An MSBuild toolset includes a microsoft.common.tasks file, a microsoft.common.targets file, and compilers such as csc.exe and vbc.exe . Most toolsets can be used to compile applications to more than one version of the .NET Framework and more than one system platform. However, the MSBuild 2.0 Toolset can be used to target only the .NET Framework 2.0.

The MSBuild ToolsVersion attribute on the Project element in Visual Studio and MSBuild project files is considered obsolete in Visual Studio 2019 and later; you can safely delete it. This article describes its use in older versions of MSBuild, or for custom toolsets. See Standard and custom Toolset configurations .

ToolsVersion attribute

Specify the Toolset in the ToolsVersion attribute on the Project element in the project file. The following example specifies that the project should be built by using the MSBuild "Current" Toolset.

<Project ToolsVersion="Current" ... </Project>
Some project types use the sdk attribute instead of ToolsVersion. For more information, see Additions to the csproj format for .NET Core.
How the ToolsVersion attribute works
When you create a project in Visual Studio, or upgrade an existing project, an attribute named ToolsVersion is automatically included in the project file and its value corresponds to the version of MSBuild that is included in the Visual Studio edition. For more information, see Framework targeting overview.
When a ToolsVersion value is defined in a project file, MSBuild uses that value to determine the values of the Toolset properties that are available to the project. One Toolset property is $(MSBuildToolsPath), which specifies the path of the .NET Framework tools. Only that Toolset property (or $(MSBuildBinPath)), is required.
Starting in Visual Studio 2013, the MSBuild Toolset version is the same as the Visual Studio version number. MSBuild defaults to this Toolset within Visual Studio and on the command line, regardless of the Toolset version specified in the project file.  This behavior can be overridden by using the -ToolsVersion flag. For more information, see Override ToolsVersion settings.
In the following example, MSBuild finds the Microsoft.CSharp.targets file by using the MSBuildToolsPath reserved property.
<Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
You can modify the value of MSBuildToolsPath by defining a custom Toolset. For more information, see Standard and custom Toolset configurations.
When you build a solution on the command line and specify a ToolsVersion for msbuild.exe, all projects and their project-to-project dependencies are built according to that ToolsVersion, even if each project in the solution specifies its own ToolsVersion. To define the ToolsVersion value on a per project basis, see Overriding ToolsVersion settings.
The ToolsVersion attribute is also used for project migration. For example, if you open a Visual Studio 2008 project in Visual Studio 2010, the project file is updated to include ToolsVersion="4.0". If you then try to open that project in Visual Studio 2008, it doesn't recognize the upgraded ToolsVersion and therefore builds the project as though the attribute was still set to 3.5.
Visual Studio 2010 and Visual Studio 2012 use a ToolsVersion of 4.0. Visual Studio 2013 uses a ToolsVersion of 12.0. Visual Studio 2015 uses ToolsVersion 14.0, and Visual Studio 2017 uses ToolsVersion 15.0. In many cases, you can open the project in multiple versions of Visual Studio without modification. Visual Studio always uses the correct Toolset, but you will be notified if the version used does not match the version in the project file. In almost all cases, this warning is benign as the Toolsets are compatible in most cases.
Sub-toolsets, which are described later in this topic, allow MSBuild to automatically switch which set of tools to use based on the context in which the build is being run. For example, MSBuild uses a newer set of tools when it's run in Visual Studio 2012 than when it's run in Visual Studio 2010, without your having to explicitly change the project file.
Toolset implementation
Implement a Toolset by selecting the paths of the various tools, targets, and tasks that make up the Toolset. The tools in the Toolset that MSBuild defines come from the following sources:
The .NET Framework folder.
Additional managed tools.
The managed tools include ResGen.exe and TlbImp.exe.
MSBuild provides two ways to access the Toolset:
By using Toolset properties
By using ToolLocationHelper methods
Toolset properties specify the paths of the tools. Starting in Visual Studio 2017, MSBuild no longer has a fixed location. By default, it is located in the MSBuild\15.0\Bin folder relative to the Visual Studio installation location. In earlier versions, MSBuild uses the value of the ToolsVersion attribute in the project file to locate the corresponding registry key, and then uses the information in the registry key to set the Toolset properties. For example, if ToolsVersion has the value 12.0, then MSBuild sets the Toolset properties according to this registry key: HKLM\Software\Microsoft\MSBuild\ToolsVersions\12.0.
These are Toolset properties:
MSBuildToolsPath specifies the path of the MSBuild binaries.
SDK40ToolsPath specifies the path of additional managed tools for MSBuild 4.x (which could be 4.0 or 4.5).
SDK35ToolsPath specifies the path of additional managed tools for MSBuild 3.5.
Alternately, you can determine the Toolset programmatically by calling the methods of the ToolLocationHelper class. The class includes these methods:
GetPathToDotNetFramework returns the path of the .NET Framework folder.
GetPathToDotNetFrameworkFile returns the path of a file in the .NET Framework folder.
GetPathToDotNetFrameworkSdk returns the path of the managed tools folder.
GetPathToDotNetFrameworkSdkFile returns the path of a file, which is typically located in the managed tools folder.
GetPathToBuildTools returns the path of the build tools.
See also
Standard and custom Toolset configurations
Multitargeting