tfsvars.targets

<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">  
<UsingTask TaskName="SetTFSVariable" TaskFactory="CodeTaskFactory" AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.v4.0.dll">

  <ParameterGroup>
    <Name ParameterType="System.String" Required="true" />
    <Value ParameterType="System.String" Required="true" />
  </ParameterGroup>

  <!-- https://github.com/Microsoft/vsts-tasks/blob/master/docs/authoring/commands.md -->
  <Task>
    <Using Namespace="System" />
    <Code Type="Fragment" Language="cs">
      <![CDATA[
        Log.LogMessage(MessageImportance.Normal, "##vso[task.setvariable variable={0};]{1}", Name, Value);
      ]]>
    </Code>
  </Task>

</UsingTask>
</Project>