How to set a binary attribute when using a AccountManagement Extension Class?

Question

I am using a custom class to expose some custom schema in Active Directory. I am storing a binary blob, per the project requirements this data must be stored in the AD, I can not use a external store (I would if I could).

When I create the user it stores the blob fine. I also can retrieve the blob back out fine too and get all my data. The issue is if I need to update the value and I am getting errors

Small example program:

using System;
using System.DirectoryServices.AccountManagement;

namespace SandboxConsole40
{
    class Program
    {
        static void Main(string[] args)
        {
            using(var context = new PrincipalContext(ContextType.Domain))
            {
                using (var clear = ExamplePrincipal.FindByIdentity(context, "example"))
                {
                    if (clear != null)
                        clear.Delete();
                }

                using (var create = new ExamplePrincipal(context, "example", "Password1", false))
                {
                    create.Save();
                }

                using (var set = ExamplePrincipal.FindByIdentity(context, "example"))
                {
                    set.BlobData = new byte[] { 0xDE, 0xAD, 0xBE, 0xEF }; //This fails with method 2.
                    set.Save();
                }

                using (var lookup = ExamplePrincipal.FindByIdentity(context, "example"))
                {
                    Console.WriteLine(BitConverter.ToString(lookup.BlobData));
                }

                using (var update = ExamplePrincipal.FindByIdentity(context, "example"))
                {
                    update.BlobData = new byte[] { 0x12, 0x34, 0x56, 0x67 };
                    update.Save(); //This save fails with method 1.
                }
            }

            Console.WriteLine("Done");
            Console.ReadLine();
        }

        [DirectoryObjectClass("user")]
        [DirectoryRdnPrefix("CN")]
        class ExamplePrincipal : UserPrincipal
        {
            public ExamplePrincipal(PrincipalContext context) : base(context) { }

            public ExamplePrincipal(PrincipalContext context, string samAccountName, string password, bool enabled)
                : base(context, samAccountName, password, enabled) { }

            public static new ExamplePrincipal FindByIdentity(PrincipalContext context, string identityValue)
            {
                return (ExamplePrincipal)FindByIdentityWithType(context, typeof(ExamplePrincipal), identityValue);
            }

            [DirectoryProperty("vwBlobData")]
            public byte[] BlobData
            {
                get
                {
                    if (ExtensionGet("vwBlobData").Length != 1)
                        return null;

                    return (byte[])ExtensionGet("vwBlobData")[0];
                }
                set
                {
                    //method 1
                    this.ExtensionSet("vwBlobData",  value );

                    //method 2
                    //this.ExtensionSet("vwBlobData", new object[] { value});
                }
            }
        }
    }
}

If I use method 1 I get the following exception on the update.Save() operation

System.DirectoryServices.AccountManagement.PrincipalOperationException was unhandled
  HResult=-2146233087
  Message=The specified directory service attribute or value already exists.

  Source=System.DirectoryServices.AccountManagement
  ErrorCode=-2147016691
  StackTrace:
       //Snip
  InnerException: System.DirectoryServices.DirectoryServicesCOMException
       HResult=-2147016691
       Message=The specified directory service attribute or value already exists.

       Source=System.DirectoryServices
       ErrorCode=-2147016691
       ExtendedError=8321
       ExtendedErrorMessage=00002081: AtrErr: DSID-030F154F, #1:
    0: 00002081: DSID-030F154F, problem 1006 (ATT_OR_VALUE_EXISTS), data 0, Att 82818fec (vwBlobData)

       StackTrace:
            //Snip
       InnerException: 

If I use method 2 I get a exception on from the this.ExtensionSet call from the set.BlobData call.

System.ArgumentException was unhandled
  HResult=-2147024809
  Message=Collections whose elements are another collection cannot be set by ExtensionClasses.
  Source=System.DirectoryServices.AccountManagement
  StackTrace:
      //Snip
  InnerException: 

---

In summary: I can set the value if it is currently not set, but if I want to overwrite a existing value I am getting a error.

Answer 1

I found a work around, by setting the value to null first it no-longer throws the exception.

using (var update = ExamplePrincipal.FindByIdentity(context, "example"))
{
    update.BlobData = null;
    update.Save();
    update.BlobData = new byte[] { 0x12, 0x34, 0x56, 0x67 };
    update.Save(); //No longer fails with method 1.
}

I am leaving the question open for a bit to see if anyone else can answer if there is a "proper" way to do this.

---

Found a 2nd work around that does not require forcing a save.

[DirectoryProperty("vwBlobData")]
public byte[] BlobData
{
    get
    {
        if (ExtensionGet("vwBlobData").Length != 1)
            return null;

        return (byte[])ExtensionGet("vwBlobData")[0];
    }
    set
    {
        ((DirectoryEntry)this.GetUnderlyingObject())
                             .Properties["vwBlobData"].Value = value;
    }
}

By casting directly to the underlying object you can set the value directly. I checked using ILSpy and disposing the AccountManagement wrapper disposes the underlying object, so no Dispose() is required for the GetUnderlyingObject() call.

---

BEST SOLUTION

I found out the 2nd work around required the object to be persisted to work, so I made a best of both worlds approach. This works when you have not yet persisted the object, when the object is null, and when the object has a value already.

[DirectoryProperty("vwBlobData")]
public byte[] BlobData
{
    get
    {
        if (ExtensionGet("vwBlobData").Length != 1)
            return null;

        return (byte[])ExtensionGet("vwBlobData")[0];
    }
    set
    {
        if(ExtensionGet("vwBlobData").Length == 0)
            this.ExtensionSet("vwBlobData", data); 
        else
            ((DirectoryEntry)this.GetUnderlyingObject())
                                 .Properties["vwBlobData"].Value = data;
    }
}