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enumprocesses (psapi)
 
.
Summary
Sayeth the MSDN: Retrieves the process identifier for each process object in the system.

C# Signature:

[DllImport("Psapi.dll", SetLastError=true)]
static extern bool EnumProcesses(
   [MarshalAs(UnmanagedType.LPArray, ArraySubType=UnmanagedType.U4)] [In][Out] UInt32[] processIds,
   UInt32 arraySizeBytes,
   [MarshalAs(UnmanagedType.U4)] out UInt32 bytesCopied
);

VB Signature:

<DllImport("psapi.dll", CharSet:=CharSet.Auto, SetLastError:=True)> _
Public Shared Function EnumProcesses(<MarshalAs(UnmanagedType.LPArray, ArraySubType:=UnmanagedType.U4), [In](), [Out]()> ByVal processIds As Integer(), ByVal size As Integer, <[Out]()> ByRef needed As Integer) As Boolean
End Function

User-Defined Types:

None.

Alternative Managed API:

System.Diagnostics.Process

Notes:

The ArraySubType field for processIds and the entire MarshalAs attribute for bytesCopied are probably unnecessary (since they're the same as the defaults).

Tips & Tricks:

Note that we can't use MarshalAsAttribute.SizeParamIndex for the array, since SizeParamIndex is like [size_is()] and operates on number of elements, not number of bytes.

Sample Code:

using System;
using System.Collections.Generic;
using System.Text;
using System.Runtime.InteropServices;

namespace Test
{
     class Program
     {
     [DllImport("psapi")]
     private static extern bool EnumProcesses(
         [MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.U4)] [In][Out] UInt32[] processIds,
           UInt32 arraySizeBytes,
           [MarshalAs(UnmanagedType.U4)] out UInt32 bytesCopied);

     static void Main(string[] args)
     {
         UInt32 arraySize = 120;
         UInt32 arrayBytesSize = arraySize * sizeof(UInt32);
         UInt32[] processIds = new UInt32[arraySize];
         UInt32 bytesCopied;

         bool success = EnumProcesses(processIds, arrayBytesSize, out bytesCopied);

         Console.WriteLine("success={0}", success);
         Console.WriteLine("bytesCopied={0}", bytesCopied);

         if (!success)
         {
         Console.WriteLine("Boo!");
         return;
         }
         if (0 == bytesCopied)
         {
         Console.WriteLine("Nobody home!");
         return;
         }

         UInt32 numIdsCopied = bytesCopied >> 2; ;

         if (0 != (bytesCopied & 3))
         {
         UInt32 partialDwordBytes = bytesCopied & 3;

         Console.WriteLine("EnumProcesses copied {0} and {1}/4th DWORDS...  Please ask it for the other {2}/4th DWORD",
             numIdsCopied, partialDwordBytes, 4 - partialDwordBytes);
         return;
         }

         for (UInt32 index = 0; index < numIdsCopied; index++)
         {
         Console.WriteLine("ProcessIds[{0}] = {1}", index, processIds[index]);
         }
     }
     }
}

Formatting... >.<

Documentation

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