T4 Include file path from project root
You should use $(ProjectDir)
<#@ include file="$(ProjectDir)\Infrastructure\Manager.ttinclude" #>
You can also use $(SolutionDir)
for the Solution root.
if you want to include file in some shared project then you can use below
<#@ include file="..\AnotherProjectFolderName\AnotherSubFolder\Shared.ttinclude" #>
First .. will be resolved to current file path So if .tt is under any sub folder of project directory then you can use
<#@ include file="..\..\AnotherProjectDirectory\AnotherSubFolder\Shared.ttinclude" #>
One ..\ means one folder level up.