Getting Feature Attachments using ArcObjects .NET (using AttachToMemory)?

Question

I am trying to retreive the Attachment of a Feature using ArcObjects in C#. The data is correct, I am able to dump the attachments to the disk using SaveToFile.

I want to work in memory and not dump to the disk and re-read the bytes as this is terribly wrong.

Also, I cannot "hack" in the database and select the BLOB directly.

Here is the relevant piece of code that I have up to now and the operation AttachToMemory which I believe is the way to go but I am unable to get it working.

IEnumAttachment enumAttachment = table.AttachmentManager.GetAttachmentsByParentIDs(ids, false);
IAttachment attachment = null;
while ((attachment = enumAttachment.Next()) != null)
{
 // THIS DOES NOT COPY THE DATA, DATA = 0円
 byte[] data = new byte[attachment.Size];
 attachment.Data.AttachToMemory(ref data[0], (uint)attachment.Size, 0);
 // THIS INDEED CREATES THE PROPER FILE WITH DATA
 attachment.Data.SaveToFile("C:\path");
}

After the AttachToMemory operation, the data does not contain the bytes, it is initialized to null bytes.

I have tried to work with unmanaged memory portions but then I get some very strange results (the few first bytes are non-null then the rest is null bytes) :

IntPtr pointer = Marshal.AllocHGlobal(attachment.Size);
byte first = (byte)pointer.ToPointer(); // <---- this looks very wrong
attachment.Data.AttachToMemory(ref first, (uint)attachment.Size, 0);
Marshal.Copy(pointer, data, 0, data.Length);
Marshal.FreeHGlobal(pointer);

One of my issues in understanding how it is supposed to work is that the AttachToMemory method accepts a ref byte as first argument. This seems very strange to me.

The reference of the ArcObject is here : http://resources.esri.com/help/9.3/arcgisengine/arcobjects/esriSystem/IMemoryBlobStream_AttachToMemory.htm

• Kadir Şahbaz: I'm confused, what is strange? That "ref byte is the first argument" or that "byte passes to method as ref"?
• Simon: Strange that you should pass a ref byte from a cast to another object that you cannot pass itself... but this type of construct is frequent when working with low-level APIs.

Answer 1

It's sensible to pass data variable by ref to method. Attachment could be 1 GB file.

When I used the code in the question, data consisted of just zeros. After some research, I added some code lines. Now attachments is retrieved correctly. (I pointed out added lines)

using ESRI.ArcGIS.Framework;
using ESRI.ArcGIS.Carto;
using ESRI.ArcGIS.Geodatabase;
using ESRI.ArcGIS.ArcMapUI;
using ESRI.ArcGIS.esriSystem;
using System;
namespace ArcMapAddin
{ 
 public class Button1 : ESRI.ArcGIS.Desktop.AddIns.Button
 {
 // to show message
 IMessageDialog messageWindow = new MessageDialogClass();
 public Button1() {} 
 protected override void OnClick()
 {
 IMxDocument mxDoc = ArcMap.Application.Document as IMxDocument;
 IMap map = mxDoc.FocusMap;
 ILayer layer = null;
 IFeatureClass featureClass = null;
 IEnumLayer enumLayer = map.Layers;
 layer = enumLayer.Next(); // map has one layer
 featureClass = (layer as IFeatureLayer).FeatureClass;
 ITableAttachments table= (ITableAttachments)featureClass;
 IAttachmentManager attachmentManager = table.AttachmentManager;
 ILongArray ids = new LongArrayClass();
 ids.Add(1); 
 IEnumAttachment enumAttachment = attachmentManager.GetAttachmentsByParentIDs(ids, false);
 IAttachment attachment = null;
 while ((attachment = enumAttachment.Next()) != null)
 {
 IMemoryBlobStream dataBlob = attachment.Data;
 byte[] data = new byte[dataBlob.Size];
 ///////////////
 // ADDED LINES
 object _data = null;
 (dataBlob as IMemoryBlobStreamVariant).ExportToVariant(out _data);
 data = (byte[])_data;
 ///////////////
 attachment.Data.AttachToMemory(ref data[0], (uint)attachment.Size, 0);
 // CONTROL
 String x = BitConverter.ToString(data, 0);
 messageWindow.DoModal("RESULT", x.ToString(), null, null, ArcMap.Application.hWnd);
 }
 } 
 protected override void OnUpdate()
 {
 Enabled = ArcMap.Application != null;
 }
 } 
}

I didn't find much about using AttachToMemorymethod.

• Simon: This is it ! The cast to IMemoryBlobStreamVariant and ExportToVariant() made it. Although in your code, no need to pre-allocate the byte[] data as it just consumes some memory for nothing. For info, how did you find this solution, is there any doc out there that explains this ?
• Kadir Şahbaz: The only thing that I've found useful is just the last paragraph in this link. There are some codes about IMemoryBlobStream, but I didn't find about AttachToMemory. I thought deleting byte[] data, but I couldn't be sure.