System.ArgumentException: 'Object of type 'myObject' cannot be converted to type 'System.Collections.Generic.List`1[myObject]'.'

Question

I need to create a generic list of T by reflection and add item of T to it by invoke a method "Add", but I have error. this is my code:

public class myObject
{
    public object FieldValue { get; set; }
    public int ID_OpenField { get; set; }
}

private void Form1_Load(object sender, EventArgs e)
{
    Type type = typeof(List<myObject>);

    var genericType = typeof(List<>).MakeGenericType(type);
    var list = Activator.CreateInstance(genericType);

    myObject obj = new myObject() { FieldValue = "TextObjectValue", ID_OpenField = 1 };
    object[] o = new object[] { obj };

    list.GetType().GetMethod("Add").Invoke(list, o);            
}

the last line gives me this error:

System.ArgumentException: 'Object of type 'myObject' cannot be converted to type 'System.Collections.Generic.List`1[myObject]'.'

@CamiloTerevinto: almost certainly practice... No one is *that* masochistic, not even me! — Andreas Bonini, Sep 24 '21 at 14:18

score 1 · Answer 1 · answered Sep 24 '21 at 14:16

You came pretty close, but MakeGenericType is being called with the wrong arguments.

This will work:

        Type type = typeof(List<myObject>);

        Type[] typeArgs = { typeof(myObject) };
              var genericType = typeof(List<>).MakeGenericType(typeArgs);
// your code: var genericType = typeof(List<>).MakeGenericType(type);
       var list = Activator.CreateInstance(genericType);

        myObject obj = new myObject() { FieldValue = "TextObjectValue", ID_OpenField = 1 };
        object[] o = new object[] { obj };

        list.GetType().GetMethod("Add").Invoke(list, o);

Refer to How to dynamically create generic C# object using reflection? for more information.

@bama71: my pleasure! :) – Andreas Bonini Sep 24 '21 at 14:43 — Andreas Bonini, Sep 24 '21 at 14:43

score 0 · Answer 2 · answered Sep 24 '21 at 14:16

Here you are creating a List<List<myObject>>, because type is List<myObject>:

Type type = typeof(List<myObject>);

var genericType = typeof(List<>).MakeGenericType(type);
var list = Activator.CreateInstance(genericType);

You should be using typeof(myObject) instead:

var genericType = typeof(List<>).MakeGenericType(typeof(myObject));
var list = Activator.CreateInstance(genericType);

System.ArgumentException: 'Object of type 'myObject' cannot be converted to type 'System.Collections.Generic.List`1[myObject]'.'

2 Answers2