Moving entities from one AutoCAD layer to another using .NET

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OK, now onto the real content. I had this question come in from António Rodrigues, an engineering student from Portugal:

I'm doing a project at school about hydraulics calculations, where i have get some data from AutoCAD. [...] I need to get all the objects from a single layer in the drawing, so i can use them to perform the calculations needed.

I put together some code for Antonio which uses Editor.SelectAll() with an appropriate SelectionFilter to get the list of entities on a particular layer. Just for fun I then extended the code to allow the user to enter a new layer name for these entities to be moved to.

Here's the C# code:

using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.EditorInput;
using Autodesk.AutoCAD.Runtime;

namespace LayerTools
{
public class Commands
{
[CommandMethod("CL")]
public void ChangeLayerOfEntitiess()
{
Document doc =
Application.DocumentManager.MdiActiveDocument;
Database db = doc.Database;
Editor ed = doc.Editor;

// Ask the user for the layer name, allowing
// spaces to be entered

PromptStringOptions pso =
new PromptStringOptions(
"/nEnter name of layer to search for: "
);
pso.AllowSpaces = true;
PromptResult pr =
ed.GetString(pso);

if (pr.Status != PromptStatus.OK)
return;
string layerName = pr.StringResult;

// We won't validate whether the layer exists -
// we'll just see what's returned by the selection.

TypedValue[] tvs = new TypedValue[1];
tvs[0] =
new TypedValue((int)DxfCode.LayerName, layerName);
SelectionFilter sf = new SelectionFilter(tvs);
PromptSelectionResult psr = ed.SelectAll(sf);

int count = 0;
if (psr.Status == PromptStatus.OK)
count = psr.Value.Count;

if (psr.Status == PromptStatus.OK ||
psr.Status == PromptStatus.Error)
{
// Display the count of entities on that layer

ed.WriteMessage(
"/nFound {0} entit{1} on layer /"{2}/".",
count,
count == 1 ? "y" : "ies",
layerName
);

// If there are some on this layer,
// prompt for the layer to move them to

if (count > 0)
{
pso.Message =
"/nEnter new layer for these entities " +
"or return to leave them alone: ";
pr =
ed.GetString(pso);

if (pr.Status != PromptStatus.OK ||
pr.StringResult == "")
return;
string newLayerName = pr.StringResult;

Transaction tr =
db.TransactionManager.StartTransaction();
using (tr)
{
// This time we do check whether
// the layer exists

LayerTable lt =
(LayerTable)tr.GetObject(
db.LayerTableId,
OpenMode.ForRead
);

if (!lt.Has(newLayerName))
ed.WriteMessage("/nLayer not found.");
else
{
int changedCount = 0;

// We have the layer table open, so let's
// get the layer ID and use that

ObjectId lid = lt[newLayerName];
foreach (ObjectId id in psr.Value.GetObjectIds())
{
Entity ent =
(Entity)tr.GetObject(id, OpenMode.ForWrite);
ent.LayerId = lid;
// Could also have used:
// ent.Layer = newLayerName;
// but this way is more efficient and cleaner
changedCount++;
}
ed.WriteMessage(
"/nChanged {0} entit{1} from " +
"layer /"{2}/" to layer /"{3}/".",
changedCount,
changedCount == 1 ? "y" : "ies",
layerName,
newLayerName
);
}
tr.Commit();
}
}
}
}
}
}

Here's what happens when you run the CL command and use it to move entities from the "Dim" layer to "My new layer":

Command: CL
Enter name of layer to search for: Dim
Found 14 entities on layer "Dim".
Enter new layer for these entities or return to leave them alone: My new layer
Changed 14 entities from layer "Dim" to layer "My new layer".