Catalog / Primitives / convert-auto-property-to-backing-field

Convert Auto-Property to Property with Backing Field

Gives an auto-property an explicit private field and accessors that read and write it, so the accessors can later gain behaviour. The reverse of Convert to Auto-Property.

Precondition

The property is an auto-property: every accessor lacks a body, and the property is not abstract, extern or an interface member.
The field name, from the name argument or _camelCase of the property, is free in the type.

Transformation

A private field of the property's type is added after the type's existing fields, or first in the type. It is static when the property is, and readonly when the property has no set accessor, since constructors and init accessors may assign a readonly field. The property's initialiser moves to the field.
A get-only property becomes an expression-bodied property returning the field. Constructors that assigned the property, in any part of a partial type, assign the field instead.
Otherwise each accessor gets an expression body, one to a line: get => _name;, set => _name = value; or init => _name = value;, keeping its modifiers.
The property keeps its documentation, attributes and accessibility.

Preserved

Every read and write of the value, including object initialisers that use an init accessor.
Nullable annotations.

Limitations

Attributes on individual accessors are not carried over.
Attributes targeting the compiler-generated field, [field: ...], are not moved to the new field.

Error codes

Code	Meaning
`not-auto-property`	the property has accessor bodies, or is abstract, extern or an interface member
`name-conflict`	the field name is taken

Cases

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

#abstract-property-rejected

Refuses an abstract property, which looks like an auto-property but has no storage

refusal

Targetsymbol P:Shop.Shape.Sides

Refusesnot-auto-property; every file is left unchanged

Shape.csinput

View fixture on GitHub

#get-only-assigned-in-partial-constructor

A get-only auto-property gets a readonly field; the constructor, in another part of the partial type, assigns the field instead of the property

success

Targetsymbol P:Shop.Product.Code

Product.Create.csmodified

Product.csmodified

View fixture on GitHub

#get-set-with-initializer

An auto-property gets a private backing field holding its initialiser, and accessors that read and write it

success

Targetsymbol P:Shop.Customer.Name

Customer.csmodified

View fixture on GitHub

#init-accessor-nullable

An init accessor gets a readonly field, which init may assign, and the nullable annotation carries over

success

Targetsymbol P:Shop.Profile.Nickname

Projectnullable enable

Profile.csmodified

View fixture on GitHub

#name-conflict-rejected

Refuses a field name another member already has

refusal

Targetsymbol P:Shop.Sample.Count

Refusesname-conflict mentioning “'_count'”; every file is left unchanged

Sample.csinput

View fixture on GitHub

#not-auto-property-rejected

Refuses a property that already has accessor bodies

refusal

Targetsymbol P:Shop.Sample.Name

Refusesnot-auto-property; every file is left unchanged

Sample.csinput

View fixture on GitHub

#private-setter-keeps-documentation

Accessor modifiers carry over to the new accessors, and the property keeps its documentation and attributes

success

Targetsymbol P:Shop.Counter.Value

Arguments

`name`	`"_current"`

Counter.csmodified

View fixture on GitHub

#static-property

A static auto-property gets a static field

success

Targetsymbol P:Shop.Registry.Instances

Registry.csmodified

View fixture on GitHub

1	1		namespace Shop
2	2		{
3	3		public partial class Product
4	4		{
5	5		public Product(string code, decimal price)
6	6		{
7		−	Code = code;
	7	+	_code = code;
8	8		_price = price;
9	9		}
10	10
11	11		public string Describe() => Code + ": " + _price;
12	12		}
13	13		}

1	1		namespace Shop
2	2		{
3	3		public partial class Product
4	4		{
5	5		private decimal _price;
	6	+	private readonly string _code;
6	7
7		−	public string Code { get; }
	8	+	public string Code => _code;
8	9
9	10		public decimal Price() => _price;
10	11		}
11	12		}

1	1		namespace Shop
2	2		{
3	3		public class Customer
4	4		{
5		−	public string Name { get; set; } = "unknown";
	5	+	private string _name = "unknown";
	6	+
	7	+	public string Name
	8	+	{
	9	+	get => _name;
	10	+	set => _name = value;
	11	+	}
6	12
7	13		public string Greeting() => "Dear " + Name;
8	14		}
9	15		}

1	1		namespace Shop
2	2		{
3	3		public class Profile
4	4		{
5		−	public string? Nickname { get; init; }
	5	+	private readonly string? _nickname;
	6	+
	7	+	public string? Nickname
	8	+	{
	9	+	get => _nickname;
	10	+	init => _nickname = value;
	11	+	}
6	12		}
7	13
8	14		public class Signup
9	15		{
10	16		public Profile Create(string name) => new Profile { Nickname = name };
11	17		}
12	18		}

1		namespace Shop
2		{
3		public class Sample
4		{
5		private int _count;
6
7		public int Count { get; set; }
8
9		public int Total() => Count + _count;
10		}
11		}

1		namespace Shop
2		{
3		public abstract class Shape
4		{
5		public abstract int Sides { get; }
6		}
7		}

1		namespace Shop
2		{
3		public class Sample
4		{
5		public string Name => "fixed";
6		}
7		}

1	1		using System;
2	2
3	3		namespace Shop
4	4		{
5	5		public class Counter
6	6		{
	7	+	private int _current;
	8	+
7	9		/// <summary>The current count.</summary>
8	10		[Obsolete("Use Next")]
9		−	public int Value { get; private set; }
	11	+	public int Value
	12	+	{
	13	+	get => _current;
	14	+	private set => _current = value;
	15	+	}
10	16
11	17		public int Next() => ++Value;
12	18		}
13	19		}

1	1		namespace Shop
2	2		{
3	3		public static class Registry
4	4		{
5		−	public static int Instances { get; set; }
	5	+	private static int _instances;
	6	+
	7	+	public static int Instances
	8	+	{
	9	+	get => _instances;
	10	+	set => _instances = value;
	11	+	}
6	12
7	13		public static void Register() => Instances++;
8	14		}
9	15		}