JSON is a text format for structured data. A document can contain an array of objects, such as records with numeric IDs and string properties. In .NET, System.Text.Json provides two practical ways to work with it: inspect and write elements directly, or map a known schema to C# objects with a serializer. section 4.2 section 4.2.1 section 4.2.3
Use a Stream to read a JSON file rather than treating it as independent lines. JsonDocument.Parse reads the input stream; RootElement.EnumerateArray visits its elements, and GetProperty retrieves values for filtering. This approach is useful when selecting or copying parts of a document without defining a C# type for every object. section 4.2.1 section 4.2.2
Listing 4.22 Using Utf8JsonWriter to write well-formed JSON
static void FindWithJsonDom(
Stream inStream,
Stream outStream,
string category)
{
var writerOptions = new JsonWriterOptions #1
{ Indented = true }; #2
using var writer = new Utf8JsonWriter(outStream, #3
writerOptions);
writer.WriteStartArray(); #4
using var jsonDoc =
JsonDocument.Parse(inStream); #5
foreach (var jokeElement in jsonDoc
.RootElement.EnumerateArray())
{
string? type = jokeElement
.GetProperty("type")
.GetString();
if (string.Equals(category, type,
StringComparison.OrdinalIgnoreCase))
{
jokeElement.WriteTo(writer); #6
}
}
writer.WriteEndArray(); #7
}
The category filter copies matching elements into a new array. Utf8JsonWriter supplies the array boundaries, separators, and optional indentation; writing raw element text alone would not produce valid JSON. section 4.2.2
When the schema is known, define a matching C# record and deserialize the stream with JsonSerializer. Filter the resulting objects with LINQ, then serialize them to an output stream. A projection can limit output to selected properties and give those properties the desired names. Case-insensitive reading helps when JSON names differ from C# names only by capitalization; JsonPropertyName handles other name differences. section 4.2.3 section 4.3
Listing 4.26 Reading and writing JSON with JsonSerializer
static void FindWithSerialization( #1
Stream inStream,
Stream outStream,
string category
)
{
var serialOptions = new JsonSerializerOptions #2
{ WriteIndented = true, #3
PropertyNameCaseInsensitive = true }; #4
var jokes = JsonSerializer.Deserialize<Joke[]>( #5
inStream, #6
serialOptions); #7
JsonSerializer.Serialize( #8
outStream,
jokes?.Where(j => string.Equals( #9
category, j.Type,
StringComparison.OrdinalIgnoreCase))
.ToArray(), #10
serialOptions); #11
}
This implementation deserializes an array of records, filters it by category, and serializes the result. Its options enable case-insensitive property matching and indented output. section 4.2.3
JSON from an HTTP response
A comic-search application obtains a JSON response stream with HttpClient and deserializes it into a Comic record. JsonPropertyName maps names such as safe_title to a differently named C# property; JsonIgnore excludes a computed date. The nullable result also requires a caller to account for the absence of a comic. section 4.3
