Application crash when using .ocx with events that have ref objects, works in Framework - System.Windows.Forms.AxHost

• .NET Core Version: 3.0 & 3.1

• Have you experienced this same bug with .NET Framework?: No

Problem description:

Using ActiveX .ocx user control that has specifically not null, ref objects passed on events crashes entire application due to corrupt memory - Same code works in Framework

Expected behavior:

ref object is not corrupt on return to OCX after calling event inside core application

Minimal repro:

using System;
using System.Windows.Forms;

namespace ConsoleApp1
{
    class Program
    {
        [STAThread]
        static void Main(string[] args)
        {
            var c = new WindowsFormsControlLibrary1.UserControl1();
            c.CreateControl();
            c.TestEventByRef += C_TestEventByRef;
            c.WithObjectByRefTestEvent();
            c.Dispose();

            var c2 = new AxAxolotl.AxTestControl();
            c2.CreateControl();
            c2.TestEventByRef += axTestControl1_TestEventByRef;
            c2.WithObjectByRefTestEvent();
            c2.Dispose();
        }

        private static void C_TestEventByRef(object sender, WindowsFormsControlLibrary1.TestEventByRefEventArgs e)
        {
            e.TestObject = new WindowsFormsControlLibrary1.TestObject { Text = "Framework Out" };
        }

        private static void axTestControl1_TestEventByRef(object sender, AxAxolotl.__TestControl_TestEventByRefEvent e)
        {
            MessageBox.Show(e.column.Text);

            e.column = new Axolotl.CTestObject { Text = "OCX Out" };
        }
    }
}

example winforms control that works in framework and core

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Data;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsControlLibrary1
{
    public class TestEventByRefEventArgs : EventArgs
    {
        public TestObject TestObject { get; set; }
    }

    public partial class UserControl1 : UserControl
    {
        public event EventHandler<TestEventByRefEventArgs> TestEventByRef;

        public UserControl1()
        {
            InitializeComponent();
        }

        public void WithObjectByRefTestEvent()
        {
            var testObject = new TestObject { Text = "In" };

            var testEventByRefEventArgs = new TestEventByRefEventArgs { TestObject = testObject };

            TestEventByRef?.Invoke(this, testEventByRefEventArgs);

            MessageBox.Show(testEventByRefEventArgs.TestObject.Text);
        }
    }
}

example OCX (in VB6) control that works in framework by crashes in core

Option Explicit

Event TestEventByRef(ByRef Column As CTestObject)

Public Sub WithObjectByRefTestEvent()
    Dim a As CTestObject
    Set a = New CTestObject
    a.Text = "OCX In"
    RaiseEvent TestEventByRef(a)
    MsgBox a.Text
End Sub

// CTestObject.cls

Option Explicit

Public Text As String

Here is a repro in a zip

https://axolotl.blob.core.windows.net/issue2863/Issue2863.zip

you have to first register the .ocx “regsvr32 Issue2863\VB6OCX\Axolotl.TestControl.ocx”

then run ConsoleApp1 (core) in debug with exceptions turned on… (ConsoleApp2 is framework and works)

Issue2863\VB6OCX\Axolotl.TestControl.ctl

MsgBox a.Text <== a is corrupt

Exception thrown at 0x11001F16 (Axolotl.TestControl.ocx) in ConsoleApp1.exe: 0xC0000005: Access violation reading location 0x00000000. occurred

[NikolaosWakem]

3.1 LTS broken but fixed in 5.0 so all Good 😄

[msftbot[bot]]

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