Transaction Null Reference and eWasOpenforWrite Error

[drobertsY4GL3]

I cannot seem to find a solution to getting the blockreference when inside an AttributeModifiedEventHandler. When using StartTransaction() I get a NullReferenceException and when using StartOpenCloseTransaction() I get a 'eWasOpenforWrite' error.

Please see below for further info and the event handler code.

Double clicking a dynamic block and selecting "OK" inside the enhanced attribute editor gives a NullReferenceException at:

using (Transaction transaction1 = transactionManager.StartTransaction())

Copying the dynamic block works however as expected.

If I change: StartTransaction() to:

using (var transaction1 = transactionManager.StartOpenCloseTransaction())

The attribute editor then works, but copying the block gives an 'eWasOpenForWrite' error at:

var blockReference = transaction1.GetObject(id, OpenMode.ForRead) as BlockReference;

Below is the full EventHandler.

private void AttributeModifiedEventHandler(object sender, Autodesk.AutoCAD.DatabaseServices.ObjectEventArgs args)
        {
            var transactionManager = currentDatabase.TransactionManager;

            AttributeReference attributeRef;
            string CadHandle = "";
            bool updateDB = false;

            if (args.DBObject is AttributeReference)
            {
                attributeRef = (AttributeReference)args.DBObject;

                using (Transaction transaction1 = transactionManager.StartTransaction())
                {                  
                    var blockTable = transaction1.GetObject(currentDatabase.BlockTableId, OpenMode.ForRead) as BlockTable;
                    var blockTableRecord = transaction1.GetObject(blockTable[BlockTableRecord.ModelSpace], OpenMode.ForRead) as BlockTableRecord;

                    foreach (ObjectId id in blockTableRecord)
                    {
                        // check if the current ObjectId is a block reference one
                        if (id.ObjectClass.DxfName == "INSERT")
                        {
                            var blockReference = transaction1.GetObject(id, OpenMode.ForRead) as BlockReference;


                            if (blockReference.Id == attributeRef.OwnerId)
                            {
                                CadHandle = blockReference.Handle.ToString();
                            }
                        };
                    }
                    transaction1.Commit();
                }
            }
        }

Any help anyone can give is extremely appreciated!

Thank you.

[norman.yuan · Accepted solution]

It looks to me that the purpose of the code in your AttributeModifiedEventHandler() does is to find the handle of attribute owner (blockreference) and assign it to a variable "CadHandle" (it also looks to me that the code in the eventhandler is not "full" - the local variable "CadHandle" is not used after being assigned value)

I do not understand is why you need to loop through entire ModelSpace for that while the agrs.DBObject has already had all information you need?

You can simply:

private void XXXXXXXHandler(object sender, ObjectEventArgs args)

{

  var att = args.DBObject As AttributeReference;

  if (att != null)

  {

    var CadHandle=att.OwnerId.Handle.ToString();

    // do something with CadHandle...

    // but do not attempt to modify anything in this event handler, in general

  }

}

[drobertsY4GL3]

Thank you Mr. Yuan.

This is my first AutoCAD plugin, so I am very grateful for the help. Your solution is a good clean example of how it should be done.