Earlier today I noticed that Visual Studio 2019 (v16.11.5) could not load any of my .NET Core projects projects. When I tried to reload any of them, I got an error saying that "The project file cannot be opened. Unable to locate the .NET SDK" in the output pane, and then nothing happened.
A google search yielded suggestions saying that the version 2.x.xxx .NET Core SDK needed to be installed. Some said that "C:\Program Files\dotnet\" needed to be part of the "PATH" environment variable. Another said that an old "global.json" file was the cause. While each of these solutions worked for some folks, none seemed to work for me. I ran "dotnet --info" on the command line, and this is what I saw:
"No SDKs were found." Then I noticed a comment about x86 version of dotnet at https://superuser.com/questions/1431833/visual-studio-2019-unable-to-locate-net-core-sdk/1441219. I realized that I was running the x86 version of dotnet. Of course, in hindsight, the "C:\Program Files (x86)" paths in the runtime locations should have been a clear indication of the problem. Then I tried running the 64 bit version:
Well, now a whole list of SDKs showed up. It turned out that the paths to both the x86 version of dotnet and the 64 bit dotnet were in my "PATH" environment variable, and the x86 version was listed first. I do not know which installer made this change, for the projects were working fine at one time. I removed the path to the x86 version of dotnet from the "PATH" environment variable, and now everything works fine.
One interesting side-note: Window 11 complained about the size of the "PATH" environment variable, saying that its length had exceeded 2048. I replaced some folder paths with environment variable references, and reduced the length to below 2048. I wondered: when installer apps increased the "PATH" environment variable length to beyond 2048, why did not Windows complain? Would Windows just ignore the part after the first 2048 characters? Or would Windows use the whole "PATH" environment variable, even though its length exceeded 2048, just because some installer app did this? It turns out that a "PATH" environment variable longer than 2048 can cause real problems: https://superuser.com/questions/355594/windows-7s-path-and-environment-variables-are-corrupted, at least for older versions of Windows. For Windows 10, there is a discussion here: https://stackoverflow.com/questions/34491244/environment-variable-is-too-large-on-windows-10. From the look of the error dialog box, this should apply to Windows 11 as well.
