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FlatSharp is Google's FlatBuffers serialization format implemented in C#, for C#. FlatBuffers is a zero-copy binary serialization format intended for high-performance scenarios. FlatSharp leverages the latest and greatest from .NET in the form of Memory<T> and Span<T>. As such, FlatSharp's safe-code implementations are often faster than other implementations using unsafe code. FlatSharp aims to provide 4 core priorities:

  • Full safety (no unsafe code or IL generation -- more on that below).
  • Speed
  • FlatBuffers schema correctness
  • Compatibility with other C#-focused projects like Unity, Blazor, and Xamarin. If it supports .NET standard 2.0, it supports FlatSharp.

All FlatSharp packages are published on

  • FlatSharp.Runtime: The runtime library. You always need this.
  • FlatSharp: Support for runtime schemas with C# attributes. Includes FlatBufferSerializer.
  • FlatSharp.Unsafe: Unsafe I/O extensions.
  • FlatSharp.Compiler: Build time compiler for generating C# from an FBS schema.

As of version 3.3.1, FlatSharp is in production use at Microsoft.

Getting Started

If you're completely new to FlatBuffers, take a minute to look over the FlatBuffer overview. Additionally, it's worth the time to understand the different elements of FlatBuffer schemas.

1. Define a schema

There are two ways to define a FlatBuffer schema with FlatSharp. The first is to use C# attributes to annotate classes, like you would with other serializers:

// FlatSharp supports enums, but makes you promise not to change the underlying type.
public enum Color : byte { Red = 1, Green, Blue }

// Tables are flexible objects meant to allow schema changes. Numeric properties can have default values,
// and all properties can be deprecated. Each index may only be used once, so once the "Parent" property is
// deprecated, index 2 cannot be used again by a different property.
public class Person : object
    [FlatBufferItem(0)] public virtual int Id { get; set; }
    [FlatBufferItem(1)] public virtual string Name { get; set; }
    [FlatBufferItem(2, Deprecated = true)] public virtual Person Parent { get; set; }
    [FlatBufferItem(3)] public virtual IList<Person> Children { get; set; }
    [FlatBufferItem(4, DefaultValue = Color.Blue)] public virtual Color FavoriteColor { get; set; } = Color.Blue;
    [FlatBufferItem(5)] public virtual Location Position { get; set; }

// Structs are really fast, but may only contain scalars and other structs. Structs
// cannot be versioned, so use only when you're sure the schema won't change.
public class Location : object
    [FlatBufferItem(0)] public virtual float Latitude { get; set; }
    [FlatBufferItem(1)] public virtual float Longitude { get; set; }

The second way to define a schema is to use an FBS schema file and run the FlatSharp compiler at build-time with your project. This enables fancy options like precompiling your serializers, interop with FlatBuffers in other languages, and GRPC definitions.

namespace MyNamespace;

enum Color : ubyte { Red = 1, Green, Blue }

table Person (PrecompiledSerializer) {
    Parent:Person (deprecated);
    FavoriteColor:Color = Blue;

struct Location {

rpc_service PersonService {

2. Serialize your data

Serialization is easy!

Person person = new Person(...);
int maxBytesNeeded = FlatBufferSerializer.Default.GetMaxSize(person);
byte[] buffer = new byte[maxBytesNeeded];
int bytesWritten = FlatBufferSerializer.Default.Serialize(person, buffer);

3. Parse your data

Deserializing is easier!

// By default, FlatSharp deserializes greedily, so everything in the Person is read from the data buffer
// and copied into the Person object, and the data buffer is no longer used after the Parse method returns.
// However, FlatSharp supports a variety of Lazy modes that read data from the buffer on demand and are
// often faster. These are covered under advanced topics below.
Person p = FlatBufferSerializer.Default.Parse<Person>(data);

Samples & Advanced Topics

FlatSharp supports some interesting features not covered here. Please visit the samples solution to see examples of:


FlatSharp works by generating subclasses of your data contracts based on the schema that you define. That is, when you attempt to deserialize a MonsterTable object, you actually get back a subclass of MonsterTable, which has properties defined in such a way as to index into the buffer, according to the deserialization mode specified (greedy, lazy, etc).


Serializers are a common vector for security issues. FlatSharp takes the following approach to security:

  • All core operations are overflow-checked
  • No unsafe code (with the exception of the Unsafe package)
  • No IL generation
  • Use standard .NET libraries for reading and writing from memory

At its core, FlatSharp is a tool to convert a FlatBuffer schema into a pile of safe C# code that depends only upon standard .NET libraries. There is no "secret sauce". Buffer overflows are intended to be impossible by design, due to the features of .NET and the CLR. A malicious input may lead to corrupt data or an Exception being thrown, but the process will not be compromised. As always, a best practice is to encrypt data at rest, in transit, and decorate it with some checksums.

Performance & Benchmarks

FlatSharp is really fast. This is primarily thanks to new changes in C# with Memory and Span, as well as FlatBuffers itself exposing a very simple type system that makes optimization simple. The FlatSharp benchmarks were run on .NET Core 3.1, using a C# approximation of Google's FlatBuffer benchmark, which can be found here. The FlatSharp benchmarks use this schema, but with the following parameters:

  • Vector length = 3 or 30
  • Traversal count = 1 or 5
  • Runtime: .NET 4.7, .NET Core 2.1, .NET Core 3.1, .NET Core 5.0 - RC

The full results for each version of FlatSharp can be viewed in the benchmarks folder. Additionally, the benchmark data contains performance data for many different configurations of FlatSharp and other features, such as sorted vectors.

The benchmarks test 3 different serialization frameworks:

  • FlatSharp
  • Protobuf.NET
  • Google's C# Flatbuffers implementation (both standard and Object API flavors)

The graphs below are generated using the default settings from each library on .NET Core 3.1:

image image

So What Packages Do I Need?

There are two main ways to use FlatSharp: Precompilation with .fbs files and runtime compilation using attributes on C# classes. Both of these produce and load the same code, so the performance will be identical. There are some good reasons to use precompilation over runtime compilation:

  • No runtime overhead -- Roslyn can take a little bit to spin up the first time
  • Fewer package dependencies
  • Better interop with other FlatBuffers languages via .fbs files
  • gRPC Support
  • Schema validation errors caught at build-time instead of runtime.
  • Better supported with other .NET toolchains

Runtime compilation is not planned to be deprecated (in fact the FlatSharp tests use Runtime compilation extensively), and can offer some compelling use cases as well, such as building more complex data structures that are shared between projects.

Framework FlatSharp.Runtime FlatSharp FlatSharp.Unsafe FlatSharp.Compiler
Unity / Blazor / Xamarin ✔️ ✔️
.NET Core (Precompiled) ✔️ ✔️
.NET Core (Runtime-compiled) ✔️ ✔️
.NET Framework (Precompiled) ✔️ ✔️
.NET Framework (Runtime-compiled) ✔️ ✔️

❔: .NET Framework does not have first-class support for Memory<T> and Span<T>, which results in degraded performance relative to .NET Core. Use of the unsafe packages has a sizeable impact on FlatSharp's speed on the legacy platform, but requires the use of unsafe code. For most cases, FlatSharp will be plenty fast without this.


FlatSharp is licensed under Apache 2.0.

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