[RESOLVED] Marshal.ReadInt32 -> AccessViolationException

[Troy Lundin]

I am getting an AccessViolationError with the following code.

Sub Blend(PosX As Int32, PosY As Int32)
            GetDrawingDimensions(PosX, PosY)
 
            Data = New BitmapData
            CanvasData = New BitmapData
 
            Select Case True
                Case Me.X < 0 AndAlso Me.Y < 0
                    Bounds = New Rectangle(Math.Abs(Me.X), Math.Abs(Me.Y), WidthToDraw, HeightToDraw)
                    CanvasBounds = New Rectangle(PosX, PosY, WidthToDraw, HeightToDraw)
                Case Me.X < 0
                    Bounds = New Rectangle(Math.Abs(Me.X), Y, WidthToDraw, HeightToDraw)
                    CanvasBounds = New Rectangle(PosX, Y + PosY, WidthToDraw, HeightToDraw)
                Case Me.Y < 0
                    Bounds = New Rectangle(X, Math.Abs(Me.Y), WidthToDraw, HeightToDraw)
                    CanvasBounds = New Rectangle(X + PosX, PosY, WidthToDraw, HeightToDraw)
                Case Else
                    Bounds = New Rectangle(X, Y, WidthToDraw, HeightToDraw)
                    CanvasBounds = New Rectangle(X + PosX, Y + PosY, WidthToDraw, HeightToDraw)
            End Select
 
            Data = TiledImage.LockBits(Bounds, ImageLockMode.ReadOnly, PixelFormat.Format32bppPArgb)
            CanvasData = bmpCanvas.LockBits(CanvasBounds, ImageLockMode.ReadWrite, PixelFormat.Format32bppPArgb)
            For y As Int32 = 0 To HeightToDraw - 1
                For x As Int32 = 0 To WidthToDraw - 1
 
                    C1 = Color.FromArgb(Marshal.ReadInt32(Data.Scan0, (Data.Stride * y) + (4 * x)))
                    C2 = Color.FromArgb(Marshal.ReadInt32(CanvasData.Scan0, (CanvasData.Stride * (y + Me.Y)) + (4 * (x + Me.X))))
 
                    If Not C1.ToArgb = Alpha.ToArgb Then
 
                        R1 = wA * (C1.R / 8) : G1 = wA * (C1.G / 8) : B1 = wA * (C1.B / 8)
                        R2 = wB * (C2.R / 8) : G2 = wB * (C2.G / 8) : B2 = wB * (C2.B / 8)
 
                        R = Math.Min(31, Math.Floor(R1 + R2)) * 8
                        G = Math.Min(31, Math.Floor(G1 + G2)) * 8
                        B = Math.Min(31, Math.Floor(B1 + B2)) * 8
 
                        Try
                            Marshal.WriteByte(CanvasData.Scan0, (CanvasData.Stride * (y + Me.Y)) + (4 * (x + Me.X)), CByte(B))
                            Marshal.WriteByte(CanvasData.Scan0, (CanvasData.Stride * (y + Me.Y)) + (4 * (x + Me.X)) + 1, CByte(G))
                            Marshal.WriteByte(CanvasData.Scan0, (CanvasData.Stride * (y + Me.Y)) + (4 * (x + Me.X)) + 2, CByte(R))
                        Catch ex As Exception
                            MessageBox.Show(ex.Message)
                        End Try
 
                    End If
                Next
                blendProgress.PerformStep(WidthToDraw)
            Next
 
            TiledImage.UnlockBits(Data)
            bmpCanvas.UnlockBits(CanvasData)
        End Sub

The error happens on the line:

C2 = Color.FromArgb(Marshal.ReadInt32(CanvasData.Scan0, (CanvasData.Stride * (y + Me.Y)) + (4 * (x + Me.X))))

I have read up on the exception and found out it comes from an invalid pointer. The weird part is the violation only happens sometimes. Even if I put that sub in a loop, it won't happen every pass. Anyone seen another error that may be causing it?