Server error in "/" Application

This problem occurs when you hit ASP. NET for an example of a validation control in the last few days:

However, we should not be unfamiliar with this problem, before the beef brisket has also appeared, then the solution is:

Find in Web. config
<appSettings>
<add key= "Aspnet:usetaskfriendlysynchronizationcontext" value= "true"/>
<add key= "Validationsettings:unobtrusivevalidationmode" value= "WebForms"/>
</appSettings>
Then delete the <add key= "Validationsettings:unobtrusivevalidationmode" value= "WebForms"/>.

Or, if there are no above two sentences: add in appsettings byte
<add key= "Validationsettings:unobtrusivevalidationmode" value= "None"/>

And then run it again.

But why does this problem occur?

Cause: vs creates an empty site without introducing the AspNet.ScriptManager.jQuery.dll file.

So there's another way we can solve this problem: find AspNet.ScriptManager.jQuery.dll and put the file under the bin file for the specified project.

Summary: There are problems, we have to learn to solve, but more to know why it appears, so that we can better understand

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Server error in "/" Application