How to Debug System.NullReferenceException When Running Transformation?

I am attempting to generate code using T4 Text Templating, but when running the script, I get the error below:

Running transformation: System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.VisualStudio.TextTemplatingB0A58A4C85EA3D7032675015C6052C89.GeneratedTextTransformation.TransformText()
   at Microsoft.VisualStudio.TextTemplating.TransformationRunner.RunTransformation(TemplateProcessingSession session, String source, ITextTemplatingEngineHost host, String& result)

As I am unfamiliar with T4, I'm not sure exactly where to look to resolve this issue.

You'll need to debug your template to find where the NullReferenceException is occurring. Tim Larson has quick overview here and Oleg Sych has a more details here, along with his other excellent blog entries on T4.

Here's the short-short version:

1. Add debug="true" to template directive: <#@ template debug="true" #>
2. Launch debugger System.Diagnostics.Debugger.Launch();
3. Break System.Diagnostics.Debugger.Break();
4. Select New instance of Microsoft Visual Studio in Visual Studio Just-In-Time Debugger dialog and click Yes
5. Debug your T4 template

Here is a simple example to help catch the NullReferenceException when calling ToString on bar:

<#@ template debug="true" language="C#" #>
<#@ output extension=".txt" #>
<#
    System.Diagnostics.Debugger.Launch();
    System.Diagnostics.Debugger.Break();

    object bar = null;  
#>
foo<#= bar.ToString() #>

Be sure to check first link though since on some versions you'll need to update the registry key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\DbgJITDebugLaunchSetting to 0x2 to get things to behave correctly.