Table of Contents

Object mapping

Constructor mapping

public record Album(int Id, string Title);

Album album = cnn.Query<Album>("SELECT AlbumId AS Id, Title FROM albums WHERE AlbumId = @albumId", new { albumId = 12 });

Constructor inputs are matched against returned columns.

Writable members

public sealed class Album
{
    public int Id { get; set; }
    public string? Title { get; set; }
}

Album album = cnn.Query<Album>("SELECT AlbumId AS Id, Title FROM albums WHERE AlbumId = @albumId", new { albumId = 12 });

A parameterless construction can fill writable fields and properties.

An init member cannot be assigned after parameterless construction.

public sealed class Album
{
    public int Id { get; init; }
    public string? Title { get; init; }
}

Album album = cnn.Query<Album>("SELECT AlbumId AS Id, Title FROM albums WHERE AlbumId = @albumId", new { albumId = 12 });
// RINKU3001

RINKU3001

Several construction paths

public sealed class Album
{
    public Album() { }

    public Album(int id, string title)
    {
        Id = id;
        Title = title;
    }

    public int Id { get; }
    public string Title { get; } = "";
}

Album album = cnn.Query<Album>("SELECT AlbumId AS Id, Title FROM albums WHERE AlbumId = @albumId", new { albumId = 12 });

A path participates when its required inputs can be satisfied by the returned shape.

Construction paths

Static factories

public interface IShape
{
    public static IShape FromCircle(double radius) => new Circle(radius);
    public static IShape FromRectangle(double width, double height) => new Rectangle(width, height);
}

public record Circle(double Radius) : IShape;
public record Rectangle(double Width, double Height) : IShape;

IShape circle = cnn.Query<IShape>("SELECT Radius FROM shapes WHERE ShapeId = @shapeId", new { shapeId = 1 });
IShape rectangle = cnn.Query<IShape>("SELECT Width, Height FROM shapes WHERE ShapeId = @shapeId", new { shapeId = 2 });

The returned shape determines which construction can participate.

Alternative construction

public record Album(int Id, string Title, string? Notes = null);

Album shortRow = cnn.Query<Album>("SELECT AlbumId AS Id, Title FROM albums WHERE AlbumId = @albumId", new { albumId = 12 });
Album fullRow = cnn.Query<Album>("SELECT AlbumId AS Id, Title, Notes FROM albums WHERE AlbumId = @albumId", new { albumId = 12 });

The declared default provides a construction path when Notes is absent.

The same mechanism lets a recursive shape terminate.

public record Employee(int Id, string Name, [Alt("Boss")] Employee? Manager = null) : IDbReadable;

Employee employee = cnn.Query<Employee>("SELECT e.EmployeeId AS Id, e.Name, m.EmployeeId AS ManagerId, m.Name AS ManagerName, b.EmployeeId AS ManagerBossId, b.Name AS ManagerBossName FROM employees e LEFT JOIN employees m ON m.EmployeeId = e.ManagerId LEFT JOIN employees b ON b.EmployeeId = m.ManagerId WHERE e.EmployeeId = @employeeId", new { employeeId = 12 });
// ManagerId maps another Employee.
// ManagerBossId maps that Employee.Manager through Alt("Boss").
// The deepest Employee finishes through the construction where Manager uses its default.

Recursive mapping · Construction paths

Complete with writable members

public sealed class Album
{
    [CanCompleteWithMembers]
    public Album(int id) => Id = id;

    public int Id { get; }
    public string? Title { get; set; }
}

Album album = cnn.Query<Album>("SELECT AlbumId AS Id, Title FROM albums WHERE AlbumId = @albumId", new { albumId = 12 });
// Id is consumed by the constructor.
// Title fills the remaining writable member.

Reading order

Conversions

public record Totals(long Widened, int Narrowed);

Totals totals = cnn.Query<Totals>("SELECT CAST(12 AS int) AS Widened, CAST(34 AS bigint) AS Narrowed");

Supported CLR conversions can participate in a slot.

public readonly struct AlbumScore
{
    public double Value { get; }
    private AlbumScore(double value) => Value = value;
    public static explicit operator AlbumScore(double value) => new(value);
}

public record RankedAlbum(int Id, AlbumScore Score);

RankedAlbum album = cnn.Query<RankedAlbum>("SELECT AlbumId AS Id, Score FROM album_scores WHERE AlbumId = @albumId", new { albumId = 12 });

[ExactType] removes conversion for that slot.

public record Amount([ExactType] int Value);
// A returned long column cannot fill Value.

Unused columns

public record Album(int Id, string Title);

Album album = cnn.Query<Album>("SELECT AlbumId AS Id, Title, ArtistName FROM album_search WHERE AlbumId = @albumId", new { albumId = 12 });
// ArtistName remains unused.

Name adaptation · Database NULL

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