Can't add assemble reference to project

Question

I have a web site on Framework 4.5.1. When I compile I have a missing assembly reference error for System.Data.DataExtensions.

So I go to add a reference tick the box close the dialog , recompile. Same assembly is missing again. Go back to references and the assembly is no longer ticked. It does add it the the web.config but the reference always disappears.

I came across this link similar issue but the solution was only having using System.Data instead of using System.Data.DataExtensions. But my using statement are already only System.Data.

Can anyone assist ?

Answer 1

Ok it seems that when the project was migrated to 4.5.1 framework the web config file wasn't update and still contains a reference to the 3.0 framework. That is what was is preventing the assembly reference being added.

Upgrading the framework seems not to update the web config with the changes.