Question: Hello  

I want to create a 64bit application in c# and I wan't to use in it a dll created in C++ builder (32bit). But when I try to load the dll the application crashes. When built in 32bit it works. Is there a way to use this dll in a 64bit app?

Answer: 
No this is not possible. A process in Windows is either 32 or 64 bit and it can only load DLL's which match. Any attempt to load a DLL which does not match will fail and produce an error. 

If you need to match 32 and 64 bit code you need to use multiple processes. In this case though I would just make the application 32 bit or the DLL 64 bit
Do you want to build a 64-bit application, or you want to build an application that will run in a 64-bit environment? Your application can run perfectly fine in a 64-bit environment even when the target platform is x86.