Associated types: A demo of what DTOs could look like as a language feature

DTO.cs

using System.ComponentModel.DataAnnotations;
using Microsoft.EntityFrameworkCore;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.MapPost("/todos", async (CreateTodo createTodo, TodoDb db) =>
{
    // This is where the implicit conversion comes in
    db.Todos.Add(createTodo);
    await db.SaveChangesAsync();
});

app.Run();

// The following is similar to https://www.typescriptlang.org/docs/handbook/utility-types.html#picktype-keys

// CreateTodo is the DTO used for creating a new Todo. 
// It only exposes the title to prevent overbinding.

// This is the proposed syntax. We want to "associate" CreateTodo with the Todo type.
// This isn't an inheritance relationship, it's one where the properties map so that
// associated class CreateTodo : Todo { Title }

// This is what it would look like without compiler syntax (something a source generator can produce)
// [Associated(typeof(Todo), nameof(Todo.Title))]
// partial class CreateTodo { }

// The compiler generates this code for CreateTodo
class CreateTodo
{
    private readonly Todo _todo = new();

    // Attributes are copied from the associated type
    [Required]
    public string Title { get => _todo.Title; set => _todo.Title = value; }

    // The conversion here lets you pass around CreateTodo to things
    // that take a Todo (the associated type)
    public static implicit operator Todo(CreateTodo createTodo)
    {
        return createTodo._todo;
    }
}

// This is the database model (EntityFramework in this case)
class Todo
{
    public int Id { get; set; }
    [Required]
    public string Title { get; set; } = default!;
    public bool IsComplete { get; set; }
}

class TodoDb : DbContext
{
    public TodoDb(DbContextOptions options)
        : base(options)
    {
    }

    public DbSet<Todo> Todos => Set<Todo>();
}

DTO.role.cs

using System.ComponentModel.DataAnnotations;
using Microsoft.EntityFrameworkCore;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.MapPost("/todos", async (CreateTodo createTodo, TodoDb db) =>
{
    // This is the magic of roles being an erased wrapper. We can get a type safe view of 
    // Todo for de-serialization, but since it's erased, we can pass CreateTodo to anything that takes
    // a Todo
    db.Todos.Add(createTodo);
    await db.SaveChangesAsync();
});

app.Run();

// CreateTodo is the DTO used for creating a new Todo. 
// It only exposes the title to prevent overposting (https://en.m.wikipedia.org/wiki/Mass_assignment_vulnerability)
role CreateTodo : Todo
{
    // This role exposes a single property Title that maps to the underlying name property
    // I made up the existing keyword but maybe matching by name is fine
    public existing string Title { get; }
}

// This is the database model (EntityFramework in this case)
class Todo
{
    public int Id { get; set; }
    [Required]
    public string Title { get; set; } = default!;
    public bool IsComplete { get; }
}

class TodoDb : DbContext
{
    public TodoDb(DbContextOptions options)
        : base(options)
    {
    }

    public DbSet<Todo> Todos => Set<Todo>();
}

Sorry, dont know if I understood what you mean by redefine the properties.