Catalog / Primitives / convert-to-extension-method

Convert to Extension Method

Makes a method callable as an extension method on the type of its first parameter.

Arguments

Argument	Meaning
`target.symbol`	the method to convert, such as `M:Shop.Text.Shout(System.String)`
`to`	optional, for an instance method: the static class to put the extension in (default `<Type>Extensions`)

Precondition

For a static method, the usual case:

It is not already an extension method.
It is declared in a static, non-generic, top-level class.
It has at least one parameter, and the first is not out, params or a pointer.

For an instance method, no precondition beyond being an ordinary method of a class.

Transformation

A static method gains this on its first parameter. Each call written as Text.Shout(value, ...) or, inside the class, Shout(value, ...) becomes value.Shout(...), with the receiver parenthesized where precedence needs it. Explicit type arguments are kept.
A call keeps its static form when the extension form would not bind the same way: its first argument is a null or default literal, is named or passed by reference, converts to the parameter only by a user-defined or numeric conversion, or the calling file does not see the class's namespace.
An instance method moves to a static class beside its type, as an extension method whose first parameter is the instance, with uses of the instance's members qualified by that parameter. The original method stays, delegating to the extension, so callers are unchanged.

Preserved

The behaviour of every call.
Nullable annotations on the first parameter.

Limitations

For an instance method, members the method uses must be accessible from the new static class; private members are not made accessible.
For an instance method the extension parameter is named after the type in camel case and typed without type arguments, so generic types are not supported.

Error codes

Code	Meaning
`already-extension`	the method is already an extension method
`not-static-class`	the static method's class is not static
`nested-or-generic-class`	the static method's class is nested or generic
`no-parameters`	the method has no parameters
`invalid-first-parameter`	the first parameter is `out`, `params` or a pointer

Cases

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

#already-extension-rejected

Refuses a method that is already an extension method

refusal

Targetsymbol M:Shop.Text.Shout(System.String)

Refusesalready-extension; every file is left unchanged

Text.csinput

View fixture on GitHub

#calls-kept-static

Calls whose first argument would not bind as a receiver, or whose file cannot see the extension, keep the static form

success

Targetsymbol M:Shop.Numbers.Twice(System.Int64)

Ledger.csmodified

Numbers.csmodified

Reports/Summary.csunchanged

View fixture on GitHub

#generic-method

A generic method becomes a generic extension method, and inferred calls stay inferred

success

Targetsymbol M:Shop.Sequences.FirstOr``1(System.Collections.Generic.IEnumerable{``0},``0)

Basket.csmodified

Sequences.csmodified

View fixture on GitHub

#instance-method

An instance method becomes an extension method in a new static class beside its type, and the original delegates to it so callers are unchanged

success

Targetsymbol M:Shop.Order.Describe

Order.csmodified

View fixture on GitHub

#no-parameters-rejected

Refuses a method without parameters, which has nothing to extend

refusal

Targetsymbol M:Shop.Clock.Today

Refusesno-parameters; every file is left unchanged

Clock.csinput

View fixture on GitHub

#not-static-class-rejected

Refuses a static method of a class that is not static, where extension methods cannot live

refusal

Targetsymbol M:Shop.Order.Parse(System.String)

Refusesnot-static-class; every file is left unchanged

Order.csinput

View fixture on GitHub

#nullable

Nullable annotations on the first parameter are kept, and a null argument keeps the static call

success

Targetsymbol M:Shop.Text.OrEmpty(System.String)

Projectnullable enable

Label.csmodified

Text.csmodified

View fixture on GitHub

#static-helper

A static method of a static class becomes an extension method on its first parameter, and calls in other files take the extension form

success

Targetsymbol M:Shop.Text.Shout(System.String)

Checkout.csmodified

Text.csmodified

View fixture on GitHub

1	1		namespace Shop
2	2		{
3	3		public class Ledger
4	4		{
5	5		public long Balance { get; set; }
6	6
7		−	public long Doubled() => Numbers.Twice(Balance);
	7	+	public long Doubled() => Balance.Twice();
8	8
9	9		public long FromInt(int small) => Numbers.Twice(small);
10	10
11	11		public long Literal() => Numbers.Twice(21);
12	12		}
13	13		}

1	1		namespace Shop
2	2		{
3	3		public static class Numbers
4	4		{
5		−	public static long Twice(long value) => value * 2;
	5	+	public static long Twice(this long value) => value * 2;
6	6		}
7	7		}

1	1		using System.Collections.Generic;
2	2
3	3		namespace Shop
4	4		{
5	5		public class Basket
6	6		{
7	7		private readonly List<int> _quantities = new List<int>();
8	8
9		−	public int First() => Sequences.FirstOr(_quantities, 0);
	9	+	public int First() => _quantities.FirstOr(0);
10	10
11		−	public string FirstName(List<string> names) => Sequences.FirstOr<string>(names, "none");
	11	+	public string FirstName(List<string> names) => names.FirstOr<string>("none");
12	12		}
13	13		}

1	1		using System.Collections.Generic;
2	2
3	3		namespace Shop
4	4		{
5	5		public static class Sequences
6	6		{
7		−	public static T FirstOr<T>(IEnumerable<T> items, T fallback)
	7	+	public static T FirstOr<T>(this IEnumerable<T> items, T fallback)
8	8		{
9	9		foreach (var item in items)
10	10		return item;
11	11		return fallback;
12	12		}
13	13		}
14	14		}

1	1		namespace Shop
2	2		{
3	3		public class Order
4	4		{
5	5		public string Number { get; set; }
6	6
7	7		public decimal Total { get; set; }
8	8
9	9		public string Describe()
10	10		{
11		−	return Number + ": " + Total;
	11	+	return OrderExtensions.Describe(this);
	12	+	}
	13	+	}
	14	+
	15	+	public static class OrderExtensions
	16	+	{
	17	+	public static string Describe(this Order order)
	18	+	{
	19	+	return order.Number + ": " + order.Total;
12	20		}
13	21		}
14	22		}

1		namespace Shop
2		{
3		public static class Text
4		{
5		public static string Shout(this string text) => text.ToUpperInvariant();
6		}
7		}

1	1	namespace Reports
2	2	{
3	3	public class Summary
4	4	{
5	5	public long Total(long value) => Shop.Numbers.Twice(value);
6	6	}
7	7	}

1		namespace Shop
2		{
3		public static class Clock
4		{
5		public static string Today() => "Monday";
6		}
7		}

1	1		namespace Shop
2	2		{
3	3		public class Label
4	4		{
5	5		public string? Caption { get; set; }
6	6
7		−	public string Show() => Text.OrEmpty(Caption);
	7	+	public string Show() => Caption.OrEmpty();
8	8
9	9		public string Blank() => Text.OrEmpty(null);
10	10		}
11	11		}

1	1		namespace Shop
2	2		{
3	3		public static class Text
4	4		{
5		−	public static string OrEmpty(string? text) => text ?? "";
	5	+	public static string OrEmpty(this string? text) => text ?? "";
6	6		}
7	7		}

1	1		namespace Shop
2	2		{
3	3		public class Checkout
4	4		{
5		−	public string Welcome(Customer customer) => Text.Shout(customer.Name);
	5	+	public string Welcome(Customer customer) => customer.Name.Shout();
6	6		}
7	7
8	8		public class Customer
9	9		{
10	10		public string Name { get; set; }
11	11		}
12	12		}

1	1		namespace Shop
2	2		{
3	3		public static class Text
4	4		{
5	5		/// <summary>Upper-cases and exclaims.</summary>
6		−	public static string Shout(string text) => text.ToUpperInvariant() + "!";
	6	+	public static string Shout(this string text) => text.ToUpperInvariant() + "!";
7	7
8		−	public static string Greet(string name) => Shout("hello " + name);
	8	+	public static string Greet(string name) => ("hello " + name).Shout();
9	9		}
10	10		}