C# Generated Code Guide

Describes exactly what C# code the protocol buffer compiler generates for protocol definitions using proto3 syntax.

You should read the proto3 language guide before reading this document.

Compiler Invocation

The protocol buffer compiler produces C# output when invoked with the --csharp_out command-line flag. The parameter to the --csharp_out option is the directory where you want the compiler to write your C# output, although depending on other options the compiler may create subdirectories of the specified directory. The compiler creates a single source file for each .proto file input, defaulting to an extension of .cs but configurable via compiler options.

Only proto3 messages are supported by the C# code generator. Ensure that each .proto file begins with a declaration of:

syntax = "proto3";

C#-specific Options

You can provide further C# options to the protocol buffer compiler using the --csharp_opt command-line flag. The supported options are:

  • file_extension: Sets the file extension for generated code. This defaults to .cs, but a common alternative is .g.cs to indicate that the file contains generated code.

  • base_namespace: When this option is specified, the generator creates a directory hierarchy for generated source code corresponding to the namespaces of the generated classes, using the value of the option to indicate which part of the namespace should be considered as the "base" for the output directory. For example, with the following command-line:

    protoc --proto_path=bar --csharp_out=src --csharp_opt=base_namespace=Example player.proto

    where player.proto has a csharp_namespace option of Example.Game the protocol buffer compiler generates a file src/Game/Player.cs being created. This option would usually correspond with the default namespace option in a C# project in Visual Studio. If the option is specified but with an empty value, the full C# namespace as used in the generated file will be used for the directory hierarchy. If the option is not specified at all, the generated files are simply written into the directory specified by --csharp_out without any hierarchy being created.

  • internal_access: When this option is specified, the generator creates types with the internal access modifier instead of public.

  • serializable: When this option is specified, the generator adds the [Serializable] attribute to generated message classes.

Multiple options can be specified by separating them with commas, as in the following example:

protoc --proto_path=src --csharp_out=build/gen --csharp_opt=file_extension=.g.cs,base_namespace=Example,internal_access src/foo.proto

File structure

The name of the output file is derived from the .proto filename by converting it to Pascal-case, treating underscores as word separators. So, for example, a file called player_record.proto will result in an output file called PlayerRecord.cs (where the file extension can be specified using --csharp_opt, as shown above).

Each generated file takes the following form, in terms of public members. (The implementation is not shown here.)

namespace [...]
  public static partial class [... descriptor class name ...]
    public static FileDescriptor Descriptor { get; }

  [... Enums ...]
  [... Message classes ...]

The namespace is inferred from the proto’s package, using the same conversion rules as the file name. For example, a proto package of example.high_score would result in a namespace of Example.HighScore. You can override the default generated namespace for a particular .proto using the csharp_namespace file option.

Each top-level enum and message results in an enum or class being declared as members of the namespace. Additionally, a single static partial class is always generated for the file descriptor. This is used for reflection-based operations. The descriptor class is given the same name as the file, without the extension. However, if there is a message with the same name (as is quite common), the descriptor class is placed in a nested Proto namespace to avoid colliding with the message.

As an example of all of these rules, consider the timestamp.proto file which is provided as part of Protocol Buffers. A cut down version of timestamp.proto looks like this:

syntax = "proto3";
package google.protobuf;
option csharp_namespace = "Google.Protobuf.WellKnownTypes";

message Timestamp { ... }

The generated Timestamp.cs file has the following structure:

namespace Google.Protobuf.WellKnownTypes
  namespace Proto
    public static partial class Timestamp
      public static FileDescriptor Descriptor { get; }

  public sealed partial class Timestamp : IMessage<Timestamp>


Given a simple message declaration:

message Foo {}

The protocol buffer compiler generates a sealed, partial class called Foo, which implements the IMessage<Foo> interface, as shown below with member declarations. See the inline comments for more information.

public sealed partial class Foo : IMessage<Foo>
  // Static properties for parsing and reflection
  public static MessageParser<Foo> Parser { get; }
  public static MessageDescriptor Descriptor { get; }

  // Explicit implementation of IMessage.Descriptor, to avoid conflicting with
  // the static Descriptor property. Typically the static property is used when
  // referring to a type known at compile time, and the instance property is used
  // when referring to an arbitrary message, such as during JSON serialization.
  MessageDescriptor IMessage.Descriptor { get; }

  // Parameterless constructor which calls the OnConstruction partial method if provided.
  public Foo();
  // Deep-cloning constructor
  public Foo(Foo);
  // Partial method which can be implemented in manually-written code for the same class, to provide
  // a hook for code which should be run whenever an instance is constructed.
  partial void OnConstruction();

  // Implementation of IDeepCloneable<T>.Clone(); creates a deep clone of this message.
  public Foo Clone();

  // Standard equality handling; note that IMessage<T> extends IEquatable<T>
  public override bool Equals(object other);
  public bool Equals(Foo other);
  public override int GetHashCode();

  // Converts the message to a JSON representation
  public override string ToString();

  // Serializes the message to the protobuf binary format
  public void WriteTo(CodedOutputStream output);
  // Calculates the size of the message in protobuf binary format
  public int CalculateSize();

  // Merges the contents of the given message into this one. Typically
  // used by generated code and message parsers.
  public void MergeFrom(Foo other);

  // Merges the contents of the given protobuf binary format stream
  // into this message. Typically used by generated code and message parsers.
  public void MergeFrom(CodedInputStream input);

Note that all of these members are always present; the optimize_for option does not affect the output of the C# code generator.

Nested Types

A message can be declared inside another message. For example:

message Foo {
  message Bar {

In this case—or if a message contains a nested enum—the compiler generates a nested Types class, and then a Bar class within the Types class, so the full generated code would be:

namespace [...]
  public sealed partial class Foo : IMessage<Foo>
    public static partial class Types
      public sealed partial class Bar : IMessage<Bar> { ... }

Although the intermediate Types class is inconvenient, it is required to deal with the common scenario of a nested type having a corresponding field in the message. You would otherwise end up with both a property and a type with the same name nested within the same class—and that would be invalid C#.


The protocol buffer compiler generates a C# property for each field defined within a message. The exact nature of the property depends on the nature of the field: its type, and whether it is singular, repeated, or a map field.

Singular Fields

Any singular field generates a read/write property. A string or bytes field will generate an ArgumentNullException if a null value is specified; fetching a value from a field which hasn’t been explicitly set will return an empty string or ByteString. Message fields can be set to null values, which is effectively clearing the field. This is not equivalent to setting the value to an "empty" instance of the message type.

Repeated Fields

Each repeated field generates a read-only property of type Google.Protobuf.Collections.RepeatedField<T> where T is the field’s element type. For the most part, this acts like List<T>, but it has an additional Add overload to allow a collection of items to be added in one go. This is convenient when populating a repeated field in an object initializer. Additionally, RepeatedField<T> has direct support for serialization, deserialization and cloning, but this is usually used by generated code instead of manually-written application code.

Repeated fields cannot contain null values, even of message types, except for the nullable wrapper types explained below.

Map Fields

Each map field generates a read-only property of type Google.Protobuf.Collections.MapField<TKey, TValue> where TKey is the field’s key type and TValue is the field’s value type. For the most part, this acts like Dictionary<TKey, TValue>, but it has an additional Add overload to allow another dictionary to be added in one go. This is convenient when populating a repeated field in an object initializer. Additionally, MapField<TKey, TValue> has direct support for serialization, deserialization and cloning, but this is usually used by generated code instead of manually-written application code. Keys in the map are not permitted to be null; values may be if the corresponding singular field type would support null values.

Oneof Fields

Each field within a oneof has a separate property, like a regular singular field. However, the compiler also generates an additional property to determine which field in the enum has been set, along with an enum and a method to clear the oneof. For example, for this oneof field definition

oneof avatar {
  string image_url = 1;
  bytes image_data = 2;

The compiler will generate these public members:

enum AvatarOneofCase
  None = 0,
  ImageUrl = 1,
  ImageData = 2

public AvatarOneofCase AvatarCase { get; }
public void ClearAvatar();
public string ImageUrl { get; set; }
public ByteString ImageData { get; set; }

If a property is the current oneof "case", fetching that property will return the value set for that property. Otherwise, fetching the property will return the default value for the property’s type—only one member of a oneof can be set at a time.

Setting any constituent property of the oneof will change the reported "case" of the oneof. As with a regular singular field, you cannot set a oneof field with a string or bytes type to a null value. Setting a message-type field to null is equivalent to calling the oneof-specific Clear method.

Wrapper Type Fields

Most of the well-known types in proto3 do not affect code generation, but the wrapper types (StringWrapper, Int32Wrapper etc) change the type and behaviour of the properties.

All of the wrapper types that correspond to C# value types (Int32Wrapper, DoubleWrapper, BoolWrapper etc) are mapped to Nullable<T> where T is the corresponding non-nullable type. For example, a field of type DoubleValue results in a C# property of type Nullable<double>.

Fields of type StringWrapper or BytesWrapper result in C# properties of type string and ByteString being generated, but with a default value of null, and allowing null to be set as the property value.

For all wrapper types, null values are not permitted in a repeated field, but are permitted as the values for map entries.


Given an enumeration definition like:

enum Color {
  COLOR_RED = 1;
  COLOR_BLUE = 1234;

The protocol buffer compiler will generate a C# enum type called Color with the same set of values. The names of the enum values are converted to make them more idiomatic for C# developers:

  • If the original name starts with the upper-cased form of the enum name itself, that is removed
  • The result is converted into Pascal case

The Color proto enum above would therefore become the following C# code:

enum Color
  Unspecified = 0,
  Red = 1,
  Green = 5,
  Blue = 1234

This name transformation does not affect the text used within the JSON representation of messages.

Note that the .proto language allows multiple enum symbols to have the same numeric value. Symbols with the same numeric value are synonyms. These are represented in C# in exactly the same way, with multiple names corresponding to the same numeric value.

A non-nested enumeration leads to a C# enum being generated as a new namespace member being generated; a nested enumeration lead to a C# enum being generated in the Types nested class within the class corresponding to the message the enumeration is nested within.


The C# code generator ignores services entirely.