All Visual Studio 2013 builds are now failing

Question

Every single project I open in Visual Studio 2013 is now all of a sudden giving me the following error:

EverySingleProject.csproj : error : The default XML namespace of the project must be the MSBuild XML namespace. If the project is authored in the MSBuild 2003 format, please add xmlns="http://schemas.microsoft.com/developer/msbuild/2003" to the element. If the project has been authored in the old 1.0 or 1.2 format, please convert it to MSBuild 2003 format. C:\Program Files (x86)\MSBuild\12.0\bin\Microsoft.Common.CurrentVersion.targets

This is in the file:

Project ToolsVersion="12.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003"

Why is this failing?

Answer 1

You also get this error if you try and open a VS 2017 RC 'simplified' .NET Core csproj file in an earlier version of Visual Studio.

These new project files look like this:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp1.0</TargetFramework>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.NETCore.App" Version="1.0.1" />
  </ItemGroup>
</Project>

(From: https://blogs.msdn.microsoft.com/dotnet/2016/12/12/updating-visual-studio-2017-rc-net-core-tooling-improvements/)

You'll need to use a recent version of VS 2017 to open them.

Answer 2

I've had this like 2 days ago.

This saved me.

Hope this helps.

EDIT:

Fortunately the solution is simple:

1. In Windows Explorer navigate to the project
2. Right Click on the .cproj file, select Properties, and un-check the "Read Only" checkbox
3. Open up the .cproj file in Notepad
4. On line 2 change xmlns="http://schemas.microsoft.com/developer/msbuild/2008" to xmlns="http://schemas.microsoft.com/developer/msbuild/2003" (notice this only difference is we changed 2008 to 2003)
5. Save your changes
6. In Visual Studio right click on the (currently unavailable) project and select "Reload Project"
7. The project will now load normally and you can get on with your life