So when it times out, we wait for it.

• Can't download the update and make it optional to install and replace the currently running XAP. A flag to just detect a newer version would be better. This would allow the UI to show the current version and available update version.

• Returns a false for the “UpdateAvailable” property for several reasons such as the new XAP is not signed, is a newer Silverlight Version, or various other errors. We must then look at ALL of the possible error class types placed in the error collection. A bunch of try-catches are therefore necessary. The try-catches do the job as long as we have every possible error type in a catch. An enum for the actual error reason may be better.

• Can't revert to a previous stable version and have it install over a newer bad version. It makes sense, but real development teams have recovery plans when updating production versions.