RenderTargetBitmap渲染未显示Grid报错：值不在预期范围的解决求助

问题原因 #

未可视化的Grid没有经过UWP的布局流程（Measure/Arrange），导致RenderTargetBitmap.RenderAsync无法获取有效的元素尺寸信息，进而抛出"Value does not fall within the expected range."异常。此外代码中存在拼写错误：scaledWhidth、scaledHeigh应为scaledWidth、scaledHeight，虽不直接引发异常，但需修正以避免逻辑问题。

解决方案 #

方案1：手动触发Grid的布局计算 #

在调用RenderAsync前，强制让Grid完成Measure和Arrange步骤，确保它有明确的尺寸信息：

private async Task<iText.Layout.Element.Image> SetImagePDF(Grid Page)
{
    var stream = new InMemoryRandomAccessStream();
    RenderTargetBitmap renderTargetBitmap = new RenderTargetBitmap();
    try
    {
        int DPI = 600;
        // 修正拼写错误
        double scaledWidth = (210 * DPI) / 25.4;
        double scaledHeight = (297 * DPI) / 25.4;

        // 手动触发Grid的布局流程
        Page.Measure(new Size(scaledWidth, scaledHeight));
        Page.Arrange(new Rect(0, 0, scaledWidth, scaledHeight));
        await Page.UpdateLayoutAsync();

        await renderTargetBitmap.RenderAsync(Page, (int)scaledWidth, (int)scaledHeight);
        IBuffer pixelBuffer = await renderTargetBitmap.GetPixelsAsync();
        byte[] pixels = pixelBuffer.ToArray();
        
        var encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.PngEncoderId, stream);
        encoder.SetPixelData(BitmapPixelFormat.Bgra8, BitmapAlphaMode.Premultiplied,
                                (uint)renderTargetBitmap.PixelWidth,
                                (uint)renderTargetBitmap.PixelHeight,
                                DPI, DPI, pixels);
        await encoder.FlushAsync();
        
        stream.Seek(0);
        var imageBytes = new byte[stream.Size];
        await stream.AsStream().ReadAsync(imageBytes, 0, imageBytes.Length);
        
        ImageData imageData = ImageDataFactory.Create(imageBytes);
        return new iText.Layout.Element.Image(imageData);
    }
    catch (Exception e) 
    { 
        // 建议添加日志而非直接返回null，便于排查问题
        // Debug.WriteLine(e.ToString());
        return null; 
    }
}

方案2：限制渲染尺寸范围 #

如果scaledWidth/scaledHeight计算后超出RenderTargetBitmap支持的最大尺寸（UWP平台通常上限为16384x16384），可调整参数：

• 降低DPI至300，此时A4尺寸约为2480x3508，远低于上限
• 若必须600DPI，可确认平台支持的最大渲染尺寸，避免参数超出范围

方案3：使用Win2D替代RenderTargetBitmap（UWP专属） #

Win2D的CanvasRenderTarget对未可视化XAML元素的渲染支持更稳定，可绕过布局依赖问题：

using Microsoft.Graphics.Canvas;
using Microsoft.Graphics.Canvas.UI.Xaml;

private async Task<iText.Layout.Element.Image> SetImagePDF(Grid Page)
{
    int DPI = 600;
    double scaledWidth = (210 * DPI) / 25.4;
    double scaledHeight = (297 * DPI) / 25.4;

    using (var canvasDevice = CanvasDevice.GetSharedDevice())
    using (var renderTarget = new CanvasRenderTarget(canvasDevice, (float)scaledWidth, (float)scaledHeight, DPI))
    {
        using (var ds = renderTarget.CreateDrawingSession())
        {
            ds.DrawElement(Page, new Rect(0, 0, scaledWidth, scaledHeight));
        }

        using (var stream = new InMemoryRandomAccessStream())
        {
            await renderTarget.SaveAsync(stream, CanvasBitmapFileFormat.Png);
            stream.Seek(0);
            var imageBytes = new byte[stream.Size];
            await stream.AsStream().ReadAsync(imageBytes, 0, imageBytes.Length);
            
            ImageData imageData = ImageDataFactory.Create(imageBytes);
            return new iText.Layout.Element.Image(imageData);
        }
    }
}

注意：使用Win2D需通过NuGet安装Microsoft.Graphics.Win2D包

内容的提问来源于stack exchange，提问作者Wolsi Wolsi