Table of Contents

• The Reverse Interop Problem
• Why You’d Want to Run C# from Java
• Four Approaches to Connect Java to C#
• 1. Proxy-Based Bridging (Java → .NET)
• 2. REST or gRPC Service Layer
• 3. COM Interop (Windows Only)
• 4. Native Hosting via coreclr
• Comparison Table
• Decision Framework: Choosing Your Java-to-C# Path
• How to Load .NET DLL in Java (Step by Step)
• How to Run C# Library from Java in Production
• Running C# in a Java App: Architecture Patterns
• Error Handling Across Runtimes
• Performance and Threading
• FAQ

The Reverse Interop Problem

Most connect Java to C# articles focus on calling Java from C#. But the reverse direction — calling C# or .NET code from Java — is equally common in enterprise environments and significantly less documented.

You might have a .NET library for tax calculations, a C# PDF renderer, or a proprietary .NET SDK that your Java application needs to consume. Rewriting it in Java is rarely justified when the .NET code already works and is actively maintained.

This guide covers how to run C# in a Java app, load .NET DLLs in Java, and run a C# library from Java using production-safe approaches.

Why You’d Want to Run C# from Java

Common scenarios include:

• Your core application is Java, but a critical library exists only in .NET. Example: a regulatory compliance engine written in C#.
• Acquisition or merger. Your company acquired a .NET product, and you need to integrate it into your Java platform.
• Vendor SDK. A third-party ships a .NET DLL as their official SDK, with no Java equivalent.
• Gradual migration. You’re moving from .NET to Java but can’t rewrite everything at once.

In each case, the goal is the same: call .NET methods from Java code, get results back, and handle errors cleanly.

Four Approaches to Connect Java to C#

1. Proxy-Based Bridging (Java → .NET)

JNBridgePro supports bidirectional interop. To call .NET from Java, you point the proxy tool at your .NET assemblies and generate Java proxy classes. These proxies appear as regular Java classes that forward calls to the CLR.

// Generated proxy for a .NET class
var renderer = new com.mycompany.dotnet.PdfRenderer();
byte[] pdf = renderer.render("template.html", data);
Files.write(Path.of("output.pdf"), pdf);

The proxy handles CLR initialization, type marshalling, and exception translation. Your Java code doesn’t know it’s talking to .NET.

Best for: Library-level integration, complex object graphs, high call frequency.

2. REST or gRPC Service Layer

Wrap the .NET library in an ASP.NET Core web API or gRPC service, then call it from Java via HTTP or gRPC client.

Best for: When the .NET component already runs as a service, or when Java and .NET are deployed on separate infrastructure.

Drawback: You must build and maintain the service wrapper, handle serialization, and accept network latency.

3. COM Interop (Windows Only)

On Windows, you can expose .NET classes as COM objects and call them from Java using a COM-to-Java bridge library (like JACOB). This was common in the 2000s but is fragile and platform-locked.

Best for: Legacy Windows-only environments where COM infrastructure already exists.

Drawback: Windows-only, brittle, complex registration, limited type support.

4. Native Hosting via coreclr

.NET provides a native hosting API (coreclr_initialize, coreclr_create_delegate) that lets native code load the CLR and call managed methods. Java can access this via JNI + native C wrapper.

Best for: Extremely specialized scenarios with dedicated native interop expertise.

Drawback: Requires writing C/C++ glue code, JNI bindings, and manual memory management. Error-prone and hard to maintain.

Comparison Table

Decision Framework: Choosing Your Java-to-C# Path

Step 1: Is the .NET code a library (DLL) or an existing service?

• Library → Proxy bridging or native hosting.
• Service → REST or gRPC.

Step 2: Do you need cross-platform support?

• Yes → Proxy bridging (JNBridgePro supports Linux), REST, or gRPC.
• Windows only → COM is an additional option.

Step 3: How many .NET types do you need in Java?

• Many (10+) → Proxy generation. Writing individual bindings for each type is prohibitive.
• Few (1–3) → Any approach works.

Step 4: What’s your team’s native interop expertise?

• Low → Use proxy bridging or a service layer. Avoid JNI/native hosting.
• High → Native hosting is feasible but still high-maintenance.

Step 5: What’s your latency requirement?

• Sub-millisecond → Proxy bridging or native hosting.
• Milliseconds acceptable → REST or gRPC.

How to Load .NET DLL in Java (Step by Step)

If your team requirement is load .NET DLL in Java, this is the direct production workflow with JNBridgePro.

// Initialize the bridge
com.jnbridge.jnbcore.JNBMain.start("bridge-config.properties");

// Use the .NET class as a Java object var taxEngine = new com.mycompany.dotnet.TaxEngine(); double result = taxEngine.calculateTax(75000.0, "Colorado"); System.out.println("Tax: " + result);

The TaxEngine proxy wraps the real .NET TaxEngine class. Method calls cross to the CLR and return Java-native types.

For detailed walkthroughs, see the JNBridgePro developer demos.

How to Run C# Library from Java in Production

If your goal is to run c# library from java at scale, production deployments require attention to:

Initialization. Start the CLR bridge early in your application lifecycle — ideally during startup, not on first use. This avoids latency spikes on the first call.

Thread safety. JNBridgePro handles thread synchronization between the JVM and CLR. However, if the underlying .NET library isn’t thread-safe, you need to synchronize on the Java side.

Memory management. Java proxy objects hold references to .NET objects. When the Java proxy is garbage-collected, the corresponding .NET object is released. Avoid holding proxies longer than necessary.

Error recovery. If the CLR throws an exception, it propagates as a Java exception. Catch and handle it normally. For transient errors, implement retry logic on the Java side.

Running C# in a Java App: Architecture Patterns

Pattern 1: Embedded library. The .NET DLL runs inside the Java process via bridging. Simplest deployment, lowest latency. Use when the .NET code is a pure library with no UI or service dependencies.

Pattern 2: Sidecar process. The .NET code runs in a separate process on the same host. Java communicates via TCP bridge or localhost REST. Useful when the .NET code requires isolation (e.g., it’s crash-prone or uses conflicting native dependencies).

Pattern 3: Remote service. The .NET code runs on a dedicated server. Java calls it via REST or gRPC. Standard microservice architecture. Best when teams manage the Java and .NET components independently.

Error Handling Across Runtimes

When you run C# in a Java app, exceptions must cross the runtime boundary. JNBridgePro translates .NET exceptions into Java exceptions:

• System.ArgumentException → java.lang.IllegalArgumentException
• System.NullReferenceException → java.lang.NullPointerException
• Custom .NET exceptions → wrapped in a com.jnbridge.JNBException with the original message and stack trace

The stack trace includes both the .NET and Java frames, making debugging straightforward.

For background on .NET exception handling internals, see Microsoft’s exception handling documentation. For Java’s side, Oracle’s exception tutorial covers the basics.

Performance and Threading

Cross-runtime calls via in-process bridging add 5–50 microseconds of overhead. This is negligible for most business logic but matters for tight loops. Strategies:

• Batch calls. Instead of fetching items one by one, call a method that returns a collection.
• Cache results. If the .NET computation is expensive but deterministic, cache on the Java side.
• Async patterns. Use Java’s CompletableFuture and dispatch bridge calls to a thread pool.

FAQ

Is there a difference between “load .NET DLL in Java” and “run C# library from Java”?

They’re usually the same implementation path. When you load .NET DLL in Java, you can then run C# library from Java through generated Java proxies.

Can Java call any .NET class, including WPF or Windows Forms?

Technically yes, but UI classes require an STA thread and a message pump. It’s practical for non-UI .NET libraries. For UI automation, use a service-based approach instead.

Does this work with .NET 8 and Java 21?

Yes. JNBridgePro supports modern .NET and Java versions. Check the overview page for the latest compatibility matrix.

Can I call asynchronous .NET methods (async/await) from Java?

The bridge calls the method synchronously from Java’s perspective. If the .NET method is async, you can call .Result on the returned Task, or use a synchronous wrapper on the .NET side.

How do I deploy the .NET runtime alongside my Java application?

On Windows, the .NET runtime is typically pre-installed. On Linux, bundle the .NET runtime or use a self-contained .NET deployment alongside your Java application.

Is bidirectional communication supported in the same session?

Yes. JNBridgePro supports callbacks and bidirectional calls — Java can call .NET, and .NET can call back into Java within the same bridge session.

What’s the maximum payload size for cross-runtime calls?

There’s no hard limit. Large arrays, byte buffers, and complex object graphs transfer based on available memory. For very large data (> 100 MB), consider streaming or file-based transfer.

Try reverse interop today. Download JNBridgePro and call .NET from Java in minutes. See the demos or contact us.

Related Articles

Continue with these related Java/.NET integration guides:

• Java/.NET proxy generation guide
• How to call C# code from Java
• Java .NET integration options compared
• Call .NET from Java landing page