If you have ever tried to mix a Java JAR into a .NET app — or call a .NET DLL from Java — you quickly hit the same wall: the files look like libraries, but the runtimes do not understand each other. For developers, the useful question is architectural: do you wrap a process, expose REST/gRPC, drop into JNI/PInvoke, translate bytecode, or use an in-process bridge? Here are the tradeoffs a working team actually needs to choose between them. TL;DR / Key Takeaways – You cannot directly reference a JAR in C# or import a .NET DLL in Java — the runtimes are incompatible. – Process wrapping is the fastest to set up but slowest at runtime (~50–200 ms per call). – REST/gRPC wrappers work well for loosely coupled, low-frequency calls. – IKVM converts Java bytecode to .NET IL but is stuck on Java 8 with no commercial support. – JNBridgePro provides in-process bridging at ~8 µs per call with full object access and enterprise support.…