Pass arguments by reference with out keyword to .NET DLL in Python 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 Python but invocation must be performed through Javonet SDK API. Javonet allows you to pass any Python 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 Python 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 Python library.

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

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

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

I code in:
# use activate only once in your app
Javonet.activate("your-license-key")

# create called runtime context
called_runtime = Javonet.in_memory().netcore()

# set up variables
library_path = resources_directory + "/TestClass.dll"
class_name = "TestClass.TestClass"

# load custom library
called_runtime.load_library(library_path)

# get type from the runtime
called_runtime_type = called_runtime.get_type(class_name).execute()
string_type = called_runtime.get_type("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
out_value_1 = called_runtime.as_out(string_type).execute()
# second way - pass initial value and type to cast on
out_value_2 = called_runtime.as_out('c', string_type).execute()
# third way - pass initial value without specific type
out_value_3 = called_runtime.as_out("Test string").execute()

# invoke type's static method with out values
called_runtime_type.invoke_static_method("OutSampleMethod", out_value_1).execute()
called_runtime_type.invoke_static_method("OutSampleMethod", out_value_2).execute()
called_runtime_type.invoke_static_method("OutSampleMethod", out_value_3).execute()

# get outs' values
result1 = out_value_1.get_ref_value().execute().get_value()
result2 = out_value_2.get_ref_value().execute().get_value()
result3 = out_value_3.get_ref_value().execute().get_value()

# write result to console
print(result1)
print(result2)
print(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 Python 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.

Additional references