Activator.CreateInstance with private sealed class

Question

I'm trying to new up a LocalCommand instance which is a private class of System.Data.SqlClient.SqlCommandSet. I seem to be able to grab the type information just fine:

Assembly sysData = Assembly.Load("System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089");
localCmdType = sysData.GetType("System.Data.SqlClient.SqlCommandSet+LocalCommand");

but Activator.CreateInstance throws an exception when I try to instantiate it:

object item = Activator.CreateInstance(localCmdType,
  new object[] { commandText, parameters, num7, commandType });

System.MissingMethodException: Constructor on type 'System.Data.SqlClient.SqlCommandSet+LocalCommand' not found.

The constructor arguments match the signature I see in Reflector. Is new'ing up a private class with an internal ctor supported with a different CreateInstance overload or what?

score 22 · Accepted Answer · edited Aug 14 '15 at 01:15

22

My first thought would be to get the ConstructorInfo using ConstructorInfo constructorInfo = Type.GetConstructor(), and then constructorInfo.Invoke() that. I suspect that Activator.CreateInstance makes it hard to call constructors you wouldn't normally have access to, although I don't remember trying it myself.

edited Aug 14 '15 at 01:15

ErikE

48,881
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answered Jan 13 '09 at 17:55

Jon Skeet

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9,194

2

The docs say CreateInstance only calls public constructors before 2.0sp1. After that, there's a bunch of permissions that are necessary. – Robert C. Barth Jan 13 '09 at 17:56
1

Good call on GetConstructor(), that does work. I find it strange considering that the GetConstructor call looks very similar to one of the Activator.CreateInstance() overloads. – Jan 13 '09 at 22:20

score 19 · Answer 2 · answered Jan 13 '09 at 19:52

I got it to work this way:

using System;
using System.Reflection;

class Test
{
    public String X { get; set; }

    Test(String x)
    {
        this.X = x;
    }
}

class Program
{
    static void Main()
    {
        Type type = typeof(Test);

        ConstructorInfo c = type.GetConstructor(BindingFlags.NonPublic | BindingFlags.Instance, 
            null, new Type[] { typeof(String) }, null);

        Object o = c.Invoke(new Object[] { "foo" });
    }
}

The trick was to go after the constructor specifically with GetConstructor rather then trying to find it in the results of GetConstructors. Go figure.

Can it be this isn't available in PCL? – Ray Aug 22 '16 at 16:16 — Ray, Aug 22 '16 at 16:16

score 15 · Answer 3 · edited May 05 '20 at 06:11

15

I might be a little late in responding, but I ran into a similar problem that fits into this topic.
I wanted to instantiate a non public constructor using Activator.CreateInstance and passing it arguments.

public class Node
{
    string name;
    Node parent;

    protected Node(string name,Node parent)
    {
       this.name = name;
       this.parent = parent;
    }

    public static Node Create(string name,Node parent)
    {
       Node result = Activator.CreateInstance(
         type: typeof(Node),
         bindingAttr: BindingFlags.Instance  | BindingFlags.NonPublic,
         binder: null, //default binder
         args: new object[] { name, parent },
         culture: null);
       return (Node)result;
    }
}

The tricky part was the binding flags.

My first instinct was to use BindingFlags.CreateInstance | BindingFlags.NonPublic, however that caused an exception to be thrown: MissingMethodException Constructor on type 'Node' not found.

edited May 05 '20 at 06:11

Shimmy Weitzhandler

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answered Aug 18 '09 at 02:51

This fails for me with the error you describe but oddly, Andrew Hare's answer does work, even though both answers share the same binding flags. – Jerther Jun 25 '14 at 20:57
Simply does not work. Like Jerther said, Andrew Hare's answer does. – Oct 01 '16 at 05:06
Worked like a charm for me! I too tried using ```BindingFlags.CreateInstance | BindingFlags.NonPublic``` and got the same error message. Changing it to ```BindingFlags.Instance | BindingFlags.NonPublic``` made it work!! – Kabua Jan 14 '17 at 17:32

score 0 · Answer 4 · answered Mar 21 '17 at 17:13

Trick is to make sure to use the right CreateInstance overload:

// WRONG
... Activator.CreateInstance(
       type,
       BindingFlags.Instance
       | BindingFlags.NonPublic
    );

This calls the params overload, which will default to Instance | Public | CreateInstance, and your binding flags will be passed as arguments to the constructor, giving the vague MissingMethodException.

Also, use Instance | Public | NonPublic if you aren't sure of the visibility of the constructor:

// right
... Activator.CreateInstance(
       type,
       BindingFlags.Instance
       | BindingFlags.Public
       | BindingFlags.NonPublic,
       null,
       new object[] { }, // or your actual constructor arguments
       null
    );

Activator.CreateInstance with private sealed class

4 Answers4

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