partial class Examples
{
  public static Rhino.Commands.Result SelLayer(Rhino.RhinoDoc doc)
  {
    // Prompt for a layer name
    string layername = doc.Layers.CurrentLayer.Name;
    Result rc = Rhino.Input.RhinoGet.GetString("Name of layer to select objects", true, ref layername);
    if (rc != Rhino.Commands.Result.Success)
      return rc;

    // Get all of the objects on the layer. If layername is bogus, you will
    // just get an empty list back
    Rhino.DocObjects.RhinoObject[] rhobjs = doc.Objects.FindByLayer(layername);
    if (rhobjs == null || rhobjs.Length < 1)
      return Rhino.Commands.Result.Cancel;

    for (int i = 0; i < rhobjs.Length; i++)
      rhobjs[i].Select(true);
    doc.Views.Redraw();
    return Rhino.Commands.Result.Success;
  }
}

import Rhino
import scriptcontext
import System.Guid, System.Drawing.Color

def SelLayer():
    # Prompt for a layer name
    layername = scriptcontext.doc.Layers.CurrentLayer.Name
    rc, layername = Rhino.Input.RhinoGet.GetString("Name of layer to select objects", True, layername)
    if rc!=Rhino.Commands.Result.Success: return rc

    # Get all of the objects on the layer. If layername is bogus, you will
    # just get an empty list back
    rhobjs = scriptcontext.doc.Objects.FindByLayer(layername)
    if not rhobjs: Rhino.Commands.Result.Cancel

    for obj in rhobjs: obj.Select(True)
    scriptcontext.doc.Views.Redraw()
    return Rhino.Commands.Result.Success

if __name__=="__main__":
    SelLayer()