using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
 
public class MyClass<T>
{
    public T MainModel { get; set; }
 
    public Type GetTheType()
    {
return typeof(T).GetInterfaces().Where(i => i.IsGenericType)
    .Single(i => i.GetGenericTypeDefinition() == typeof(IEnumerable<>))
    .GetGenericArguments().First();
    }
}
 
public class Test
{
	public static void Main()
	{
var mainInstance = new MyClass<List<string>>();
Console.Out.WriteLine(mainInstance.GetTheType().FullName);		
	}
}

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