Downcastly - A C# Library for Safe Downcasting with Compile-Time Type Checking

Downcastly is a C# library that provides a safe and efficient way to downcast objects with compile-time type checking. It automatically generates downcasting constructors for your classes to copy all the properties from the base class to the derived class, ensuring type safety and reducing boilerplate code.

usage example

    public record ParentRecord
    {
        public int Id { get; init; }

        public string Name { get; init; }
    }

    [Downcast]
    public partial record ChildRecord : ParentRecord
    {
        public string Status { get; init; }
    }

    public class Program
    {
        public static void Main()
        {
            ParentRecord parent = new()
            {
                Id = 1,
                Name = "Parent"
            };

            // Safe downcasting using Downcastly
            ChildRecord child = new ChildRecord(parent) { Status = "active" };
            Console.WriteLine($"Id: {child.Id}, Name: {child.Name}, Status: {child.Status}");
        }
    }

installation

dotnet add package Downcastly

usefull tips

Add following line to your PropertyGroup of your .csproj file so you can see generated files in your obj folder:

<CompilerGeneratedFilesOutputPath>$(BaseIntermediateOutputPath)\Generated</CompilerGeneratedFilesOutputPath>

why not to use AutoMapper, Mapster or similar libraries instead?

Those libraries are great for mapping between different types, but with that strength comes complexity. It's easy to start using them not only for downcasting but also for other mapping scenarios, which can lead to overcomplication of your codebase, complicated configurations, unnecessary abstractions, and debudding overhead. We all know that using mappers is very easy until it suddenly becomes really hard.