C# 设置和获取 JPG 中的 PropertyItem 'Property not found' 异常

Question

在 C# 中，我试图在 jpg 文件中设置以下属性。但是当我尝试取回它们时，我得到了 Property not found 异常。此外，SetPropertyItem 调用无法报告成功\失败，因此很难理解哪里出了问题。

0x5111PropertyTagPixelPerUnitX

0x5112PropertyTagPixelPerUnitY

string file = "New.jpg";
double x = 24524.5555598;
double y = 234123.4123423;
Image img = Image.FromFile(file);
PropertyItem[] props = img.PropertyItems;
PropertyItem newProp1 = props.FirstOrDefault();
newProp1.Id = 0x5111;
newProp1.Type = 1;
newProp1.Value = BitConverter.GetBytes(x);
newProp1.Len = newProp1.Value.Length;

PropertyItem newProp2 = props.FirstOrDefault();
newProp2.Id = 0x5112;
newProp2.Type = 1;
newProp2.Value = BitConverter.GetBytes(y);
newProp2.Len = newProp1.Value.Length;

img.SetPropertyItem(newProp1);
img.SetPropertyItem(newProp2);
img.Save("New1.jpg", ImageFormat.Jpeg);

以及检索它们的代码，

string file = "New1.jpg";
Image img = Image.FromFile(file);
PropertyItem prop = img.GetPropertyItem(0x5111);

上面这行抛出异常'Property not found'

Answer 1

我不明白为什么他们将 PropertyItem 构造函数设为内部，并且不提供任何其他创建属性项的方法。然而，您可以只使用反射来解决这个奇怪的问题，它会起作用:

        string file = @"New1.jpg";
        double x = 24524.5555598;
        double y = 234123.4123423;
        var img = System.Drawing.Image.FromFile(file);
        // note how to force Activator.CreateInstance to call internal constructor, 
        // it's important to call this overload
        var newProp1 = (PropertyItem) Activator.CreateInstance(typeof(PropertyItem), BindingFlags.Instance | BindingFlags.NonPublic, null, new object[0], CultureInfo.InvariantCulture);
        newProp1.Id = 0x5111;
        newProp1.Type = 1;
        newProp1.Value = BitConverter.GetBytes(x);
        newProp1.Len = newProp1.Value.Length;

        var newProp2 = (PropertyItem)Activator.CreateInstance(typeof(PropertyItem), BindingFlags.Instance | BindingFlags.NonPublic, null, new object[0], CultureInfo.InvariantCulture);
        newProp2.Id = 0x5112;
        newProp2.Type = 1;
        newProp2.Value = BitConverter.GetBytes(y);
        newProp2.Len = newProp1.Value.Length;

        img.SetPropertyItem(newProp1);
        img.SetPropertyItem(newProp2);
        img.Save("New1.jpg", ImageFormat.Jpeg);