Develop Plugin for Windows Desktop

Question

I am developing a Cordova plugin for windows desktop and need to use some native functions, I will have to use Windows Runtime. I started by following a tutorial and I have the following structure:

 - /src
  - /EchoRuntime
  - /bin
  - /obj
  - /Properties
  - EchoPluginRT.cs
  - EchoRuntimeComponent.csproj
  - echopluginProxy.js
 - /www
  - echoplugin.js
 - plugin.xml 

At code level I have the following:

xml plugin.

<?xml version="1.0" encoding="UTF-8"?>

<plugin xmlns="http://apache.org/cordova/ns/plugins/1.0" id="com.teste.echoplugin" version="0.1.0">
<name>EchoPlug</name>

<description>Echo demo plugin for Apache Cordova</description>
<license>MIT</license>
<keywords></keywords>
<repo></repo>
<issue></issue>

<js-module src="www/echoplugin.js" name="echoplugin">
    <clobbers target="window.echoplugin" />
</js-module>

<!-- windows8 -->
<platform name="windows">
    <js-module src="src/windows/echopluginProxy.js" name="EchoProxy">
        <merges target="" />
    </js-module>

  <framework src="src/windows/EchoRuntime/EchoRuntimeComponent.csproj" custom="true" type="projectReference"/>
</platform>

/www/echoplugin.js

var EchoPlugin = {
    // the echo function calls successCallback with the provided text in strInput
    // if strInput is empty, it will call the errorCallback
    echo:function(successCallback, errorCallback, strInput) {
        cordova.exec(successCallback,errorCallback,"EchoPlugin","echo",[strInput]);
    }
}

module.exports = EchoPlugin;

/src/echopluginProxy.js

cordova.commandProxy.add("EchoPlugin",{
    echo:function(successCallback,errorCallback,strInput) {
        var res = EchoRuntimeComponent.EchoPluginRT.echo(strInput);

        if(res.indexOf("Error") == 0) {
            errorCallback(res);
        }
        else {
            successCallback(res);
        }
    }
});

/src/Echoruntime/Echopluginrt.Cs

    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using System.Threading.Tasks;

    namespace EchoRuntimeComponent
    {
      public sealed class EchoPluginRT
     {
        public static string echo(string valor)
         {
            return " PASSOU AQUI";
         }
     }
    }

Only with Javascript can I get it working now with the C# function is not. Someone can help?