Debugging .net application with windbg, cannot insert breakpoints

I use windbg, to debug a simple c# application, which consits just of empty form like this:

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

namespace WindowsFormsApp2
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }
    }
}

I compile, run this app, attach to it with windbg, then run in windbg:

0:009> .cordll -ve -u -l
Automatically loaded SOS Extension
CLRDLL: Loaded DLL C:\Windows\Microsoft.NET\Framework64\v2.0.50727\mscordacwks.dll
CLR DLL status: Loaded DLL C:\Windows\Microsoft.NET\Framework64\v2.0.50727\mscordacwks.dll

And thein i load SOS extension and verify that its loaded:

0:009> .loadby sos mscorwks
0:009> .chain
Extension DLL search Path:
    C:\Program Files (x86)\Windows Kits\10\Debuggers\x64\WINXP;C:\Program Files (x86)\Windows Kits\10\Debuggers\x64\winext;C:\Program Files (x86)\Windows Kits\10\Debuggers\x64\winext\arcade;C:\Program Files (x86)\Windows Kits\10\Debuggers\x64\pri;C:\Program Files (x86)\Windows Kits\10\Debuggers\x64;C:\Users\username\AppData\Local\Dbg\EngineExtensions;C:\Program Files (x86)\Windows Kits\10\Debuggers\x64;C:\WINDOWS\system32;C:\WINDOWS;C:\WINDOWS\System32\Wbem;C:\WINDOWS\System32\WindowsPowerShell\v1.0\;C:\Program Files\PuTTY\;C:\Program Files (x86)\Bitvise SSH Client;C:\Program Files\nodejs\;C:\Program Files\Microsoft SQL Server\130\Tools\Binn\;C:\Program Files\Git\cmd;C:\Users\username\AppData\Local\Android\sdk\platform-tools;C:\Program Files (x86)\Nox\bin\;C:\Users\username\AppData\Local\Microsoft\WindowsApps;C:\Users\username\AppData\Roaming\npm;C:\Program Files (x86)\Nmap;C:\Program Files (x86)\mitmproxy\bin
Extension DLL chain:
    C:\Windows\Microsoft.NET\Framework64\v2.0.50727\sos: image 2.0.50727.8794, API 1.0.0, built Tue Jun 20 23:15:41 2017
        [path: C:\Windows\Microsoft.NET\Framework64\v2.0.50727\SOS.dll]
    C:\Windows\Microsoft.NET\Framework64\v2.0.50727\SOS.dll: image 2.0.50727.8794, API 1.0.0, built Tue Jun 20 23:15:41 2017
        [path: C:\Windows\Microsoft.NET\Framework64\v2.0.50727\SOS.dll]
    dbghelp: image 10.0.15063.468, API 10.0.6, built Thu Jan  1 03:00:00 1970
        [path: C:\Program Files (x86)\Windows Kits\10\Debuggers\x64\dbghelp.dll]
    ext: image 10.0.15063.468, API 1.0.0, built Thu Jan  1 03:00:00 1970
        [path: C:\Program Files (x86)\Windows Kits\10\Debuggers\x64\winext\ext.dll]
    exts: image 10.0.15063.468, API 1.0.0, built Thu Jan  1 03:00:00 1970
        [path: C:\Program Files (x86)\Windows Kits\10\Debuggers\x64\WINXP\exts.dll]
    uext: image 10.0.15063.468, API 1.0.0, built Thu Jan  1 03:00:00 1970
        [path: C:\Program Files (x86)\Windows Kits\10\Debuggers\x64\winext\uext.dll]
    ntsdexts: image 10.0.15063.468, API 1.0.0, built Thu Jan  1 03:00:00 1970
        [path: C:\Program Files (x86)\Windows Kits\10\Debuggers\x64\WINXP\ntsdexts.dll]

Then i set the debugging symbols path to the folder where .pdb located, and sources folder.

But when i open the source file and set the breakpoint on a source line, and then continue the process i get error in windbg:

0:009> g
Unable to insert breakpoint 3 at 00000000`008f001a, Win32 error 0n998
    "Invalid access to memory."
bp3 at 00000000`008f001a failed
WaitForEvent failed

I expected it would break, and show me the .net assembler instructions, but it fails all the time. How do you breakpoint the .net application using windbg then?

You can't set breakpoints in .NET using F9 on a source file. Instead you need the SOS !bpmd or SOSEX !mbp command.

The SOS syntax is

!BPMD <module name> <method name>
!BPMD -md <MethodDesc>

and it cannot use line numbers.

The SOSEX syntax is

!sosex.mbp <source file> <line number> [<column number>] [Options]

and can use line numbers if the PDBs are available.

!mbc clears a managed breakpoint. !mbd disables a managed breakpoint and !mbl lists managed breakpoints.

If I remember well, there's a difference: !bpmd only works if the method was already JIT compiled, so that a breakpoint can be set in executable machine code. !mbp can set breakpoints although the method was not JIT-compiled. It will set the breakpoint itself as soon as the method is compiled.

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