AutoCAD GripOverrule API sample, demonstrates how to add a direction grip that will change the direction of a polyline when clicked.

clsDirectionGrip.vb

Option Explicit On

Imports Autodesk.AutoCAD.ApplicationServices
Imports Autodesk.AutoCAD.Geometry
Imports Autodesk.AutoCAD.GraphicsInterface
Imports Autodesk.AutoCAD.DatabaseServices

Public Class DirectionGrip
    Inherits GripData

    Public Delegate Sub ComputeGripPosition()
    Public Delegate Sub EntityAction(id As ObjectId)

    Private ReadOnly _Crv As Curve
    Private ReadOnly _GripAction As EntityAction
    Private ReadOnly _AssignPosition As ComputeGripPosition
    Private _GripDirection As Vector3d
    Private _GripOffset As Double = 4.0#

    Public Sub New(crv As Curve, gripAction As EntityAction)
        If crv Is Nothing OrElse gripAction Is Nothing Then Throw New ArgumentNullException("Neither 'crv' or 'gripAction' arguments can be null.")

        _Crv = crv
        _AssignPosition = AddressOf AssignStartPosition
        _GripAction = gripAction
    End Sub

    Public Sub New(crv As Curve, atStart As Boolean)
        If crv Is Nothing Then Throw New ArgumentNullException("The 'crv' argument cannot be null.")

        _Crv = crv
        _GripAction = AddressOf ExecuteReverse

        If atStart Then
            _AssignPosition = AddressOf AssignStartPosition
        Else
            _AssignPosition = AddressOf AssignEndPosition
        End If

        _AssignPosition()
    End Sub

    Public Sub UpdateGripPoint()
        _AssignPosition.Invoke()
    End Sub

#Region "Overriden Methods"

    Public Overrides Function OnHotGrip(entityId As ObjectId, contextFlags As Context) As ReturnValue
        If contextFlags <> 0 Then Return ReturnValue.Failure

        Try
            _GripAction(_Crv.Id)

            Return ReturnValue.GetNewGripPoints
        Catch
            Return ReturnValue.Failure
        Finally
            ' A little hack to help ensure the entity and grips are regenerated.
            Autodesk.AutoCAD.Internal.Utils.RegenEntity(_Crv.Id)
        End Try
    End Function

    Public Overrides Function WorldDraw(wd As WorldDraw, entityId As ObjectId, type As DrawType, imageGripPoint As Point3d?, dGripSize As Double) As Boolean
        Try : DrawDirection(wd, type, dGripSize)
        Catch
        End Try

        Return True
    End Function

#End Region

    Private Sub DrawDirection(wd As WorldDraw, dt As DrawType, size As Double)
        Dim a As Double = _GripDirection.GetAngleTo(Vector3d.XAxis)

        If _GripDirection.Y < 0 Then a = Math.PI * 2 - a

        Using pts As New Point3dCollection
            Dim v As Vector3d = Me.GripPoint.GetAsVector() - New Vector3d(1.5# * size, 0#, 0#)

            pts.Add(New Point3d(0, 0, 0) + v)
            pts.Add(New Point3d(1.5 * size, 1.5 * size, 0) + v)
            pts.Add(New Point3d(1.5 * size, 0.65 * size, 0) + v)
            pts.Add(New Point3d(3 * size, 0.65 * size, 0) + v)
            pts.Add(New Point3d(3 * size, -0.65 * size, 0) + v)
            pts.Add(New Point3d(1.5 * size, -0.65 * size, 0) + v)
            pts.Add(New Point3d(1.5 * size, -1.5 * size, 0) + v)

            For index As Integer = 0 To pts.Count - 1
                pts(index) = pts(index).RotateBy(a, Vector3d.ZAxis, Me.GripPoint)
            Next

            wd.SubEntityTraits.Color = If(dt = DrawType.WarmGrip, 82, 11)
            wd.SubEntityTraits.FillType = FillType.FillAlways
            wd.Geometry.Polygon(pts)

            wd.SubEntityTraits.Color = If(dt = DrawType.WarmGrip, 3, 1)
            wd.SubEntityTraits.FillType = FillType.FillNever
            wd.Geometry.Polygon(pts)
        End Using
    End Sub

    ''' <summary>
    ''' Assign the grip position from the start of the curve.
    ''' </summary>
    Private Sub AssignStartPosition()
        Dim v As Vector3d = _Crv.GetFirstDerivative(_Crv.StartParam) * -1.0#

        _GripDirection = v / v.Length
        Me.GripPoint = _Crv.StartPoint + _GripDirection * _GripOffset
    End Sub

    ''' <summary>
    ''' Assign the grip position from the end of the curve.
    ''' </summary>
    Private Sub AssignEndPosition()
        Dim v As Vector3d = _Crv.GetFirstDerivative(_Crv.EndParam)

        v = v / v.Length

        Me.GripPoint = _Crv.EndPoint + v * _GripOffset

        _GripDirection = v * -1.0#
    End Sub

    ''' <summary>
    ''' Reverse the direction of the curve.
    ''' </summary>
    Private Shared Sub ExecuteReverse(id As ObjectId)
        Dim dwg As Document = Application.DocumentManager.GetDocument(id.Database)

        Using l = dwg.LockDocument(DocumentLockMode.ExclusiveWrite, "Reverse", "Reverse", False)
            Using tr As Transaction = dwg.TransactionManager.StartTransaction()
                Try
                    Dim crv As Curve = tr.GetObject(id, OpenMode.ForWrite, True, True)
                    crv.ReverseCurve()
                Finally
                    tr.Commit()
                End Try
            End Using
        End Using
    End Sub

End Class

clsSampleGrip.vb

Option Explicit On

Imports System.ComponentModel

Imports Autodesk.AutoCAD.DatabaseServices
Imports Autodesk.AutoCAD.Geometry
Imports Autodesk.AutoCAD.Runtime

Public Class SampleGrip
    Inherits GripOverrule
    Implements IRuleEnabled

    Public Sub New()
        MyBase.SetCustomFilter()
    End Sub

#Region "IRuleEnabled Implements"

    Public ReadOnly Property Name As String Implements IRuleEnabled.Name
        Get
            Return "Polyline Direction Grip"
        End Get
    End Property

    Public Property Enabled As Boolean = False Implements IRuleEnabled.Enabled

#End Region

#Region "Overridden Methods"

    Public Overrides Function IsApplicable(overruledSubject As RXObject) As Boolean
        Return _Enabled AndAlso TypeOf overruledSubject Is Polyline AndAlso DirectCast(overruledSubject, Polyline).NumberOfVertices > 1
    End Function

    Public Overrides Sub GetGripPoints(entity As Entity, grips As GripDataCollection, curViewUnitSize As Double, gripSize As Integer, curViewDir As Vector3d, bitFlags As GetGripPointsFlags)
        Try : ComputeGrips(entity, grips, curViewUnitSize, gripSize, curViewDir, bitFlags)
        Catch
        End Try
    End Sub

#End Region

    Private Sub ComputeGrips(entity As Entity, grips As GripDataCollection, curViewUnitSize As Double, gripSize As Integer, curViewDir As Vector3d, bitFlags As GetGripPointsFlags)
        MyBase.GetGripPoints(entity, grips, curViewUnitSize, gripSize, curViewDir, bitFlags)

        Dim crv As Curve = DirectCast(entity, Curve)

        grips.Add(New DirectionGrip(entity, True))
        If Not crv.Closed Then grips.Add(New DirectionGrip(entity, False))
    End Sub

End Class

nfIRuleEnabled.vb

Public Interface IRuleEnabled

    ReadOnly Property Name As String
    Property Enabled As Boolean

End Interface