R – ny good replacement for SharePoint Designer

If you open the .aspx file and switch between design view and html view and back it will prompt VS to check the controls and add any that are missing to the designer file.

In VS2013-15 there is a Convert to Web Application command under the Project menu. Prior to VS2013 this option was available in the right-click context menu for as(c/p)x files. When this is done you should see that you now have a *.Designer.cs file available and your controls within the Design HTML will be available for your control.

PS: This should not be done in debug mode, as not everything is "recompiled" when debugging.

Some people have also reported success by (making a backup copy of your .designer.cs file and then) deleting the .designer.cs file. Re-create an empty file with the same name.

There are many comments to this answer that add tips on how best to re-create the designer.cs file.

I only use SharePoint Designer when I want to see the effect of a change (e.g. to a master page) immediately, without having to retract/deploy a solution just to do so.

While it takes longer, I author all my HTML in Visual Studio, and paste into SharePoint Designer if I need to. I NEVER paste from SharePoint Designer back into Visual Studio, since the tool tends to butcher the HTML and add to it just so it can open it.

Having said that, it's important to understand when (and when not) to use SharePoint Designer. You should be creating SharePoint Solutions for all your assets, e.g. master pages and CSS files.

If you open and edit one of these in SharePoint Designer, remember that you are unghosting/customizing them. This means that the file now lives in the content database instead of on disk. This might affect the performance of your website.

Also remember that SharePoint Designer doesn't understand server side code. So if you have a Page Layout that has a code behind, editing it in SharePoint Designer will break it.