StringResult

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

def AddLayer():
    # Cook up an unused layer name
    unused_name = scriptcontext.doc.Layers.GetUnusedLayerName(False)

    # Prompt the user to enter a layer name
    gs = Rhino.Input.Custom.GetString()
    gs.SetCommandPrompt("Name of layer to add")
    gs.SetDefaultString(unused_name)
    gs.AcceptNothing(True)
    gs.Get()
    if gs.CommandResult()!=Rhino.Commands.Result.Success:
        return gs.CommandResult()

    # Was a layer named entered?
    layer_name = gs.StringResult().Trim()
    if not layer_name:
        print "Layer name cannot be blank."
        return Rhino.Commands.Result.Cancel

    # Is the layer name valid?
    if not Rhino.DocObjects.Layer.IsValidName(layer_name):
        print layer_name, "is not a valid layer name."
        return Rhino.Commands.Result.Cancel

    # Does a layer with the same name already exist?
    layer_index = scriptcontext.doc.Layers.Find(layer_name, True)
    if layer_index>=0:
        print "A layer with the name", layer_name, "already exists."
        return Rhino.Commands.Result.Cancel

    # Add a new layer to the document
    layer_index = scriptcontext.doc.Layers.Add(layer_name, System.Drawing.Color.Black)
    if layer_index<0:
        print "Unable to add", layer_name, "layer."
        return Rhino.Commands.Result.Failure

    return Rhino.Commands.Result.Success


if __name__=="__main__":
    AddLayer()

Partial Class Examples
  Public Shared Function AddLayer(ByVal doc As Rhino.RhinoDoc) As Rhino.Commands.Result
    ' Cook up an unused layer name
    Dim unused_name As String = doc.Layers.GetUnusedLayerName(False)

    ' Prompt the user to enter a layer name
    Dim gs As New Rhino.Input.Custom.GetString()
    gs.SetCommandPrompt("Name of layer to add")
    gs.SetDefaultString(unused_name)
    gs.AcceptNothing(True)
    gs.Get()
    If gs.CommandResult() <> Rhino.Commands.Result.Success Then
      Return gs.CommandResult()
    End If

    ' Was a layer named entered?
    Dim layer_name As String = gs.StringResult().Trim()
    If String.IsNullOrEmpty(layer_name) Then
      Rhino.RhinoApp.WriteLine("Layer name cannot be blank.")
      Return Rhino.Commands.Result.Cancel
    End If

    ' Is the layer name valid?
    If Not Rhino.DocObjects.Layer.IsValidName(layer_name) Then
      Rhino.RhinoApp.WriteLine(layer_name & " is not a valid layer name.")
      Return Rhino.Commands.Result.Cancel
    End If

    ' Does a layer with the same name already exist?
    Dim layer_index As Integer = doc.Layers.Find(layer_name, True)
    If layer_index >= 0 Then
      Rhino.RhinoApp.WriteLine("A layer with the name {0} already exists.", layer_name)
      Return Rhino.Commands.Result.Cancel
    End If

    ' Add a new layer to the document
    layer_index = doc.Layers.Add(layer_name, System.Drawing.Color.Black)
    If layer_index < 0 Then
      Rhino.RhinoApp.WriteLine("Unable to add {0} layer.", layer_name)
      Return Rhino.Commands.Result.Failure
    End If
    Return Rhino.Commands.Result.Success
  End Function
End Class