This seems to be a hard/repeatable failure when mixing F#, C# projects in a VS2008 solution. Following is a step by step recipe for reproducing the problem(s).

1.) Using VS2008 (Version 9.0.30729. 1 SP), Win XP Pro SP1, create a new solution with 2 projects:

o CsharpLib (a c# library)
o FSharpLib (an f# library)

2.) To the references of CSharpLib add a reference to the FSharpLib *project*.
o This should work OK.
3.) Remove the reference created in 2.

4.) To the references of FSharpLib add a reference to the CSharpLib *project*.
o This pops an error dialog "The given key was not present in the dictionary", but the reference is added in spite of this.
5.) Remove the reference created in 4.

6.) Right click the solution node and select Add.[New Solution Folder].
o New folder is named NewFolder1
7.) Drag and drop CSharpLib project into the new folder.
8.) To the references of FSharpLib add a reference to the CSharpLib project.
o Operation fails with error dialog "Object reference not set to an instance of an object."

9.) To the references of the CSharpLib add a reference to the FSharpLib
o This works as expected.

So, referencing a C# Project from an F# project is problematic (case 4) and fails entirely when the C# project is in a solution folder.

This seems to be a fairly major barrier to using VS2008 to develop mixed language solutions involving C# and F# -- at least it's a barrier for us!

Any suggestions/fixes?

Thanks in advance,

Bill