c# - Converting inherited class with generic class property -

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IListModel discloses a general inventory property, which is an abstract type of item item item item. But when I try to convert any Derivative orbit to IListModel, I get the "Object reference not set for an instance of an object" error. 

  Public abstract class list itemModel {public string username {get; Set; } Public string SomeProperty {get; Set; }} Public Interface Illustudel & lt; T & gt; Where T: ListItemModel {list & lt; T & gt; Item {get; Set; }} Public Category UserListModel: IListModel & lt; UserListItemModel & gt; {Public string query {get; Set; } Public Intel TotalUsers {get; Set; } Public listing & lt; UserListItemModel & gt; Item {get; Set; }} Public class UserListItemModel: ListItemModel {public string first name {get; Set; } Public String LastName {get; Set; }} Var users = new UserListModel {query = "a", total user = 1111, item = new list & lt; UserListItemModel & gt; {New UserListItemModel {FirstName = "a", last name = "B"}}}; // Later application will be passed to users as an object which will be sent to LICTML & lt; ListItemModel & gt; To use your properties, but converted the converted null value = users to ILISTODEL & lt; ListItemModel & gt; Forex Currency (Various Items in Converted Items) Item) {item.SomeProperty = DoSomethingHere (item.UserName); }   
 I'm trying to get here which is able to populate some proprietary list items from the itemmodel baseclass.   
 
 
 please see:  
 
 However, if the conversion is not possible, So instead of raising the exception, as a return tap.   
 Actually, what is happening is that you are doing an invalid artist, and the result of that illegal artist depends on the form of castings used.  
  var changed = users as elitodels & lt; ListItemModel & gt; // Converted Null variation = (Eististellel & lt; ListItemModel & gt;) user; // raises the exception   
 Is it really necessary?  UserListModel: IListModel & lt; UserListItemModel & gt;  This indicates that this is a type of IListModel you want, so you should be able to provide it only in the pressure block and work with it, do not you?

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