Passing C++ argument with out keyword to .NET DLL

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 C++ but invocation must be performed through Javonet SDK API. Javonet allows you to pass any C++ 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 .NET DLL with out C++ argument

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 C++ 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 C++ code:

#include "Javonet.h"
#include <iostream>
#include <vector>

using namespace JavonetNS::Cpp::Sdk;

std::string resourcesDirectory = ".";

int main(int argc, char* argv[]) {

    // use Activate only once in your app
    Javonet::Activate("your-license-key");
    
    // create called runtime context
    auto calledRuntime = Javonet::InMemory()->Netcore();
    
    // set up variables
    auto libraryPath = resourcesDirectory + "/TestClass.dll";
    auto className = "TestClass.TestClass";
    
    // load custom library
    calledRuntime->LoadLibrary(libraryPath);
    
    // get type from the runtime
    auto calledRuntimeType = calledRuntime->GetType(className)->Execute();
    auto 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
    auto outValue_1 = calledRuntime->AsOut(stringType)->Execute();
    // second way - pass initial value and type to cast on
    auto outValue_2 = calledRuntime->AsOut({ 'c', stringType })->Execute();
    // third way - pass initial value without specific type
    auto outValue_3 = calledRuntime->AsOut("Test string")->Execute();
    
    // invoke type's static method with out values
    calledRuntimeType->InvokeStaticMethod("OutSampleMethod", outValue_1)->Execute();
    calledRuntimeType->InvokeStaticMethod("OutSampleMethod", outValue_2)->Execute();
    calledRuntimeType->InvokeStaticMethod("OutSampleMethod", outValue_3)->Execute();
    
    // get outs' values
    auto result1 = std::any_cast<std::string>(outValue_1->GetRefValue()->Execute()->GetValue());
    auto result2 = std::any_cast<std::string>(outValue_2->GetRefValue()->Execute()->GetValue());
    auto result3 = std::any_cast<std::string>(outValue_3->GetRefValue()->Execute()->GetValue());
    
    // write result to console
    std::cout << result1 << std::endl;
    std::cout << result2 << std::endl;
    std::cout << result3 << std::endl;
}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
auto calledRuntime = Javonet::InMemory()->Netcore();

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

// load custom library
calledRuntime->LoadLibrary(libraryPath);

// get type from the runtime
auto calledRuntimeType = calledRuntime->GetType(className)->Execute();
auto 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
auto outValue_1 = calledRuntime->AsOut(stringType)->Execute();
// second way - pass initial value and type to cast on
auto outValue_2 = calledRuntime->AsOut({ 'c', stringType })->Execute();
// third way - pass initial value without specific type
auto outValue_3 = calledRuntime->AsOut("Test string")->Execute();

// invoke type's static method with out values
calledRuntimeType->InvokeStaticMethod("OutSampleMethod", outValue_1)->Execute();
calledRuntimeType->InvokeStaticMethod("OutSampleMethod", outValue_2)->Execute();
calledRuntimeType->InvokeStaticMethod("OutSampleMethod", outValue_3)->Execute();

// get outs' values
auto result1 = std::any_cast<std::string>(outValue_1->GetRefValue()->Execute()->GetValue());
auto result2 = std::any_cast<std::string>(outValue_2->GetRefValue()->Execute()->GetValue());
auto result3 = std::any_cast<std::string>(outValue_3->GetRefValue()->Execute()->GetValue());

// write result to console
std::cout << result1 << std::endl;
std::cout << result2 << std::endl;
std::cout << result3 << std::endl;

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 C++ 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.

Passing C++ argument with out keyword to .NET DLL

Custom .NET DLL with out C++ argument

Additional references