class DrawArrowHeadsConduit : Rhino.Display.DisplayConduit
{
  private readonly Line m_line;
  private readonly int m_screen_size;
  private readonly double m_world_size;

  public DrawArrowHeadsConduit(Line line, int screenSize, double worldSize)
  {
    m_line = line;
    m_screen_size = screenSize;
    m_world_size = worldSize;
  }

  protected override void DrawForeground(Rhino.Display.DrawEventArgs e)
  {
    e.Display.DrawArrow(m_line, System.Drawing.Color.Black, m_screen_size, m_world_size);
  }
}

partial class Examples
{
  static DrawArrowHeadsConduit m_draw_conduit;

  public static Result ConduitArrowHeads(RhinoDoc doc)
  {
    if (m_draw_conduit != null)
    {
      RhinoApp.WriteLine("Turn off existing arrowhead conduit");
      m_draw_conduit.Enabled = false;
      m_draw_conduit = null;
    }
    else
    {
      // get arrow head size
      var go = new GetOption();
      go.SetCommandPrompt("ArrowHead length in screen size (pixels) or world size (percentage of arrow length)?");
      go.AddOption("screen");
      go.AddOption("world");
      go.Get();
      if (go.CommandResult() != Result.Success)
        return go.CommandResult();

      int screen_size = 0;
      double world_size = 0.0;
      if (go.Option().EnglishName == "screen")
      {
        var gi = new GetInteger();
        gi.SetLowerLimit(0, true);
        gi.SetCommandPrompt("Length of arrow head in pixels");
        gi.Get();
        if (gi.CommandResult() != Result.Success)
          return gi.CommandResult();
        screen_size = gi.Number();
      }
      else
      {
        var gi = new GetInteger();
        gi.SetLowerLimit(0, true);
        gi.SetUpperLimit(100, false);
        gi.SetCommandPrompt("Length of arrow head in percentage of total arrow length");
        gi.Get();
        if (gi.CommandResult() != Result.Success)
          return gi.CommandResult();
        world_size = gi.Number() / 100.0;
      }


      // get arrow start and end points
      var gp = new GetPoint();
      gp.SetCommandPrompt("Start of line");
      gp.Get();
      if (gp.CommandResult() != Result.Success)
        return gp.CommandResult();
      var start_point = gp.Point();

      gp.SetCommandPrompt("End of line");
      gp.SetBasePoint(start_point, false);
      gp.DrawLineFromPoint(start_point, true);
      gp.Get();
      if (gp.CommandResult() != Result.Success)
        return gp.CommandResult();
      var end_point = gp.Point();

      var v = end_point - start_point;
      if (v.IsTiny(Rhino.RhinoMath.ZeroTolerance))
        return Result.Nothing;

      var line = new Line(start_point, end_point);

      m_draw_conduit = new DrawArrowHeadsConduit(line, screen_size, world_size);
      // toggle conduit on/off
      m_draw_conduit.Enabled = true;
      RhinoApp.WriteLine("Draw arrowheads conduit enabled.");
    }
    doc.Views.Redraw();
    return Result.Success;
  }
}

Partial Friend Class Examples
  Private Shared m_draw_conduit As DrawArrowHeadsConduit

  Public Shared Function ConduitArrowHeads(ByVal doc As RhinoDoc) As Result
	If m_draw_conduit IsNot Nothing Then
	  RhinoApp.WriteLine("Turn off existing arrowhead conduit")
	  m_draw_conduit.Enabled = False
	  m_draw_conduit = Nothing
	Else
	  ' get arrow head size
	  Dim go = New GetOption()
	  go.SetCommandPrompt("ArrowHead length in screen size (pixels) or world size (percentage of arrow length)?")
	  go.AddOption("screen")
	  go.AddOption("world")
	  go.Get()
	  If go.CommandResult() <> Result.Success Then
		Return go.CommandResult()
	  End If

	  Dim screen_size As Integer = 0
	  Dim world_size As Double = 0.0
	  If go.Option().EnglishName = "screen" Then
		Dim gi = New GetInteger()
		gi.SetLowerLimit(0, True)
		gi.SetCommandPrompt("Length of arrow head in pixels")
		gi.Get()
		If gi.CommandResult() <> Result.Success Then
		  Return gi.CommandResult()
		End If
		screen_size = gi.Number()
	  Else
		Dim gi = New GetInteger()
		gi.SetLowerLimit(0, True)
		gi.SetUpperLimit(100, False)
		gi.SetCommandPrompt("Length of arrow head in percentage of total arrow length")
		gi.Get()
		If gi.CommandResult() <> Result.Success Then
		  Return gi.CommandResult()
		End If
'INSTANT VB WARNING: Instant VB cannot determine whether both operands of this division are integer types - if they are then you should use the VB integer division operator:
		world_size = gi.Number() / 100.0
	  End If


	  ' get arrow start and end points
	  Dim gp = New GetPoint()
	  gp.SetCommandPrompt("Start of line")
	  gp.Get()
	  If gp.CommandResult() <> Result.Success Then
		Return gp.CommandResult()
	  End If
	  Dim start_point = gp.Point()

	  gp.SetCommandPrompt("End of line")
	  gp.SetBasePoint(start_point, False)
	  gp.DrawLineFromPoint(start_point, True)
	  gp.Get()
	  If gp.CommandResult() <> Result.Success Then
		Return gp.CommandResult()
	  End If
	  Dim end_point = gp.Point()

	  Dim v = end_point - start_point
	  If v.IsTiny(Rhino.RhinoMath.ZeroTolerance) Then
		Return Result.Nothing
	  End If

	  Dim line = New Line(start_point, end_point)

	  m_draw_conduit = New DrawArrowHeadsConduit(line, screen_size, world_size)
	  ' toggle conduit on/off
	  m_draw_conduit.Enabled = True
	  RhinoApp.WriteLine("Draw arrowheads conduit enabled.")
	End If
	doc.Views.Redraw()
	Return Result.Success
  End Function
End Class

import Rhino
import System.Drawing
import scriptcontext
import rhinoscriptsyntax as rs

class DrawArrowHeadsConduit(Rhino.Display.DisplayConduit):
    def __init__(self, line, screenSize, worldSize):
        self.line = line
        self.screenSize = screenSize
        self.worldSize = worldSize

    def DrawForeground(self, e):
        e.Display.DrawArrow(self.line, System.Drawing.Color.Black, self.screenSize, self.worldSize)

def RunCommand():
    # get arrow head size
    go = Rhino.Input.Custom.GetOption()
    go.SetCommandPrompt("ArrowHead length in screen size (pixles) or world size (percentage of arrow lenght)?")
    go.AddOption("screen")
    go.AddOption("world")
    go.Get()
    if (go.CommandResult() != Rhino.Commands.Result.Success):
        return go.CommandResult()

    screenSize = 0
    worldSize = 0.0
    if (go.Option().EnglishName == "screen"):
        gi = Rhino.Input.Custom.GetInteger()
        gi.SetLowerLimit(0,True)
        gi.SetCommandPrompt("Length of arrow head in pixels")
        gi.Get()
        if (gi.CommandResult() != Rhino.Commands.Result.Success):
            return gi.CommandResult()
        screenSize = gi.Number()
    else:
        gi = Rhino.Input.Custom.GetInteger()
        gi.SetLowerLimit(0, True)
        gi.SetUpperLimit(100, False)
        gi.SetCommandPrompt("Lenght of arrow head in percentage of total arrow lenght")
        gi.Get()
        if (gi.CommandResult() != Rhino.Commands.Result.Success):
            return gi.CommandResult()
        worldSize = gi.Number()/100.0


    # get arrow start and end points
    gp = Rhino.Input.Custom.GetPoint()
    gp.SetCommandPrompt("Start of line")
    gp.Get()
    if (gp.CommandResult() != Rhino.Commands.Result.Success):
        return gp.CommandResult()
    ptStart = gp.Point()

    gp.SetCommandPrompt("End of line")
    gp.SetBasePoint(ptStart, False)
    gp.DrawLineFromPoint(ptStart, True)
    gp.Get()
    if (gp.CommandResult() != Rhino.Commands.Result.Success):
        return gp.CommandResult()
    ptEnd = gp.Point()


    v = ptEnd - ptStart
    if (v.IsTiny(Rhino.RhinoMath.ZeroTolerance)):
        return Rhino.Commands.Result.Nothing

    line = Rhino.Geometry.Line(ptStart, ptEnd)

    conduit = DrawArrowHeadsConduit(line, screenSize, worldSize)
    conduit.Enabled = True
    scriptcontext.doc.Views.Redraw()
    rs.GetString("Pausing for user input")
    conduit.Enabled = False
    scriptcontext.doc.Views.Redraw()

    return Rhino.Commands.Result.Success

if __name__=="__main__":
    RunCommand()