Why is the process I am writing the memory to crash?

So I recently stumbled onto a blog that described how to run your WinForms application from another process. I instantly thought this was something really cool an unique and wanted to test it out myself. While there was no code it explained how to do so and this is what I've done.

class Program
    {

        [Flags]
        public enum ThreadAccess : int
        {
            TERMINATE = (0x0001),
            SUSPEND_RESUME = (0x0002),
            GET_CONTEXT = (0x0008),
            SET_CONTEXT = (0x0010),
            SET_INFORMATION = (0x0020),
            QUERY_INFORMATION = (0x0040),
            SET_THREAD_TOKEN = (0x0080),
            IMPERSONATE = (0x0100),
            DIRECT_IMPERSONATION = (0x0200)
        }

        [DllImport("kernel32.dll")]
        static extern IntPtr OpenThread(ThreadAccess dwDesiredAccess, bool bInheritHandle, uint dwThreadId);
        [DllImport("kernel32.dll")]
        static extern uint SuspendThread(IntPtr hThread);
        [DllImport("kernel32.dll")]
        static extern int ResumeThread(IntPtr hThread);
        [DllImport("kernel32", CharSet = CharSet.Auto, SetLastError = true)]
        static extern bool CloseHandle(IntPtr handle);


        [Flags]
        public enum ProcessAccessFlags : uint
        {
            All = 0x001F0FFF,
            Terminate = 0x00000001,
            CreateThread = 0x00000002,
            VirtualMemoryOperation = 0x00000008,
            VirtualMemoryRead = 0x00000010,
            VirtualMemoryWrite = 0x00000020,
            DuplicateHandle = 0x00000040,
            CreateProcess = 0x000000080,
            SetQuota = 0x00000100,
            SetInformation = 0x00000200,
            QueryInformation = 0x00000400,
            QueryLimitedInformation = 0x00001000,
            Synchronize = 0x00100000
        }


        [DllImport("kernel32.dll", SetLastError = true)]
        public static extern bool WriteProcessMemory(
            IntPtr hProcess,
            IntPtr lpBaseAddress,
            byte[] lpBuffer,
            Int32 nSize,
            out IntPtr lpNumberOfBytesWritten);

        [DllImport("kernel32.dll")]
        static extern IntPtr CreateRemoteThread(IntPtr hProcess,
            IntPtr lpThreadAttributes, uint dwStackSize, IntPtr lpStartAddress,
            IntPtr lpParameter, uint dwCreationFlags, out IntPtr lpThreadId);


        [DllImport("kernel32.dll", SetLastError = true)]
        public static extern IntPtr OpenProcess(
            ProcessAccessFlags processAccess,
            bool bInheritHandle,
            int processId
        );
        public static IntPtr OpenProcess(Process proc, ProcessAccessFlags flags)
        {
            return OpenProcess(flags, false, proc.Id);
        }

        static void Main(string[] args)
        {
            Process notepad = null;

            Process[] processes = Process.GetProcesses();
            foreach (var process in processes)
            {
                if (process.ProcessName.ToLower() == "notepad")
                {
                    notepad = process;
                }
            }

            OpenProcess(notepad, ProcessAccessFlags.All);
            byte[] payload = File.ReadAllBytes(@"C:\Users\developer\source\repos\WindowsFormsApp2\WindowsFormsApp2\bin\Debug\WindowsFormsApp2.exe");
            foreach (ProcessThread pt in notepad.Threads)
            {
                IntPtr openThread = OpenThread(ThreadAccess.SUSPEND_RESUME, false, (uint)pt.Id);
                SuspendThread(openThread);
                CloseHandle(openThread);
            }
            WriteProcessMemory(notepad.Handle, notepad.Modules[0].BaseAddress, payload, payload.Length, out IntPtr lpNumberOfBytesWritten);
            CreateRemoteThread(notepad.Handle, IntPtr.Zero, 0x0, notepad.Modules[0].BaseAddress, IntPtr.Zero, 0x0, out _);

        }
    }

It's something very simple that is supposed to suspend the process and all of it's threads. And then write the bytearray from my other application which is a simple WinForms application witha button that when you click on it, it shows a MessageBox saying Hello World But as soon as I run the code, notepad crashes and nothing shows up, where did I go wrong?

Check out this Memory problem with application in C# Windows Forms and also this High memory usage of a windows forms method. I'm led to believe you may have a caching issue or are not writing safe code for memory management. I would start by looking at your entry point to the program and debugging the foreach blocks.