Pass Base64 encoded string using JsonTextReader value as new stream

问题

We are consuming large JSON streams from an HTTP Post request. The goal is to stream the incoming body as JSON using JsonTextReader and extract the embedded base64 encoded binary files to disk. In XML, an equivalent method might be XMLReader.ReadElementContentAsBase64Async.

Using JSON.NET, as we iterative how do we send the each item of the encodedImages array into a FileStream without holding the whole string in memory.

Example JSON Object:

{
    "company":"{clientCompany}",
    "batchName":"{clientBatchName}",
    "fileType":"{clientFileType}",
    "encodedImages":[
        "{base64encodedimage}",
        "{base64encodedimage}",
        "{base64encodedimage}"
    ], 
    "customFields":{
        "{clientCustomField1}":"{clientCustomValue}",
        "{clientCustomField2}":"{clientCustomValue}",
        "{clientCustomField3}":"{clientCustomValue}",
        "{clientCustomField4}":"{clientCustomValue}"
    }
}

回答1:

It seems like your problem can be addressed in two parts: 1) How to parse and process the JSON in a memory efficient way, and 2) How to perform base-64 decoding iteratively

1) Memory efficient JSON parsing:

Assuming you can use the Newtonsoft JSON.net library, the ReadAsBytes or ReadAsBytesAsync methods of the JsonReader class are going to be your best friends, as they allow iterative stream-based processing which will allows you to minimize your memory footprint during JSON parsing and processing. To avoid writing low-level parsing code for your entire document, you might consider writing a JsonConverter implementation for the encodedImages node of your example.

2) iterative base-64 decoding

Most base-64 decoding implementations decode a string in its entirety. Support for iterative buffered decoding (as support by the ReadElementContentAsBase64Async method of XmlReader) requires some state to be maintained. Digging into the implementation of that class, you will find the internal Base64Decoder class which does precisely what you need.

来源：https://stackoverflow.com/questions/46670994/pass-base64-encoded-string-using-jsontextreader-value-as-new-stream

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