This was way harder to figure out than it should have been, so for the next person that needs to reference an input file in a T4 template by its relative path, here is what you need to do…

1. Include T4Toolbox.tt…

<#@ include file=”T4Toolbox.tt” #>

2. Add hostspecific=”True” to your template setting…

<#@ template language=”C#” hostspecific=”True” #>

3. Import EnvDTE…

<#@ import namespace=”EnvDTE” #>

4. Now TransformationContext.Host.ResolvePath(“SomeFile.sdf”) will return the absolute path of “SomeFile.sdf”.