您的基本要求是将包含属性"fileContent": "...base64..." 的JSON 转换为"fileRoute": "/route/to/file"，同时还将fileContent 的值写入单独的二进制文件而不将fileContent 的值作为完整的具体化字符串。
目前尚不清楚这是否可以通过 System.Text.Json 的 .NET Core 3.1 实现来完成。即使可以，也不容易。简单地从Stream 生成Utf8JsonReader 需要工作，请参阅Parsing a JSON file with .NET core 3.0/System.text.Json。完成此操作后，有一个方法 Utf8JsonReader.ValueSequence 将最后处理的令牌的原始值作为输入有效负载的 ReadOnlySequence<byte> 切片返回。但是，该方法似乎并不容易使用，因为它仅在令牌包含在多个段中时才有效，不能保证值的格式正确，并且不会转义 JSON 转义序列。
Newtonsoft 在这里根本不起作用，因为 JsonTextReader 总是完全实现每个原始字符串值。
作为替代方案，您可以考虑JsonReaderWriterFactory 返回的读者和作者。这些读取器和写入器由DataContractJsonSerializer 使用，并将JSON 即时转换为XML，因为它是read 和written。由于这些读取器和写入器的基类是XmlReader 和XmlWriter，因此它们支持通过XmlReader.ReadValueChunk(Char[], Int32, Int32) 读取块中的字符串值。更好的是，它们支持通过 XmlReader.ReadContentAsBase64(Byte[], Int32, Int32) 读取块中的 Base64 二进制值。
鉴于这些读取器和写入器，我们可以使用流式转换算法将 fileContent 节点转换为 fileRoute 节点，同时将 Base64 二进制文件提取到单独的二进制文件中。
首先介绍以下XML流转换方法，由Mark Fussell和this answer松散地基于Combining the XmlReader and XmlWriter classes for simple streaming transformations到Automating replacing tables from external files：
public static class XmlWriterExtensions
{
// Adapted from this answer https://stackoverflow.com/a/28903486
// to https://stackoverflow.com/questions/28891440/automating-replacing-tables-from-external-files/
// By https://stackoverflow.com/users/3744182/dbc
/// <summary>
/// Make a DEEP copy of the current xmlreader node to xmlwriter, allowing the caller to transform selected elements.
/// </summary>
/// <param name="writer"></param>
/// <param name="reader"></param>
/// <param name="shouldTransform"></param>
/// <param name="transform"></param>
public static void WriteTransformedNode(this XmlWriter writer, XmlReader reader, Predicate<XmlReader> shouldTransform, Action<XmlReader, XmlWriter> transform)
{
if (reader == null || writer == null || shouldTransform == null || transform == null)
throw new ArgumentNullException();
int d = reader.NodeType == XmlNodeType.None ? -1 : reader.Depth;
do
{
if (reader.NodeType == XmlNodeType.Element && shouldTransform(reader))
{
using (var subReader = reader.ReadSubtree())
{
transform(subReader, writer);
}
// ReadSubtree() places us at the end of the current element, so we need to move to the next node.
reader.Read();
}
else
{
writer.WriteShallowNode(reader);
}
}
while (!reader.EOF && (d < reader.Depth || (d == reader.Depth && reader.NodeType == XmlNodeType.EndElement)));
}
/// <summary>
/// Make a SHALLOW copy of the current xmlreader node to xmlwriter, and advance the XML reader past the current node.
/// </summary>
/// <param name="writer"></param>
/// <param name="reader"></param>
public static void WriteShallowNode(this XmlWriter writer, XmlReader reader)
{
// Adapted from https://docs.microsoft.com/en-us/archive/blogs/mfussell/combining-the-xmlreader-and-xmlwriter-classes-for-simple-streaming-transformations
// By Mark Fussell https://docs.microsoft.com/en-us/archive/blogs/mfussell/
// and rewritten to avoid using reader.Value, which fully materializes the text value of a node.
if (reader == null)
throw new ArgumentNullException("reader");
if (writer == null)
throw new ArgumentNullException("writer");
switch (reader.NodeType)
{
case XmlNodeType.None:
// This is returned by the System.Xml.XmlReader if a Read method has not been called.
reader.Read();
break;
case XmlNodeType.Element:
writer.WriteStartElement(reader.Prefix, reader.LocalName, reader.NamespaceURI);
writer.WriteAttributes(reader, true);
if (reader.IsEmptyElement)
{
writer.WriteEndElement();
}
reader.Read();
break;
case XmlNodeType.Text:
case XmlNodeType.Whitespace:
case XmlNodeType.SignificantWhitespace:
case XmlNodeType.CDATA:
case XmlNodeType.XmlDeclaration:
case XmlNodeType.ProcessingInstruction:
case XmlNodeType.EntityReference:
case XmlNodeType.DocumentType:
case XmlNodeType.Comment:
//Avoid using reader.Value as this will fully materialize the string value of the node. Use WriteNode instead,
// it copies text values in chunks. See: https://referencesource.microsoft.com/#system.xml/System/Xml/Core/XmlWriter.cs,368
writer.WriteNode(reader, true);
break;
case XmlNodeType.EndElement:
writer.WriteFullEndElement();
reader.Read();
break;
default:
throw new XmlException(string.Format("Unknown NodeType {0}", reader.NodeType));
}
}
}
public static partial class XmlReaderExtensions
{
// Taken from this answer https://stackoverflow.com/a/54136179/3744182
// To https://stackoverflow.com/questions/54126687/xmlreader-how-to-read-very-long-string-in-element-without-system-outofmemoryex
// By https://stackoverflow.com/users/3744182/dbc
public static bool CopyBase64ElementContentsToFile(this XmlReader reader, string path)
{
using (var stream = File.Create(path))
{
byte[] buffer = new byte[8192];
int readBytes = 0;
while ((readBytes = reader.ReadElementContentAsBase64(buffer, 0, buffer.Length)) > 0)
{
stream.Write(buffer, 0, readBytes);
}
}
return true;
}
}
接下来，使用JsonReaderWriterFactory，引入以下方法从一个JSON文件流式传输到另一个JSON文件，根据需要重写fileContent节点：
public static class JsonPatchExtensions
{
public static string[] PatchFileContentToFileRoute(string oldJsonFileName, string newJsonFileName, FilenameGenerator generator)
{
var newNames = new List<string>();
using (var inStream = File.OpenRead(oldJsonFileName))
using (var outStream = File.Open(newJsonFileName, FileMode.Create))
using (var xmlReader = JsonReaderWriterFactory.CreateJsonReader(inStream, XmlDictionaryReaderQuotas.Max))
using (var xmlWriter = JsonReaderWriterFactory.CreateJsonWriter(outStream))
{
xmlWriter.WriteTransformedNode(xmlReader,
r => r.LocalName == "fileContent" && r.NamespaceURI == "",
(r, w) =>
{
r.MoveToContent();
var name = generator.GenerateNewName();
r.CopyBase64ElementContentsToFile(name);
w.WriteStartElement("fileRoute", "");
w.WriteAttributeString("type", "string");
w.WriteString(name);
w.WriteEndElement();
newNames.Add(name);
});
}
return newNames.ToArray();
}
}
public abstract class FilenameGenerator
{
public abstract string GenerateNewName();
}
// Replace the following with whatever algorithm you need to generate unique binary file names.
public class IncrementalFilenameGenerator : FilenameGenerator
{
readonly string prefix;
readonly string extension;
int count = 0;
public IncrementalFilenameGenerator(string prefix, string extension)
{
this.prefix = prefix;
this.extension = extension;
}
public override string GenerateNewName()
{
var newName = Path.ChangeExtension(prefix + (++count).ToString(), extension);
return newName;
}
}
然后调用如下：
var binaryFileNames = JsonPatchExtensions.PatchFileContentToFileRoute(
oldJsonFileName,
newJsonFileName,
// Replace the following with your actual binary file name generation algorithm
new IncrementalFilenameGenerator("Question59839437_fileContent_", ".bin"));
来源：
演示小提琴here.