Mailman 3 How do you add an element to a List in a read-only property? - Python.NET

2 Mar 2020

      Hello,

I think this is a really basic question, but I haven't found a solution.  I've been asked to use a .NET assembly from python.  I have programed in other languages, but not C#.  In the brief time I've used it, PythonNet has been fantastic... with one little wrinkle.  

Here's a pared down version of the C#:

namespace Service.API
{
    public class ServiceAPI 
    {
        private static ServiceAPI m_Instance = new ServiceAPI();

        public static ServiceAPI Instance
        {
            get
            {
                return m_Instance;
            }
        }

        public ServiceAPI() 
        {
        }

        ~ServiceAPI() {}

        ServiceInformation m_ServiceInformation = new ServiceInformation();

        public List<string> ServiceHosts
        {
            get
            {
				return m_ServiceInformation.ServiceHosts;
            }
        }
	}

    public class ServiceInformation 
    {
        public List<string> ServiceHosts { get; set; }

        public ServiceInformation()
        {
            ServiceHosts = new List<string>();
        }
	}
}

I need to add strings to the List "ServiceHosts" in ServiceAPI.  The author placed the List inside another class, "ServiceInformation" and did not provide a set method, so that the ServiceHosts could not be altered to another object, while allowing strings to be added and removed from it.  

I think adding a string to that list is trivial in C#: (I've been assured by the assembly author that this works)
	ServiceAPI.Instance.ServiceHosts.Add('1.1.1.1');

So I try it in Python:
...
...
...
import clr	# Pyton.NET package
clr.AddReference("Service.API")	
import Service.API
import System	# This is a .NET package.  
Service.API.ServiceAPI.Instance.ServiceHosts.Add('1.1.1.1')
Traceback (most recent call last):
  File "<interactive input>", line 1, in <module>
AttributeError: 'ServiceAPI' object has no attribute 'Add'
This is a List that PythonNet is mapping onto a python list:
...
...
...
Service.API.ServiceAPI.Instance.ServiceHosts
[]
So try using a python method:
...
...
...
Service.API.ServiceAPI.Instance.ServiceHosts.append('1.1.1.1')
Service.API.ServiceAPI.Instance.ServiceHosts
[]
Nope.  And I start suspecting that I've added that string to a boxed version of the list.  But because there's no set accessor, I can't do this:
...
...
...
hosts = Service.API.ServiceAPI.Instance.ServiceHosts
hosts.append('1.1.1.1')
Service.API.ServiceAPI.Instance.ServiceHosts = hosts
Traceback (most recent call last):
  File "<interactive input>", line 1, in <module>
TypeError: property is read-only
So what about calling the unbound list method, passing in the list?
...
...
...
System.Collections.Generic.List[String].Add(
       Service.API.ServiceAPI.Instance.ServiceHosts, '1.1.1.1')
Traceback (most recent call last):
  File "<interactive input>", line 1, in <module>
AttributeError: 'ServiceAPI' object has no attribute 'Add'
The author of this assembly has offered to create a specific method so that I can access the list, but he assures me that this is a common C# pattern that I will encounter frequently.  

I can't just create an instance of ServiceInformation and add the hosts to that, ServiceAPI has no method for getting or setting m_ServiceInformation.  

Any help in setting me in the right direction would be appreciated.

Thanks,

Mark Brighton

How do you add an element to a List in a read-only property?

mark＠brightsoftwaredesign.com

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