Catalog / Primitives / convert-class-to-record

Convert Class to Record

Turns a class whose state is fixed by its constructor into a record: a positional record where the constructor only assigns its parameters to get-only properties, otherwise the same members with class changed to record.

Arguments

None. The target is the class, by symbol.

Precondition

A record changes three behaviours of a class: Equals, GetHashCode and == compare values instead of references, and ToString lists the members instead of printing the type name. The conversion is refused wherever the solution could observe the difference.

The type is a non-static class, and the project's language version is C# 9 or later.
The class neither derives from a class other than object nor is derived from, since records only derive from records.
Its state is fixed at construction: every instance field is readonly and no instance property has a set accessor.
It does not override Equals(object), which a record generates.
Nothing in the solution observes equality: no == or != between instances (comparing with null is fine), no Equals or GetHashCode call on or with an instance, no use as a dictionary key or set element (a type argument for a TKey type parameter, or the element type of a hash set), and no call to Contains, IndexOf, LastIndexOf, Remove, Distinct, Union, Intersect, Except, ToHashSet or SequenceEqual over the type.
Unless the class overrides ToString, nothing formats an instance: no ToString call, interpolation or string concatenation of one.

Transformation

class becomes record on every declaration, keeping modifiers, type parameters, base types and constraints.
The record is positional when the class has one declaration and a single public constructor whose body only assigns each parameter to a public get-only auto-property of the same type, and neither the constructor nor those properties carry attributes or documentation. The parameter list, named after the properties and keeping default values, follows the name and type parameters; the constructor and the properties are removed, and a record left with no members ends with ;.
Named arguments at every call of the constructor in the solution are renamed to the property names.
Otherwise the constructor and properties stay in the body.

Preserved

Construction, member access and every observable behaviour the precondition checks.

Limitations

Positional properties get init accessors, so object initializers and with can set them afterwards; nothing that compiled before changes.
An instance passed as object to a method that formats or compares it, such as string.Format or a non-generic collection, is not detected.
Records also gain Deconstruct, a copy constructor and EqualityContract; a class already declaring a conflicting member is refused by the compile check.

Error codes

Code	Meaning
`unsupported-type`	the type is a static class, a struct or an interface
`language-version`	the project's language version predates records
`class-hierarchy`	the class derives from, or is derived by, another class
`mutable-state`	an instance field is not readonly or a property has a setter
`declares-equality`	the class overrides `Equals(object)`
`equality-observed`	the solution compares, hashes or looks up instances
`to-string-observed`	the solution formats an instance and the class does not override `ToString`

Cases

Diffs show each file from before to after. /*[*/ … /*]*/ marks a selection and /*^*/ a caret; the runner removes them before the refactoring runs.

#base-class-rejected

Refuses a class that derives from another class, since a record can only derive from a record

refusal

Targetsymbol T:Shop.Discount

Refusesclass-hierarchy; every file is left unchanged

Discount.csinput

1		namespace Shop;
2
3		public abstract class Adjustment
4		{
5		}
6
7		public sealed class Discount : Adjustment
8		{
9		public Discount(decimal amount)
10		{
11		Amount = amount;
12		}
13
14		public decimal Amount { get; }
15		}

1		namespace Shop;
2
3		public sealed class Point
4		{
5		public Point(int x, int y)
6		{
7		X = x;
8		Y = y;
9		}
10
11		public int X { get; }
12
13		public int Y { get; }
14		}

1		namespace Shop;
2
3		public class Route
4		{
5		public Point Start { get; } = new Point(0, 0);
6
7		public bool EndsAt(Point point) => point == Start;
8		}

1		namespace Shop;
2
3		public abstract class Adjustment
4		{
5		protected Adjustment(decimal amount)
6		{
7		Amount = amount;
8		}
9
10		public decimal Amount { get; }
11		}
12
13		public sealed class Discount : Adjustment
14		{
15		public Discount(decimal amount) : base(amount)
16		{
17		}
18		}

1		namespace Shop;
2
3		public sealed class Sku
4		{
5		public Sku(string code)
6		{
7		Code = code;
8		}
9
10		public string Code { get; }
11		}

1		using System.Collections.Generic;
2
3		namespace Shop;
4
5		public class Stock
6		{
7		private readonly Dictionary<Sku, int> _levels = new();
8
9		public void Add(Sku sku, int quantity) => _levels[sku] = quantity;
10		}

1	1		namespace Shop;
2	2
3		−	public class Customer
	3	+	public record Customer
4	4		{
5	5		public Customer(string name)
6	6		{
7	7		Name = name;
8	8		}
9	9
10	10		/// <summary>The name printed on invoices.</summary>
11	11		public string Name { get; }
12	12		}

1	1		using System;
2	2
3	3		namespace Shop;
4	4
5		−	public sealed class Pair<T> where T : IComparable<T>
	5	+	public sealed record Pair<T>(T First, T Second) where T : IComparable<T>
6	6		{
7		−	public Pair(T first, T second)
8		−	{
9		−	First = first;
10		−	Second = second;
11		−	}
12		−
13		−	public T First { get; }
14		−
15		−	public T Second { get; }
16		−
17	7		public T Larger => First.CompareTo(Second) >= 0 ? First : Second;
18	8		}

1		namespace Shop
2		{
3		public sealed class Point
4		{
5		public Point(int x, int y)
6		{
7		X = x;
8		Y = y;
9		}
10
11		public int X { get; }
12
13		public int Y { get; }
14		}
15		}

1		namespace Shop;
2
3		public class Basket
4		{
5		public Basket(string owner)
6		{
7		Owner = owner;
8		}
9
10		public string Owner { get; }
11
12		public int Count { get; set; }
13		}

1	1		namespace Shop;
2	2
3	3		public class Parcel
4	4		{
5		−	public Size Box { get; } = new Size(width: 2m, height: 3m);
	5	+	public Size Box { get; } = new Size(Width: 2m, Height: 3m);
6	6
7	7		public Size Flat() => new(Box.Width);
8	8
9	9		public decimal Area() => Box.Width * Box.Height;
10	10		}

1	1		namespace Shop;
2	2
3		−	partial class Tag
	3	+	partial record Tag
4	4		{
5	5		public string Display => "#" + Name;
6	6		}

1	1		namespace Shop;
2	2
3		−	public sealed class Size
4		−	{
5		−	public Size(decimal width, decimal height = 1m)
6		−	{
7		−	this.Width = width;
8		−	Height = height;
9		−	}
10		−
11		−	public decimal Width { get; }
12		−
13		−	public decimal Height { get; }
14		−	}
	3	+	public sealed record Size(decimal Width, decimal Height = 1m);

1	1		namespace Shop;
2	2
3		−	public sealed class Contact
4		−	{
5		−	public Contact(string name, string? email)
6		−	{
7		−	Name = name;
8		−	Email = email;
9		−	}
10		−
11		−	public string Name { get; }
12		−
13		−	public string? Email { get; }
14		−	}
	3	+	public sealed record Contact(string Name, string? Email);

1	1		namespace Shop;
2	2
3		−	public sealed partial class Tag
	3	+	public sealed partial record Tag
4	4		{
5	5		public Tag(string name)
6	6		{
7	7		Name = name;
8	8		}
9	9
10	10		public string Name { get; }
11	11		}

1	1		using System;
2	2
3	3		namespace Shop;
4	4
5	5		/// <summary>A point on the warehouse floor.</summary>
6		−	public sealed class Point
	6	+	public sealed record Point(int X, int Y)
7	7		{
8		−	public Point(int x, int y)
9		−	{
10		−	X = x;
11		−	Y = y;
12		−	}
13		−
14		−	public int X { get; }
15		−
16		−	public int Y { get; }
17		−
18	8		// Straight-line distance from the loading bay.
19	9		public double Length() => Math.Sqrt(X * X + Y * Y);
20	10		}

1		namespace Shop;
2
3		public static class Pricing
4		{
5		public static decimal Rate => 0.2m;
6		}

1	1		using System;
2	2
3	3		namespace Shop;
4	4
5		−	public sealed class Money
	5	+	public sealed record Money
6	6		{
7	7		private readonly int _scale = 2;
8	8
9	9		public Money(decimal amount, string currency)
10	10		{
11	11		if (amount < 0)
12	12		throw new ArgumentOutOfRangeException(nameof(amount));
13	13
14	14		Amount = amount;
15	15		Currency = currency;
16	16		}
17	17
18	18		public decimal Amount { get; }
19	19
20	20		public string Currency { get; }
21	21
22	22		public decimal Rounded => decimal.Round(Amount, _scale);
23	23		}

Convert Class to Record

Arguments

Precondition

Transformation

Preserved

Limitations

Error codes

Cases

#base-class-rejected

#compared-with-operator-rejected

#derived-class-rejected

#dictionary-key-rejected

#documented-properties-stay-in-body

#generic-with-constraint

#language-version-rejected

#mutable-state-rejected

#named-arguments-in-another-file

#nullable-enabled

#overrides-equals-rejected

#partial-class

#positional-with-methods

#printed-rejected

#static-class-rejected

#validating-constructor-stays-in-body