private void ConvertToDiff_Click(object sender, System.EventArgs e) { if (FilesList.SelectedIndices.Count == 0) { return; } int firstIdx = FilesList.SelectedIndices[0]; Sprite firstSprite = (Sprite)SPack.Sprites[firstIdx]; foreach (int idx in FilesList.SelectedIndices) { if (idx <= 0) { continue; } Sprite sprite = (Sprite)SPack.Sprites[idx]; sprite.Subtract(ref firstSprite); } }