WriteableBitmap.Unlock Method

// The DrawPixel method updates the WriteableBitmap by using 
// unsafe code to write a pixel into the back buffer. 
static void DrawPixel(MouseEventArgs e)
{
    int column = (int)e.GetPosition(i).X;
    int row = (int)e.GetPosition(i).Y;

    // Reserve the back buffer for updates.
    writeableBitmap.Lock();

    unsafe
    {
        // Get a pointer to the back buffer. 
        int pBackBuffer = (int)writeableBitmap.BackBuffer;

        // Find the address of the pixel to draw.
        pBackBuffer += row * writeableBitmap.BackBufferStride;
        pBackBuffer += column * 4;

        // Compute the pixel's color. 
        int color_data = 255 << 16; // R
        color_data |= 128 << 8;   // G
        color_data |= 255 << 0;   // B 

        // Assign the color data to the pixel.
        *((int*) pBackBuffer) = color_data;
    }

    // Specify the area of the bitmap that changed.
    writeableBitmap.AddDirtyRect(new Int32Rect(column, row, 1, 1));

    // Release the back buffer and make it available for display.
    writeableBitmap.Unlock();
}