您想要使用UIButton的TitleEdgeInsets和ImageEdgeInsets属性来控制文字和影像的内缩开始位置：

button.TitleEdgeInsets = new UIEdgeInsets(0, -button.ImageView.Frame.Size.Width, 0, button.ImageView.Frame.Size.Width);
button.ImageEdgeInsets = new UIEdgeInsets(0, button.TitleLabel.Frame.Size.Width, 0, -button.TitleLabel.Frame.Size.Width);

完整示例：

button = new UIButton(UIButtonType.System);
    button.Frame = new CGRect(150, 20, 200, 50);
    button.Layer.CornerRadius = 5;
    button.BackgroundColor = UIColor.Black;
    button.SetTitleColor(UIColor.White, UIControlState.Normal);
    button.SetImage(UIImage.FromFile("ratingstar.png"), UIControlState.Normal);
    button.SetTitle("StackOverflow", UIControlState.Normal);
    button.TitleEdgeInsets = new UIEdgeInsets(0, -button.ImageView.Frame.Size.Width, 0, button.ImageView.Frame.Size.Width);
    button.ImageEdgeInsets = new UIEdgeInsets(0, button.TitleLabel.Frame.Size.Width, 0, -button.TitleLabel.Frame.Size.Width);