Try and Catch

[Anonymous]

I have a very basic piece of code to understand how the try and catch works. I think I understand it somewhat but not entirely. Here is the code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.EditorInput;
using Autodesk.AutoCAD.Geometry;
using Autodesk.AutoCAD.Runtime;

using AcAp = Autodesk.AutoCAD.ApplicationServices.Application;


namespace AddCirc2
{
    public class Commands
    {
        [CommandMethod("AddCir")]
        public void Test()
        {
            var doc = AcAp.DocumentManager.MdiActiveDocument;
            var db = doc.Database;
            var ed = doc.Editor;
            using (var trans = db.TransactionManager.StartTransaction())
            {
                BlockTable blkTbl = (BlockTable)trans.GetObject(db.BlockTableId, OpenMode.ForRead);
                BlockTableRecord blkTblRec = (BlockTableRecord)trans.GetObject(blkTbl[BlockTableRecord.ModelSpace], OpenMode.ForWrite);
                try
                {
                    Point3d cirCen = new Point3d(5, 5, 0);
                    Circle cir = new Circle(cirCen, new Vector3d(0, 0, 1), 6.3);
                    Line lin = new Line(cirCen, new Point3d(cir.Center.X + 6.3, cir.Center.Y, 0));

                    blkTblRec.AppendEntity(cir);
                    blkTblRec.AppendEntity(cir);
                    blkTblRec.AppendEntity(lin);
                    trans.AddNewlyCreatedDBObject(cir, true);
                    trans.AddNewlyCreatedDBObject(lin, true);

                    trans.Commit();
                }
                catch (System.Exception ex)
                {
                    ed.WriteMessage("\nError: " + ex.Message);
                    trans.Abort();
                }
            }
        }
    }
}

Questions:

1. If I call the trans in a using statement does it really matter if I have a try and catch block?

2. I saw in an example online that the trans.abort() was called in the catch block. I ignored it and also added it, both ways I do not see any difference. The error is still thrown.

3. The blkTbl and blkTblRec initialization is happening outside the try but inside the using. Why does it matter? can I initialize it even before the using statement is called, that is right after var ed = doc.Editor;

var ed = doc.Editor;

I am also trying to understand where does the try and catch blocks begin? In other words, why can I not have everything included in the try and catch. So try goes right at the top and the using statement is called in the try block and then have a catch block at the end. Is that acceptable?