System.Text.Json vs Newtonsoft.Json: Key Differences

In C# and .NET development, handling JSON data typically comes down to two major libraries: the third-party standard Newtonsoft.Json (Json.NET) and Microsoft's built-in System.Text.Json. While Newtonsoft.Json provides a mature, highly flexible ecosystem suited for complex serialization scenarios, System.Text.Json is engineered for modern runtimes to deliver maximum throughput and minimal memory overhead. This article outlines the key technical differences between these two libraries, examining their performance, feature sets, default behaviors, and ideal use cases.

Origins and Architecture

  • Newtonsoft.Json (Json.NET): Introduced in 2006, it served as the default JSON serializer in ASP.NET for years. It was built around deep object graph traversal, reflection, and rich developer convenience, supporting virtually any C# object structure.
  • System.Text.Json: Introduced in .NET Core 3.0 as part of the Base Class Library (BCL), it was built from scratch to leverage modern runtime features such as Span<T>, ReadOnlySpan<byte>, and UTF-8 byte processing directly without converting strings first.

Performance and Memory Allocation

Performance is the most prominent differentiator between the two libraries:

  • System.Text.Json avoids unnecessary allocations by processing UTF-8 byte streams directly using low-level structs like Utf8JsonReader and Utf8JsonWriter. This results in significantly faster execution speeds and substantially lower garbage collection (GC) pressure.
  • Newtonsoft.Json allocates intermediate string objects and complex metadata structures during serialization and deserialization, making it slower and more memory-intensive under high-throughput workloads.

Default Behavior and Strictness

System.Text.Json adheres strictly to the RFC 8259 JSON specification by default, whereas Newtonsoft.Json favors developer convenience.

  • Case Sensitivity: System.Text.Json is case-sensitive by default when matching properties. Newtonsoft.Json is case-insensitive.
  • Comments and Trailing Commas: System.Text.Json rejects JSON comments and trailing commas unless explicitly allowed via JsonSerializerOptions. Newtonsoft.Json tolerates comments and malformed elements more leniently.
  • String-to-Number Conversions: Newtonsoft.Json automatically coerces strings containing numbers into numeric types. System.Text.Json strictly requires numbers to be represented as JSON numbers unless JsonNumberHandling is configured.

Feature Completeness and Extensibility

Newtonsoft.Json remains the more feature-rich library, particularly for edge cases:

  • Document Object Model (DOM): Newtonsoft utilizes JObject, JArray, and JToken, which provide a flexible, mutable API for reading, modifying, and querying JSON via JSONPath. System.Text.Json offers JsonNode (mutable) and JsonDocument (read-only, high-performance), but with fewer querying conveniences.
  • Polymorphism and Type Handling: Newtonsoft.Json easily handles polymorphic types through TypeNameHandling. System.Text.Json supports polymorphism starting in .NET 7, but it requires explicit type discriminators and deliberate configuration to avoid security vulnerabilities.
  • Private Members: Newtonsoft.Json can serialize and deserialize private fields and properties out of the box using attributes. System.Text.Json focuses primarily on public properties, requiring custom converters or parameterless constructors to populate private state.
  • Dictionary Key Types: Newtonsoft.Json handles non-string dictionary keys (such as integer or object keys) automatically. System.Text.Json historically requires custom converters for complex dictionary keys.

Attributes and Mapping

Both libraries provide mapping attributes, but they reside in different namespaces:

  • Property renaming: [JsonProperty("name")] (Newtonsoft) vs. [JsonPropertyName("name")] (System.Text.Json)
  • Ignoring fields: [JsonIgnore] (used by both, but under separate namespaces)
  • Constructor matching: [JsonConstructor] (used by both, under separate namespaces)

Choosing the Right Library

Choose System.Text.Json if:

  • You are building high-performance web APIs, microservices, or cloud applications where memory efficiency and low latency are critical.
  • You are working with standard Data Transfer Objects (DTOs) that do not require complex, non-standard mapping rules.
  • You are running modern .NET (.NET 6+) and want to avoid external package dependencies.

Choose Newtonsoft.Json if:

  • You are maintaining legacy .NET Framework applications.
  • Your application relies heavily on dynamic manipulation via JObject, JToken, or complex JSONPath queries.
  • You require advanced serialization features, such as deep polymorphic hierarchies, object reference loops, or deserialization into classes with non-standard private constructors.