When using LINQ with C# and adding a Join method to an expression, there is no Intellisense working for the second lambda (fourth parameter). It isn't picking up that the type of the lambda variable is the base type from the first parameter.
Well, more correctly, it knows the type of the variable but provides no intellisense for the members of the variable, nor does it list any members in the Ctrl-Space menu.
This is caused by a bug in the Roslyn APIs:https://github.com/dotnet/roslyn/issues/38041
Given that it's been fixed in the Roslyn source code, the fix will find its way into LINQPad in due course.
Should you want to confirm this, go to Edit | Preferences > Query and click "Use Roslyn Daily Build". This will download the latest Roslyn beta assemblies from the daily build server and bind them to LINQPad.