Pass arguments by reference with out keyword to .NET DLL in Java app

This article provides an introduction to cross-technology invocation of both static and instance methods which contains argument of type out. Parameter modifier out in C# (.NET) technology is used in a method signature to pass an argument by reference. It is described in details in this article.

Javonet allows you to reference and use modules or packages written in (Java/Kotlin/Groovy/Clojure, C#/VB.NET, Ruby, Perl, Python, JavaScript/TypeScript) like they were created in your technology. If have not yet created your first project check Javonet overview and quick start guides for your technology.

With Javonet you can interact with methods with out parameter modifier from .NET DLL like they were available in Java but invocation must be performed through Javonet SDK API. Javonet allows you to pass any Java value type as argument to method from .NET DLL. For reference type arguments (instances of other classes) you can create such instance with Javonet and pass the Invocation Context variable referencing that object as argument of method invocation.

Custom Java with out keyword

With Javonet it is possible to reference any custom .NET DLL and interact with its methods declared on types defined within that module almost the same as with any other Java library.

Snippet below represents the sample code from .NET DLL which contains methods with out parameter modifier.

namespace TestClass
{
	public class TestClass
	{
		public TestClass() { }

        public static void OutSampleMethod(out string outStr)
        {
        	outStr = "String from OutSampleMethod";
        }
	}
}
public static void OutSampleMethod(out string outStr)
{
	outStr = "String from OutSampleMethod";
}

It is possible to invoke the declared methods from .NET DLL using following Java code:

import com.javonet.sdk.*;

public class SampleProgram {

    private static final String resourcesDirectory = ".";
    public static void main(String []args) {

        // use activate only once in your app
        Javonet.activate("your-license-key");
        
        // create called runtime context
        RuntimeContext calledRuntime = Javonet.inMemory().netcore();
        
        // set up variables
        String libraryPath = resourcesDirectory + "/TestClass.dll";
        String className = "TestClass.TestClass";
        
        // load custom library
        calledRuntime.loadLibrary(libraryPath);
        
        // get type from runtime
        InvocationContext calledRuntimeType = calledRuntime.getType(className).execute();
        InvocationContext stringType = calledRuntime.getType("System.String").execute();
        
        // create values for outs
        // out variable should have specific type to be able to invoke methods on it
        // first way - pass only type
        InvocationContext outValue1 = calledRuntime.asOut(stringType).execute();
        // second way - pass initial value and type to cast on
        InvocationContext outValue2 = calledRuntime.asOut('c', stringType).execute();
        // third way - pass initial value without specific type
        InvocationContext outValue3 = calledRuntime.asOut("Test string").execute();
        
        // invoke type's static method with out values
        calledRuntimeType.invokeStaticMethod("OutSampleMethod", outValue1).execute();
        calledRuntimeType.invokeStaticMethod("OutSampleMethod", outValue2).execute();
        calledRuntimeType.invokeStaticMethod("OutSampleMethod", outValue3).execute();
        
        // get outs' values
        String result1 = (String) outValue1.getRefValue().execute().getValue();
        String result2 = (String) outValue2.getRefValue().execute().getValue();
        String result3 = (String) outValue3.getRefValue().execute().getValue();
        
        // write result to console
        System.out.println(result1);
        System.out.println(result2);
        System.out.println(result3);
     }
}namespace TestClass
{
	public class TestClass
	{
		public TestClass() { }

        public static void OutSampleMethod(out string outStr)
        {
        	outStr = "String from OutSampleMethod";
        }
	}
}
// use activate only once in your app
Javonet.activate("your-license-key");

// create called runtime context
RuntimeContext calledRuntime = Javonet.inMemory().netcore();

// set up variables
String libraryPath = resourcesDirectory + "/TestClass.dll";
String className = "TestClass.TestClass";

// load custom library
calledRuntime.loadLibrary(libraryPath);

// get type from runtime
InvocationContext calledRuntimeType = calledRuntime.getType(className).execute();
InvocationContext stringType = calledRuntime.getType("System.String").execute();

// create values for outs
// out variable should have specific type to be able to invoke methods on it
// first way - pass only type
InvocationContext outValue1 = calledRuntime.asOut(stringType).execute();
// second way - pass initial value and type to cast on
InvocationContext outValue2 = calledRuntime.asOut('c', stringType).execute();
// third way - pass initial value without specific type
InvocationContext outValue3 = calledRuntime.asOut("Test string").execute();

// invoke type's static method with out values
calledRuntimeType.invokeStaticMethod("OutSampleMethod", outValue1).execute();
calledRuntimeType.invokeStaticMethod("OutSampleMethod", outValue2).execute();
calledRuntimeType.invokeStaticMethod("OutSampleMethod", outValue3).execute();

// get outs' values
String result1 = (String) outValue1.getRefValue().execute().getValue();
String result2 = (String) outValue2.getRefValue().execute().getValue();
String result3 = (String) outValue3.getRefValue().execute().getValue();

// write result to console
System.out.println(result1);
System.out.println(result2);
System.out.println(result3);

This snippet uses in memory runtime bridging to load the .NET DLL and next retrieves reference to specific type.
Next, two reference type arguments are created. An argument that is passed to a out parameter do not have to be initialized before it's passed to a method. Three ways of initialization are presented in create values for out section of the snippet.
Fist way (outValue1) is initializing using asOut() method with specifying type of argument.
Second way (outValue2) is initializing using asOut() method with specifying value and type of argument.
Third way (outValue3) is initializing using asOut() method with specifying value of argument.

All these values can be passed to OutSampleMethod(out string outStr) method using invokeStaticMethod(...).execute() invocation.

Each reference type argument is get as regular Java value using getRefValue() and can be used for further processing.

The same operation can be performed remotely by just changing the new Runtime Context invocation from in memory to tcp that will create and interact with your .NET DLL objects on any remote node, container or service that hosts Javonet Code Gateway. This way you can preserve the same logic in your application and instantly switch between monolithic and microservices architecture without the need to implement the integration layer based on web services or other remote invocation methods.

Read more about use cases and software architecture scenarios where Javonet runtime bridging technology can support your development process.

Pass arguments by reference with out keyword to .NET DLL in Java app

Custom Java with out keyword

Additional references