Process32FirstW always returns false

It's really a charset problem. You use the ***W version of api. The actual type of PROCESSENTRY32W.szExeFile is wchar_t, which is 2 bytes, so the structure size you passed is 260 bytes shorter(you can simply try p32Iw.dwSize + 260, Process32FirstW will return ture, if we are regardless of the subsequent execution). Fix the struct declaration and run the "C/C++ Programming" C# code.

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
public struct PROCESSENTRY32 
   { 
      public uint dwSize; 
      public uint cntUsage; 
      public uint th32ProcessID; 
      public IntPtr th32DefaultHeapID; 
      public uint th32ModuleID; 
      public uint cntThreads; 
      public uint th32ParentProcessID; 
      public int pcPriClassBase; 
      public uint dwFlags; 
      [MarshalAs(UnmanagedType.ByValTStr, SizeConst=260)] public string szExeFile; 
    };

But according to the document, Marshal.StructureToPtr and Marshal.PtrToStructure are out of date and we should avoid using it. And this way of passing values through pointers is usually replaced by references in C#.

First, declare charset and reference parameters for Process32First and Process32Next

[DllImport("kernel32", EntryPoint = "Process32First", SetLastError = true, CharSet = CharSet.Auto)]
[return: MarshalAs(UnmanagedType.Bool)]
public static extern bool Process32First(IntPtr hSnapshot, ref PROCESSENTRY32W lppe);

[DllImport("kernel32", EntryPoint = "Process32Next", CharSet = CharSet.Auto)]
[return: MarshalAs(UnmanagedType.Bool)]
public static extern bool Process32Next(IntPtr hSnapshot, ref PROCESSENTRY32W lppe);

Then, Pass the ref type of PROCESSENTRY32W:

bool blFirstProcess = Process32First(HANLDE_Processes, ref p32Iw); // returns false no matter what?
if (blFirstProcess)
{
   do
   {
      //Marshal.PtrToStructure(p32IntPtr, p32Iw);
      int PID = (int)p32Iw.th32ProcessID;
      returnable_processes_.Add(PID, new Process());
      Console.WriteLine(p32Iw.szExeFile);
   }
   while (Process32Next(HANLDE_Processes, ref p32Iw));
 }

Summarize, a minimal, compilable example:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Runtime.InteropServices;
using System.Diagnostics;
namespace ConsoleApp
{
    class Program
    {
        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
         public struct PROCESSENTRY32W
        {
            public uint dwSize;
            public uint cntUsage;
            public uint th32ProcessID;
            public IntPtr th32DefaultHeapID;
            public uint th32ModuleID;
            public uint cntThreads;
            public uint th32ParentProcessID;
            public int pcPriClassBase;
            public uint dwFlags;
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 260)] public string szExeFile;
        };
        [DllImport("kernel32.dll", EntryPoint = "CreateToolhelp32Snapshot")]
        public static extern IntPtr CreateToolhelp32SnapshotRtlMoveMemory(UInt32 dwFlagsdes, UInt32 th32ProcessID);

        [DllImport("kernel32", EntryPoint = "Process32First", SetLastError = true, CharSet = CharSet.Auto)]
        [return: MarshalAs(UnmanagedType.Bool)]
        public static extern bool Process32First(IntPtr hSnapshot, ref PROCESSENTRY32W lppe);

        [DllImport("kernel32", EntryPoint = "Process32Next", CharSet = CharSet.Auto)]
        [return: MarshalAs(UnmanagedType.Bool)]
        public static extern bool Process32Next(IntPtr hSnapshot, ref PROCESSENTRY32W lppe);

        [DllImport("kernel32", EntryPoint = "CloseHandle")]
        [return: MarshalAs(UnmanagedType.Bool)]
        public static extern bool CloseHandle(IntPtr handle);
        public static Dictionary<int, Process> get_process_list()
        {
            //Dictionary<string, Process> returnable_processes_ = new Dictionary<string, Process>();
            Dictionary<int, Process> returnable_processes_ = new Dictionary<int, Process>();
            IntPtr HANLDE_Processes = CreateToolhelp32SnapshotRtlMoveMemory(2, 0);

            PROCESSENTRY32W p32Iw = new PROCESSENTRY32W();
            int size = System.Runtime.InteropServices.Marshal.SizeOf(typeof(PROCESSENTRY32W));
            p32Iw.dwSize = Convert.ToUInt32(size);
            //IntPtr p32IntPtr = Marshal.AllocHGlobal(Marshal.SizeOf(p32Iw));
            //Marshal.StructureToPtr(p32Iw, p32IntPtr, false);
            bool blFirstProcess = Process32First(HANLDE_Processes, ref p32Iw); // returns false no matter what?
            int x = Marshal.GetLastWin32Error();

            if (blFirstProcess)
            {

                do
                {
                    int PID = (int)p32Iw.th32ProcessID;
                    returnable_processes_.Add(PID, new Process());
                    Console.WriteLine(p32Iw.szExeFile);

                }
                while (Process32Next(HANLDE_Processes, ref p32Iw));
            }
            return returnable_processes_;
        }
        static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");
            get_process_list();
            Console.ReadKey();
        }
    }
}