C# Instantiate Class from String given an Interface

Question

I am trying to make an instance of a class based on a string that will be retrieved from the User Interface, and then I want to access the properties of the instance of the class.

Here is an overview of what I have so far -

namespace MamdaAdapter
{
    public interface IExchange
    {
        string GetTransport();
    }
}


namespace MamdaAdapter
{
    public class Exchange
    {
        public class Arca : IExchange
        {
            private const string _Transport = "tportname";

            public string GetTransport()
            {
                return _Transport;
            }
        }


        public static IExchange DeriveExchange(string ExchangeName)
        {
            IExchange SelectedExchange = (IExchange)Activator.CreateInstance(Type.GetType(ExchangeName));

            return SelectedExchange;
        }
    }
}



namespace MyUserInterface
{
    public class MainForm
    {
        private void simpleButton1_Click(object sender, EventArgs e)
        {
            IExchange SelectedExchange = Exchange.DeriveExchange("Exchange.Arca");

            Console.WriteLine(SelectedExchange.GetTransport());
        }
    }
}

UPDATE: Right now, I'm getting an Exception that says the "Value cannot be null" which to me means that it is unable to create the instance of the class given the string provided -

Answer 1

The problem here is how you specify the name of your class:

First, specify the namespace. Second, since Arca is an inner class you must use '+' instead of '.'

(...) = Exchange.DeriveExchange("MamdaAdapter.Exchange+Arca");

Answer 2

Assuming you UI doesnt expose the full type name, you typically want a dictionary to associate the display name to the type:

Dictionary<string, Type> _associations = new Dictionary<string, Type>(); 

Then, you simply instantiate the new object:

if(_associations.ContainsKey(someString))
{
   Type selectedType = _associations[someString];

   return Activator.CreateInstance(selectedType) as IExchange;
}

throw new ApplicationException("No type defined for that string yo");

If the string is not known at compile time, you basically need to check for the existance of the type:

var type = Type.GetType(someString);

if(type != null)
{
    // Do Stuff
}

Answer 3

I wrote a small c# console application to simulate your need, tested ok, hope it helps:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using MamdaAdapter;
using System.Reflection;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            IExchange SelectedExchange = Exchange.DeriveExchange("MamdaAdapter.Arca");
            Console.WriteLine(SelectedExchange.GetTransport());
        }
    }
}

namespace MamdaAdapter
{
    public interface IExchange
    {
        string GetTransport();
    }
}


namespace MamdaAdapter
{
    public class Arca : IExchange
    {
        private const string _Transport = "tportname";

        public string GetTransport()
        {
            return _Transport;
        }
    }
}

namespace MamdaAdapter
{
    public class Exchange
    {
        public static IExchange DeriveExchange(string ExchangeName)
        {
            IExchange SelectedExchange = (IExchange)Assembly.GetAssembly(typeof(IExchange)).CreateInstance(ExchangeName, false, BindingFlags.CreateInstance, null, null, null, null);
            return SelectedExchange;
        }
    }

}

Answer 4

If the Type you are looking for is not defined in the same assembly that is executing Type.GetType you must use the AssemblyQualifiedName (something like MyNamespace.MyClass, MyAssembly, Version=1.3.0.0, Culture=neutral, PublicKeyToken=b17a5c561934e089), even the FullName is not enough. Otherwise you could first get the assembly containing the class and then execute the GetType method of the Assembly class.