List AppDomains in Process

You may want to look at this post

using System.Runtime.InteropServices;
// Add the following as a COM reference - C:\WINDOWS\Microsoft.NET\Framework\vXXXXXX\mscoree.tlb
using mscoree;                              

        public static IList<AppDomain> GetAppDomains()
        {
            IList<AppDomain> _IList = new List<AppDomain>();
            IntPtr enumHandle = IntPtr.Zero
            CorRuntimeHostClass host = new mscoree.CorRuntimeHostClass();
            try
            {
                host.EnumDomains(out enumHandle);
                object domain = null;
                while (true)
                {
                    host.NextDomain(enumHandle, out domain);
                    if (domain == null) break;
                    AppDomain appDomain = (AppDomain)domain;
                    _IList.Add(appDomain);
                }
                return _IList;
            }
            catch (Exception e)
            {
                Console.WriteLine(e.ToString());
                return null;
            }
            finally
            {
                host.CloseEnum(enumHandle);
                Marshal.ReleaseComObject(host);
            }
        } 
    }

UPDATE

You can add the interface ICorRuntimeHost as:

using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

namespace mscoree
{
    [CompilerGenerated]
    [Guid("CB2F6722-AB3A-11D2-9C40-00C04FA30A3E")]
    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    [TypeIdentifier]
    [ComImport]
    [CLSCompliant(false)]
    public interface ICorRuntimeHost
    {
        void _VtblGap1_11();

        void EnumDomains(out IntPtr enumHandle);

        void NextDomain([In] IntPtr enumHandle, [MarshalAs(UnmanagedType.IUnknown)] out object appDomain);

        void CloseEnum([In] IntPtr enumHandle);
    }
}

And add a method:

private static ICorRuntimeHost GetCorRuntimeHost()
{
    return (ICorRuntimeHost)Activator.CreateInstance(Marshal.GetTypeFromCLSID(new Guid("CB2F6723-AB3A-11D2-9C40-00C04FA30A3E")));
}

Then you don't need the mscoree.tlb reference. And make some changes as:

using mscoree;
ICorRuntimeHost host = null;
host = GetCorRuntimeHost();

It's my current code for this problem.

---

Original answer

I refine it as this:

using System.Runtime.InteropServices;
using mscoree;

public static IEnumerable<AppDomain> EnumAppDomains()
{
    IntPtr enumHandle = IntPtr.Zero;
    CorRuntimeHostClass host = null;

    try
    {
        host = new CorRuntimeHostClass();
        host.EnumDomains(out enumHandle);
        object domain = null;

        host.NextDomain(enumHandle, out domain);
        while (domain != null)
        {
            yield return (AppDomain)domain;
            host.NextDomain(enumHandle, out domain);
        }
    }
    finally
    {
        if (host != null)
        {
            if (enumHandle != IntPtr.Zero)
            {
                host.CloseEnum(enumHandle);
            }

            Marshal.ReleaseComObject(host);
        }
    }
}

Call it as this:

foreach (AppDomain appDomain in EnumAppDomains())
{
    // use appDomain
}

Remember to reference COM object \WINDOWS\Microsoft.NET\Framework\vXXX\mscoree.tlb, set reference mscoree "Embed Interop Types" as "False".