Create a message contract

Create an assembly

Create a new project, using the class library template.

dotnet new classlib -o ./src/MyProject.Contracts
dotnet sln add ./src/MyProject.Contracts

Add message contract

The message examples below show the same command to update a customer address using each of the supported contract types. Messages must be public reference types and can be defined using records, interfaces, or classes.

While non-public message types are allowed, they might be created using temporary broker entities where supported and may cause developer confusion.

Using a record

namespace Company.Application.Contracts
{
  using System;

  public record UpdateCustomerAddress
  {
    public Guid CommandId { get; init; }
    public DateTime Timestamp { get; init; }
    public string CustomerId { get; init; }
    public string HouseNumber { get; init; }
    public string Street { get; init; }
    public string City { get; init; }
    public string State { get; init; }
    public string PostalCode { get; init; }
  }
}

Using an interface

namespace Company.Application.Contracts
{
  using System;

  public interface UpdateCustomerAddress
  {
    Guid CommandId { get; }
    DateTime Timestamp { get; }
    string CustomerId { get; }
    string HouseNumber { get; }
    string Street { get; }
    string City { get; }
    string State { get; }
    string PostalCode { get; }
  }
}

When defining a message type using an interface, MassTransit will create a dynamic class implementing the interface for serialization, allowing the interface with get-only properties to be presented to the consumer. To create an interface message, use a message initializer.

Using a class

namespace Company.Application.Contracts
{
  using System;

  public class UpdateCustomerAddress
  {
    public Guid CommandId { get; set; }
    public DateTime Timestamp { get; set; }
    public string CustomerId { get; set; }
    public string HouseNumber { get; set; }
    public string Street { get; set; }
    public string City { get; set; }
    public string State { get; set; }
    public string PostalCode { get; set; }
  }
}

Properties with private set; are not recommended as they are not serialized by default when using System.Text.Json.

Customize using attributes

Attribute	Description
EntityName	The exchange or topic name
ExcludeFromTopology	Don’t create an exchange or topic unless it is directly consumed or published
ExcludeFromImplementedTypes	Don’t create a middleware filter for the message type
MessageUrn	The message urn