The default XML namespace of the project must be the MSBuild XML namespace

The projects you are trying to open are in the new .NET Core csproj format. This means you need to use Visual Studio 2017 which supports this new format.

For a little bit of history, initially .NET Core used project.json instead of *.csproj. However, after some considerable internal deliberation at Microsoft, they decided to go back to csproj but with a much cleaner and updated format. However, this new format is only supported in VS2017.

If you want to open the projects but don’t want to wait until March 7th for the official VS2017 release, you could use Visual Studio Code instead.

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