C# 如何从函数返回新图像？

我加了一行

if (newWidth >= 1 && newHeight >= 1)

现在我得到了错误

错误1“Images\u Batch\u Resize.Form1.ResizeImagestring，int，int”：并非所有代码路径都返回值

private static Bitmap ResizeImage(String filename, int maxWidth, int maxHeight)
        {
            using (Image originalImage = Image.FromFile(filename))
            {
                //Caluate new Size
                int newWidth = originalImage.Width;
                int newHeight = originalImage.Height;
                double aspectRatio = (double)originalImage.Width / (double)originalImage.Height;
                if (aspectRatio <= 1 && originalImage.Width > maxWidth)
                {
                    newWidth = maxWidth;
                    newHeight = (int)Math.Round(newWidth / aspectRatio);
                }
                else if (aspectRatio > 1 && originalImage.Height > maxHeight)
                {
                    newHeight = maxHeight;
                    newWidth = (int)Math.Round(newHeight * aspectRatio);
                }
                if (newWidth >= 1 && newHeight >= 1)
                {
                    Bitmap newImage = new Bitmap(newWidth, newHeight);
                    using (Graphics g = Graphics.FromImage(newImage))
                    {
                        //--Quality Settings Adjust to fit your application
                        g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBilinear;
                        g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
                        g.PixelOffsetMode = System.Drawing.Drawing2D.PixelOffsetMode.HighQuality;
                        g.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
                        g.DrawImage(originalImage, 0, 0, newImage.Width, newImage.Height);
                        return newImage;
                    }
                }
            }

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您之所以会收到该错误，是因为函数并不总是返回值。如果newWidth不是>=1或newHeight不是>=1，则它将不会返回任何内容。如果它不满足这些要求，您将希望返回null或类似的值，这样您就知道有问题了

您只返回其中一个if in last中的值。将return语句添加到other if或在该函数的末尾添加类似returnnull的内容

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在您的代码中，如果代码没有最后命中，如果它不会返回任何值，但是你要减速，这个方法会在最后返回位图，所以编译希望在任何情况下，对于该函数的用户，某种类型的位图都会返回，即使是空的

如果newWidth或newHeight为零，你想返回什么？因为…当测试失败时会发生什么？它经过了那个代码块，然后呢？您需要在最后返回…返回null；可能

private static Bitmap ResizeImage(String filename, int maxWidth, int maxHeight)
    {
        using (Image originalImage = Image.FromFile(filename))
        {
            //Caluate new Size
            int newWidth = originalImage.Width;
            int newHeight = originalImage.Height;
            double aspectRatio = (double)originalImage.Width / (double)originalImage.Height;
            if (aspectRatio <= 1 && originalImage.Width > maxWidth)
            {
                newWidth = maxWidth;
                newHeight = (int)Math.Round(newWidth / aspectRatio);
            }
            else if (aspectRatio > 1 && originalImage.Height > maxHeight)
            {
                newHeight = maxHeight;
                newWidth = (int)Math.Round(newHeight * aspectRatio);

            }
            if (newWidth >= 1 && newHeight >= 1)
            {
                Bitmap newImage = new Bitmap(newWidth, newHeight);
                using (Graphics g = Graphics.FromImage(newImage))
                {
                    //--Quality Settings Adjust to fit your application
                    g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBilinear;
                    g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
                    g.PixelOffsetMode = System.Drawing.Drawing2D.PixelOffsetMode.HighQuality;
                    g.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
                    g.DrawImage(originalImage, 0, 0, newImage.Width, newImage.Height);
                    return newImage;
                }
            }
            return null ( or something ) 
        }