[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?

[Niya]

It means most likely your math is off somewhere and this caused you to read memory that doesn't belong to you. I'd review the math in that procedure to make sure there aren't any off-by-one type errors.

[Troy Lundin]

You were absolutely right, I was trying to read memory out of my range. I got it now. Thanks for your response.