How to Deserialize Nested JSON to a Nested Dictionary with Custom Type Mapping in C#?

Deserializing Nested JSON to Nested Dictionary with Custom JSON Type Mapping

Deserializing JSON objects into a nested Dictionary is a common task, but the default System.Text.Json deserialization often results in all nested objects being represented as JsonElement instances. To achieve more specific type mapping, a custom JSON converter is required.

Custom JSON Converter for Type Mapping

The following custom JSON converter, named ObjectAsPrimitiveConverter, provides the desired type mapping:

• String -> string
• Number -> int/double
• Object -> Dictionary

Converter Implementation

The Read and Write methods of the converter perform the custom mapping:

• Read:

  • Parses the incoming JSON token based on its type.
  • Returns a specific .Net type (string, int, double, Dictionary, etc.) or throws an exception if the type cannot be parsed.

• Write:

  • Handles object types by writing an empty JSON object, as required by the custom conversion.

Usage of the Converter

To use the custom converter, it must be added to the JsonSerializerOptions during deserialization:

var options = new JsonSerializerOptions
{
    Converters = { new ObjectAsPrimitiveConverter() },
    WriteIndented = true,
};

dynamic d = JsonSerializer.Deserialize<dynamic>(json, options);

Configuration Options

The converter provides several configuration options:

• FloatFormat: Specifies whether floating-point numbers should be parsed as double or decimal.
• UnknownNumberFormat: Determines how to handle numbers that cannot be parsed into a .Net primitive type.
• ObjectFormat: Configures the output type for JSON objects as either a Dictionary or an ExpandoObject.

Sample Fiddle

A live demonstration is available here: [Sample Fiddle](https://dotnetfiddle.net/tD9pPI)

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