How to remove a line which is drawn on a form - vb.net

Question

I'm trying to draw a triangle like this:
Dim triangle As Graphics
Dim pen1 As New Pen(Color.LimeGreen, 2)
Dim lside As Integer
Dim wside As Integer
Dim dside As Integer
triangle = Me.CreateGraphics()
triangle.DrawLine(pen1, wside, 420, 640, 420)
triangle.DrawLine(pen1, 640, lside, 640, 420)
triangle.DrawLine(pen1, dside, 420, 640, lside)
lside, wside and dside stand for length side, width side and diagonal side.
I've got 4 textboxes, for the length, width, diagonal side and one for the angle.
The purpose is to fill in 2 of the values, and then a rectangular triangle gets drawn following Pythagoras' theorem. I want to draw a line for Angle as well later on. But I first want to get this to work.
But every time I click the button to draw a new triangle, the previous one should get deleted. And that's the problem.
I've tried multiple methods, like triangle.Dispose triangle.Restore triangle.Clear and more. None of them work.
Why am I not drawing them in a picturebox you might ask. Well, when I drew a line in a picturebox, the picturebox sort of went in front of the line, making the line invisible. And I didn't know how to fix that.

Answer 1

Try using Me.Invalidate(), it basically clears, then draws the shape in the area you're painting on. Reference.
Private Sub ClearCanvas_Click(sender As Object, e As EventArgs) Handles Button1.Click
Me.Invalidate()
End Sub
Priavte DrawTriangle_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim triangle As Graphics
Dim pen1 As New Pen(Color.LimeGreen, 2)
Dim lside As Integer
Dim wside As Integer
Dim dside As Integer
triangle = Me.CreateGraphics()
triangle.DrawLine(pen1, wside, 420, 640, 420)
triangle.DrawLine(pen1, 640, lside, 640, 420)
triangle.DrawLine(pen1, dside, 420, 640, lside)
End Sub

Answer 2

‘Draw select delete multiple lines on Picturebox
Imports System
Imports System.Drawing
Imports System.Drawing.Drawing2D
Public Class Form1
Dim drawrec, undo_delete As Boolean
Dim index_arrary_line_tobe_deleted(10000) As Integer
Dim ptA, ptB As Point ' starting and ending point
Dim down As Boolean
Dim k, Last_index_line_tobe_selected As Integer
Private temp As line
Dim List_of_line_tobe_deleted As New List(Of line)
Dim List_of_line_to_Undo As New List(Of line)
Private m_Lines As New List(Of line)
Private m_Pt As Point
Private m_Pt2 As Point
Private m_tracking As Boolean
Private Sub B2_index_arrary_line_tobe_deletedete_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles B2_Delete.Click
Try
m_Lines.RemoveAll(AddressOf List_of_line_tobe_deleted.Contains)
Catch ex As Exception
End Try
PictureBox1.Refresh()
End Sub
Private Sub B3_Undodelete_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles B3_Undodelete.Click
undo_delete = True
Try
m_Lines.AddRange(List_of_line_tobe_deleted)
Catch ex As Exception
End Try
PictureBox1.Refresh()
End Sub
Private Sub B1_Select_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles B1_Select.Click
drawrec = True
End Sub
Private Sub PictureBox1_Paint(ByVal sender As Object, ByVal e As PaintEventArgs) Handles PictureBox1.Paint
Dim r As New Rectangle(m_Pt, New Size(m_Pt2.X - m_Pt.X, m_Pt2.Y - m_Pt.Y))
If m_tracking = True And drawrec = True Then
k = -1
For i As Integer = 0 To m_Lines.Count - 1
If m_Lines(i).ContainsCompletely(r) = True Then
k = k + 1
index_arrary_line_tobe_deleted(i) = k
Debug.Print("Index of NOT selected lines " + i.ToString + "Index of selected lines " + Last_index_line_tobe_selected.ToString) 'to compare idex of two lists !!!!
index_arrary_line_tobe_deleted(k) = i
List_of_line_tobe_deleted.Add(m_Lines(i))
End If
Next
Last_index_line_tobe_selected = k 'so far no use, just to know
e.Graphics.DrawRectangle(Pens.Cyan, r)
End If
If undo_delete = False Then
For i As Integer = 0 To m_Lines.Count - 1
Me.m_Lines(i).Draw(e.Graphics, r)
Debug.Print("Index of remaining lines " + i.ToString)
Next
End If
If undo_delete = True Then
For i As Integer = 0 To m_Lines.Count - 1
Me.m_Lines(i).Re_Draw(e.Graphics)
Debug.Print("Index of remaining lines " + i.ToString)
Next
End If
End Sub
Private Sub Form1_Load(ByVal sender As Object, ByVal e As EventArgs) Handles MyBase.Load
drawrec = False
down = False
undo_delete = False
For i As Integer = 0 To index_arrary_line_tobe_deleted.Length - 1
index_arrary_line_tobe_deleted(0) = -1
Next i
k = -1
Last_index_line_tobe_selected = -1
End Sub
Private Sub PictureBox1_MouseDown(ByVal sender As Object, ByVal e As MouseEventArgs) Handles PictureBox1.MouseDown
down = True
If down = True And drawrec = False Then
ptA = e.Location
temp = New line
temp.StartPoint = e.Location
End If
If e.Button = MouseButtons.Left Then
ResetSelected(Me.m_Lines)
m_Pt = e.Location
End If
End Sub
Private Sub ResetSelected(ByVal m_Lines As List(Of line))
For i As Integer = 0 To m_Lines.Count - 1
m_Lines(i).Selected = False
Next
End Sub
Private Sub PictureBox1_MouseMove(ByVal sender As Object, ByVal e As MouseEventArgs) Handles PictureBox1.MouseMove
If down = True Then
End If
If e.Button = MouseButtons.Left Then
If down = True And drawrec = False Then
ptB = e.Location
temp.EndPoint = e.Location
End If
m_Pt2 = e.Location
m_tracking = True
Me.PictureBox1.Invalidate()
End If
End Sub
Private Sub PictureBox1_MouseUp(ByVal sender As Object, ByVal e As MouseEventArgs) Handles PictureBox1.MouseUp
down = False
If drawrec = False Then
temp.EndPoint = e.Location
m_Lines.Add(temp)
temp = Nothing
Me.PictureBox1.Invalidate()
End If
m_tracking = False
Me.PictureBox1.Invalidate()
End Sub
End Class
Public Class line
Public StartPoint As Point
Public EndPoint As Point
Private m_selected As Boolean
Public Property Selected() As Boolean
Get
Return m_selected
End Get
Set(ByVal value As Boolean)
m_selected = value
End Set
End Property
Public Sub Draw(ByVal g As Graphics, ByVal r As Rectangle)
Dim myPen1 As New Pen(Color.Red, 1)
g.SmoothingMode = SmoothingMode.AntiAlias
If Me.ContainsCompletely(r) OrElse Me.Selected Then
Me.Selected = True
g.DrawLine(myPen1, Me.StartPoint, Me.EndPoint)
Else
Dim myPen2 As New Pen(Color.Blue, 1)
g.DrawLine(myPen2, Me.StartPoint, Me.EndPoint)
End If
End Sub
Public Sub Re_Draw(ByVal g As Graphics)
g.SmoothingMode = SmoothingMode.AntiAlias
Dim myPen2 As New Pen(Color.Blue, 1)
g.DrawLine(myPen2, Me.StartPoint, Me.EndPoint)
End Sub
Public Function ContainsCompletely(ByVal r As Rectangle) As Boolean
If r.Contains(Me.StartPoint) AndAlso r.Contains(Me.EndPoint) Then
Return True
End If
Return False
End Function
End Class

Answer 3

Imports System
Imports System.Drawing
Imports System.Drawing.Drawing2D
Public Class Form1
Dim drawrec As Boolean
Dim del As Integer
Dim ptA, ptB As Point ' starting and ending point
Dim down As Boolean
Private temp As line
Private m_Lines As New List(Of line)
Private m_rnd As New Random
Private m_Pt As Point
Private m_Pt2 As Point
Private m_tracking As Boolean
Private Sub Button4_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Bt_Delete.Click
Try
m_Lines.RemoveAt(del)
Catch ex As Exception
End Try
PictureBox1.Refresh()
End Sub
Private Sub Form1_Load(ByVal sender As Object, ByVal e As EventArgs) Handles MyBase.Load
drawrec = False
down = False
del = -1
End Sub
Private Sub PictureBox1_MouseDown(ByVal sender As Object, ByVal e As MouseEventArgs) Handles PictureBox1.MouseDown
down = True
If down = True And drawrec = False Then
ptA = e.Location
temp = New line
temp.StartPoint = e.Location
End If
If e.Button = MouseButtons.Left Then
ResetSelected(Me.m_Lines)
m_Pt = e.Location
End If
End Sub
Private Sub ResetSelected(ByVal m_Lines As List(Of line))
For i As Integer = 0 To m_Lines.Count - 1
m_Lines(i).Selected = False
Next
End Sub
Private Sub PictureBox1_MouseMove(ByVal sender As Object, ByVal e As MouseEventArgs) Handles PictureBox1.MouseMove
If down = True Then
End If
If e.Button = MouseButtons.Left Then
If down = True And drawrec = False Then
ptB = e.Location
temp.EndPoint = e.Location
End If
m_Pt2 = e.Location
m_tracking = True
Me.PictureBox1.Invalidate()
End If
End Sub
Private Sub PictureBox1_MouseUp(ByVal sender As Object, ByVal e As MouseEventArgs) Handles PictureBox1.MouseUp
down = False
If drawrec = False Then
temp.EndPoint = e.Location
m_Lines.Add(temp)
temp = Nothing
Me.PictureBox1.Invalidate()
End If
m_tracking = False
Me.PictureBox1.Invalidate()
End Sub
Private Sub PictureBox1_Paint(ByVal sender As Object, ByVal e As PaintEventArgs) Handles PictureBox1.Paint
Dim r As New Rectangle(m_Pt, New Size(m_Pt2.X - m_Pt.X, m_Pt2.Y - m_Pt.Y))
If m_tracking And drawrec = True Then
For i As Integer = 0 To m_Lines.Count - 1
If m_Lines(i).ContainsCompletely(r) = True Then
Debug.Print("KKKKKKKKKKKKKKK " + i.ToString)
del = i
End If
Next
e.Graphics.DrawRectangle(Pens.Cyan, r)
End If
For i As Integer = 0 To m_Lines.Count - 1
Me.m_Lines(i).Draw(e.Graphics, r)
Next
End Sub
Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Bt_Select.Click
drawrec = True
End Sub
End Class
Public Class line
Public StartPoint As Point
Public EndPoint As Point
Public Filled As Boolean
Public ShapeColor As Color
Public PenWidth As Integer
Private m_selected As Boolean
Public Property Selected() As Boolean
Get
Return m_selected
End Get
Set(ByVal value As Boolean)
m_selected = value
End Set
End Property
Public Sub Draw(ByVal g As Graphics, ByVal r As Rectangle)
g.SmoothingMode = SmoothingMode.AntiAlias
If Me.ContainsCompletely(r) OrElse Me.Selected Then
Me.Selected = True
g.DrawLine(Pens.Red, Me.StartPoint, Me.EndPoint)
Else
g.DrawLine(Pens.Blue, Me.StartPoint, Me.EndPoint)
End If
End Sub
Public Function ContainsCompletely(ByVal r As Rectangle) As Boolean
If r.Contains(Me.StartPoint) AndAlso r.Contains(Me.EndPoint) Then
Return True
End If
Return False
End Function
End Class