Rhino layers (CRhinoLayer) are stored on a layer table (CRhinoLayerTable) which is located on the active document. The process for modifying an exising layer, such as changing it's name, is as follows.

1. Get the exising layer.
2. Make a copy of it.
3. Modify the copy.
4. Call CRhinoLayerTable::ModifyLayer().

The following code sample demonstrates how to rename an existing layer.

C++

  CRhinoCommand::result CCommandTestSdk::RunCommand(const CRhinoCommandContext& context)
  {
    // Get the layer name
    CRhinoGetString gs;
    gs.SetCommandPrompt( L"Name of layer to rename" );
    gs.GetString();
    if( gs.CommandResult() != CRhinoCommand::success )
      return gs.CommandResult();


    // Validate the string
    ON_wString layer_name = gs.String();
    layer_name.TrimLeftAndRight();
    if( layer_name.IsEmpty() )
      return CRhinoCommand::nothing;


    // Get a reference to the layer table        
    CRhinoLayerTable& layer_table = context.m_doc.m_layer_table;


    // Find the layer
    int layer_index = layer_table.FindLayer( layer_name );
    if( layer_index < 0 )
    {
      RhinoApp().Print( L"Layer \"%s\" does not exist.\n", layer_name );
      return CRhinoCommand::cancel;
    }


    // Get the new layer name        
    gs.SetCommandPrompt( L"New layer name" );
    gs.GetString();
    if( gs.CommandResult() != CRhinoCommand::success )
      return gs.CommandResult();


    // Validate the string
    ON_wString new_name = gs.String();
    layer_name.TrimLeftAndRight();
    if( layer_name.IsEmpty() )
      return CRhinoCommand::nothing;


    // Compare both names        
    if( layer_name.CompareNoCase(new_name) == 0 )
      return CRhinoCommand::nothing;


    // Get the layer
    const CRhinoLayer& layer = layer_table[layer_index];


    // Make a copy of it, and modify the name
    ON_Layer onlayer( layer );
    onlayer.SetLayerName( new_name );


    // Modify the exising layer with the new definition        
    CRhinoCommand::result rc = CRhinoCommand::cancel;
    if( layer_table.ModifyLayer(onlayer, layer_index) )
      rc = CRhinoCommand::success;


    return rc;
  }

VB.NET (Rhino 4)

  Public Overrides Function RunCommand(ByVal context As IRhinoCommandContext) _
    As IRhinoCommand.result


    ' Get the layer name
    Dim gs As New MRhinoGetString()
    gs.SetCommandPrompt("Name of layer to rename")
    gs.GetString()
    If (gs.CommandResult() <> IRhinoCommand.result.success) Then
      Return gs.CommandResult()
    End If


    ' Validate the string
    Dim layer_name As String = gs.String().Trim()
    If (String.IsNullOrEmpty(layer_name)) Then
      Return IRhinoCommand.result.nothing
    End If


    ' Get a reference to the layer table        
    Dim layer_table As MRhinoLayerTable = context.m_doc.m_layer_table
    ' Find the layer
    Dim layer_index As Integer = layer_table.FindLayer(layer_name)
    If (layer_index < 0) Then
      RhUtil.RhinoApp.Print(String.Format("Layer {0} does not exist."+vbCrLf, layer_name))
      Return IRhinoCommand.result.cancel
    End If


    ' Get the new layer name        
    gs.SetCommandPrompt("New layer name")
    gs.GetString()
    If (gs.CommandResult() <> IRhinoCommand.result.success) Then
      Return gs.CommandResult()
    End If


    ' Validate the string
    Dim new_name As String = gs.String().Trim()
    If (String.IsNullOrEmpty(new_name)) Then
      Return IRhinoCommand.result.nothing
    End If


    ' Compare both names        
    If (layer_name.Equals(new_name, StringComparison.OrdinalIgnoreCase)) Then
      Return IRhinoCommand.result.nothing
    End If


    ' Get the layer
    Dim layer As IRhinoLayer = layer_table(layer_index)
    ' Make a copy of it, and modify the name
    Dim copy_layer As New OnLayer(layer)
    copy_layer.SetLayerName(new_name)


    ' Modify the exising layer with the new definition        
    Dim rc As IRhinoCommand.result = IRhinoCommand.result.cancel
    If (layer_table.ModifyLayer(copy_layer, layer_index)) Then
      rc = IRhinoCommand.result.success
    End If
    Return rc
  End Function

C# (Rhino 4)

  public override IRhinoCommand.result RunCommand(IRhinoCommandContext context)
  {
    // Get the layer name
    MRhinoGetString gs = new MRhinoGetString();
    gs.SetCommandPrompt("Name of layer to rename");
    gs.GetString();
    if(gs.CommandResult() != IRhinoCommand.result.success)
      return gs.CommandResult();


    // Validate the string
    string layer_name = gs.String().Trim();
    if(string.IsNullOrEmpty(layer_name))
      return IRhinoCommand.result.nothing;


    // Get a reference to the layer table        
    MRhinoLayerTable layer_table = context.m_doc.m_layer_table;
    // Find the layer
    int layer_index = layer_table.FindLayer(layer_name);
    if(layer_index < 0)
    {
      RhUtil.RhinoApp().Print(string.Format("Layer {0} does not exist.\n", layer_name));
      return IRhinoCommand.result.cancel;
    }


    // Get the new layer name        
    gs.SetCommandPrompt("New layer name");
    gs.GetString();
    if(gs.CommandResult() != IRhinoCommand.result.success)
      return gs.CommandResult();


    // Validate the string
    string new_name = gs.String().Trim();
    if(string.IsNullOrEmpty(new_name))
      return IRhinoCommand.result.nothing;


    // Compare both names        
    if(layer_name.Equals(new_name, System.StringComparison.OrdinalIgnoreCase))
      return IRhinoCommand.result.nothing;


    // Get the layer
    IRhinoLayer layer = layer_table[layer_index];
    // Make a copy of it, and modify the name
    OnLayer copy_layer = new OnLayer(layer);
    copy_layer.SetLayerName(new_name);


    // Modify the exising layer with the new definition        
    IRhinoCommand.result rc = IRhinoCommand.result.cancel;
    if(layer_table.ModifyLayer(copy_layer, layer_index))
      rc = IRhinoCommand.result.success;
    return rc;
  }