EditBlockAtt

csharp

[CommandMethod("EditBlockAtt")]   
public void EditBlockAtt()   
{      
        var acDb = HostApplicationServices.WorkingDatabase;
        var acEd = AcadApplication.DocumentManager.MdiActiveDocument.Editor;
        var blockNamePrompt = acEd.GetString(Environment.NewLine + "Enter block name: ");
        if (blockNamePrompt.Status != PromptStatus.OK) return;
        var blockName = blockNamePrompt.StringResult;
        var attNamePrompt = acEd.GetString(Environment.NewLine + "Enter attribute name: ");
        if (attNamePrompt.Status != PromptStatus.OK) return;
        var attName = attNamePrompt.StringResult;
        var acPo = new PromptStringOptions(Environment.NewLine + "Enter new attribute value: "){ AllowSpaces = true };
        var newValuePrompt = acEd.GetString(acPo);
        if (newValuePrompt.Status != PromptStatus.OK) return;
        var newValue = newValuePrompt.StringResult;
        using (var acTrans = acDb.TransactionManager.StartTransaction())
        {
            var acBlockTable = acTrans.GetObject(acDb.BlockTableId, OpenMode.ForRead) as BlockTable;
            if (acBlockTable == null) return;
            var acBlockTableRecord =                acTrans.GetObject(acBlockTable[BlockTableRecord.ModelSpace], OpenMode.ForRead) as BlockTableRecord;
            if (acBlockTableRecord == null) return;
            foreach (var blkId in acBlockTableRecord)
            {
                var acBlock = acTrans.GetObject(blkId, OpenMode.ForRead) as BlockReference;
                if (acBlock == null) continue;
                if (!acBlock.Name.Equals(blockName, StringComparison.CurrentCultureIgnoreCase)) continue;
                foreach (ObjectId attId in acBlock.AttributeCollection)
                {
                    var acAtt = acTrans.GetObject(attId, OpenMode.ForRead) as AttributeReference;
                    if (acAtt == null) continue;
                    if (!acAtt.Tag.Equals(attName, StringComparison.CurrentCultureIgnoreCase)) continue;
                    acAtt.UpgradeOpen();
                    acAtt.TextString = newValue;
                }
            }
            acTrans.Commit();
        }
    }

Then from an interop AcadApplication, netload the dll and call the method from the commandline in this format:
(Command "EditBlockAtt" "BlockName" "AttributeName" "NewValue")However if you want to go pure Interop, this may get you what you need given you have an AcadDocument object at runtime:

foreach (AcadEntity ent in acadDoc.ModelSpace)
{
    var block = ent as AcadBlockReference;
    if (block == null) continue;
    {
        if (!block.Name.Equals("BlockName", StringComparison.CurrentCultureIgnoreCase)) continue;
        var atts = block.GetAttributes() as object[];
        if (atts == null) continue;
        foreach (var attribute in atts.OfType<AcadAttributeReference>()
            .Where(attribute => attribute.TagString.Equals("AttributeName",
                                 StringComparison.CurrentCultureIgnoreCase)))
        {
            attribute.TextString = "New Value";
        }
    }
}