If you are searching for call C# from Java jni4net, you are looking at one of jni4net’s strongest historical use cases. Unlike one-way Java-to-.NET conversion tools, jni4net was designed as a bidirectional bridge. It could expose .NET classes to Java through generated proxies, so Java code could call C#/.NET code without rewriting the .NET side.

That idea is still valuable. Many enterprises have Java applications that need to reuse .NET business logic, call a C# DLL, reach a .NET service library, or preserve years of C# code during modernization.

The question is not whether Java-to-.NET calls matter. They do. The question is whether jni4net is the right foundation for that work in production today.

For old Windows/.NET Framework prototypes, jni4net may still be enough. For production Java-to-.NET interoperability across modern runtimes, supported deployment models, and enterprise operations, JNBridgePro is the safer bridge.

How jni4net calls .NET from Java

jni4net’s Java-to-.NET model is clever. The original project materials describe generated Java proxies with native methods that connect back to .NET implementations (Java to .NET design). SourceForge describes jni4net as a fast, object-oriented, intraprocess bridge between JVM and CLR, and the project is explicitly bidirectional (SourceForge).

That means it is not accurate to say jni4net cannot call C# from Java. It can. If you have a compatible old stack, generated proxies, correct native libraries, and a narrow local integration boundary, it may work.

The problems usually appear around everything surrounding that basic bridge:

• old release history;
• Windows/in-process assumptions;
• native DLL loading;
• x86/x64 matching;
• classpath and generated proxy packaging;
• .NET Framework-era expectations;
• limited modern Java/.NET support signals;
• lack of commercial support for production incidents.

When Java calling C# becomes business-critical, those surrounding issues matter as much as the method call itself.

Common Java-to-.NET use cases

Teams usually need to call .NET from Java for practical reasons:

• reuse an existing C# DLL from a Java application;
• call .NET pricing, risk, workflow, reporting, or rules code;
• expose .NET business logic to a Java service layer;
• migrate gradually without rewriting C# logic in Java;
• integrate Java applications with .NET-only vendor libraries;
• preserve tested C# code while modernizing the Java side.

A bridge is attractive because it avoids a rewrite. Java developers can keep Java code. .NET developers can keep C# code. The bridge becomes the connection point.

That is also why supportability matters. If the bridge fails, two teams and two runtime ecosystems are affected.

Where jni4net becomes risky for Java calling C#

jni4net’s public project signals point to a legacy project. The latest GitHub release is 0.8.8.0 from September 2014, and public docs/build notes include older runtime assumptions such as .NET SDK 3.5/4.0 and JDK 1.5 references (GitHub, GitHub releases). The README also states that Linux/Mono was not supported at the time (GitHub).

For Java-to-.NET production calls, that creates several concerns.

Runtime modernization

If Java needs to move to Java 17, Java 21, or Java 25, or .NET needs to move toward .NET 8, .NET 9, or .NET 10, jni4net becomes a compatibility question your team must answer alone.

Deployment topology

jni4net is intraprocess-focused. That can be efficient locally, but it limits architectural freedom. Production teams may want Java and .NET isolated, deployed separately, restarted independently, or connected across hosts.

Operational debugging

Java-to-.NET calls through jni4net can fail because of native bridge loading, bitness mismatch, classpath, proxy generation, stale wrappers, JAVA_HOME, or assembly loading. Those failures are bridge infrastructure failures, not business-code failures.

Support ownership

If production breaks, who supports the bridge? With jni4net, the answer is usually your own team.

How JNBridgePro handles Java-to-.NET calls

JNBridgePro is built for bidirectional Java/.NET interoperability. Its generated proxies expose APIs across runtime boundaries, so Java code can access .NET classes and .NET code can access Java classes (How It Works). The JNBridgePro user guide also documents both directions and the distinction between proxy generation and proxy use (User Guide).

For a Java application that needs to call C# or .NET, that means the bridge is not an unsupported side experiment. It is a productized path.

JNBridgePro currently lists support for .NET Framework 4.8, .NET 8, .NET 9, .NET 10, Windows, 64-bit Linux, and JDK 8 through 25 (System Requirements). That matters if the Java application is modern, if the .NET side is changing, or if production must run outside a developer workstation.

JNBridgePro also supports multiple deployment models: same process, separate processes, or networked communication (Overview). That lets architects choose the operational shape instead of being forced into a single intraprocess model.

Design the boundary before choosing the bridge

Whether you use jni4net, JNBridgePro, REST, gRPC, or another approach, the best Java-to-.NET integrations avoid overly chatty calls.

A bad bridge design looks like this:

• Java loops over thousands of objects.
• Each iteration calls one tiny C# method.
• Objects cross the runtime boundary repeatedly.
• Performance and debugging depend on tiny-call overhead.

A better bridge design looks like this:

• Java calls a coarse-grained .NET operation.
• The .NET side performs meaningful work locally.
• Data crosses the boundary in batches or purposeful DTOs.
• The bridge boundary reflects a business operation, not every internal helper method.

The May 22 benchmark for the opposite direction — .NET calling Java — reinforced this principle. JNBridgePro won the production-shaped coarse batch workload while jni4net won some local in-process microbenchmarks. The lesson is not that every direction and every call shape has the same numbers. The lesson is that production bridge design should emphasize coarse-grained operations and supported runtime execution.

Decision table: call C# from Java

Migration approach

If Java currently calls C# through jni4net, migrate in stages:

1. List the .NET classes and methods exposed to Java.
2. Identify calls that are too chatty and group them into business operations.
3. Decide whether Java and .NET should share a process or run separately.
4. Generate JNBridgePro proxies for the .NET APIs Java needs.
5. Rebuild the Java call sites against the supported proxy model.
6. Test object lifetime, exceptions, callbacks, strings, arrays, and deployment startup.
7. Benchmark production-shaped workloads, not only tiny calls.

This preserves the important part: Java can still call the .NET logic. The difference is that the interop layer becomes supportable.

FAQ

Can jni4net call C# from Java?

Yes. jni4net is bidirectional and can expose .NET code to Java through generated proxies. The concern is not directionality; it is whether the bridge is current and supportable for your production environment.

Can Java call a .NET DLL?

Yes, with a Java/.NET bridge. jni4net historically supported this kind of scenario in constrained environments. JNBridgePro is the stronger production option for modern Java and .NET runtimes.

Is JNBridgePro a jni4net replacement for Java-to-.NET calls?

Yes, it can replace the bridge role, but it is not a drop-in DLL swap. You should regenerate proxies, review the Java/.NET boundary, and test the target deployment topology.

Should I use REST or gRPC instead?

Use REST or gRPC if you want a service boundary and can expose the .NET logic as a service. Use JNBridgePro if you need object-level Java/.NET interoperability, generated proxies, and direct runtime integration.

Next step

If your Java application only needs to maintain an old local bridge, document the jni4net environment and keep it stable. If Java-to-.NET calls are part of a production modernization plan, review How JNBridgePro works and test the integration with the JNBridgePro download.