If you're inquiring about using the "built-in" Microsoft VB functions from
C# then you'll need to add a reference to the "Microsoft.VisualBasic.dll"
assembly. From there you'll be able to access certain functions such as
MsgBox...
Microsoft.VisualBasic.Interaction.MsgBox(...);
If you're asking how to call you're own functions that you've authored in
VB.NET, then you'll need to compile them into a class lib project and then
add the resulting assembly as a reference in your C# project.