Catalog / Primitives / extract-local-variable

Extract Local Variable

Also known as Introduce Variable. Declares a local holding a selected expression just before the statement that contains it, and uses the local in place of the expression. The reverse of Inline Local Variable.

Target

The expression, by a selection. The argument name is the local's name.

Precondition

The selection is an expression inside a statement of a block-bodied member.
The expression has a value.
The expression is evaluated every time the statement runs, and only then: it is not in a loop condition or increment, the right of &&, || or ??, a branch of ?:, the part after ?., a switch expression arm, or a lambda.
It reads no variable declared inside the statement outside the selection, such as the parameter of an enclosing lambda.
name is not a local or parameter visible at the statement, and does not appear in the rest of the block, where the local would clash with or hide it.

Transformation

The local is declared with the expression's type, written as briefly as the scope allows, with its nullable annotation. An anonymous type is declared with var.
The declaration goes directly before the statement. A statement that is the body of an if, loop or similar without braces is wrapped in a block to hold it.
The selected expression is replaced by the local, dropping parentheses that only grouped it. When the expression has no side effects, identical expressions elsewhere in the same statement that are evaluated whenever it is are replaced as well. Other statements are not touched.

Preserved

Behaviour: the expression is evaluated once, at the same point in the statement's evaluation or earlier, and only where nothing it reads can have changed.
Comments above the statement stay above the new declaration; a trailing comment stays on the statement.

Limitations

Moving the evaluation of an expression with side effects to before the statement runs it ahead of the parts of the statement that preceded it.
Occurrences in other statements are not replaced, even when they would see the same value.
Expression-bodied members are refused rather than converted to a block body.

Error codes

Code	Meaning
`expression-bodied-member`	the expression is in an expression-bodied member
`not-in-statement`	the expression is not inside a statement, such as a field initializer
`in-loop-condition`	the expression is in a loop condition or increment
`conditionally-evaluated`	the expression runs only on some paths, or later
`declared-in-statement`	the expression reads a variable declared inside the statement
`name-conflict`	`name` is already declared or used where the local would be
`void-expression`	the expression has no value
`not-an-expression`	the selection is not an expression

Cases

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

#conditionally-evaluated-rejected

Refuses an expression evaluated only on some paths, which would then run on every path

refusal

TargetSample.cs, the /*[*/ … /*]*/ selection

Arguments

`name`	`"extracted"`

Refusesconditionally-evaluated; every file is left unchanged

Sample.csinput

1		public class Sample
2		{
3		public bool HasItems(string text)
4		{
5		return text != null && /[/text.Length/]/ > 0;
6		}
7		}

1		using System.Linq;
2
3		public class Sample
4		{
5		public int[] Doubled(int[] values)
6		{
7		return values.Select(v => /[/v * 2/]/).ToArray();
8		}
9		}

1	1		public class Sample
2	2		{
3	3		public decimal Tax(bool taxable, decimal total, decimal rate)
4	4		{
5	5		if (taxable)
6		−	return /[/total * rate/]/;
	6	+	{
	7	+	decimal tax = total * rate;
	8	+	return tax;
	9	+	}
7	10		return 0;
8	11		}
9	12		}

1		public class Sample
2		{
3		public int Calc(int a, int b) => /[/a * b/]/ + 1;
4		}

1	1		using System.Collections.Generic;
2	2
3	3		public class Sample
4	4		{
5	5		public int Count<T>(T item)
6	6		{
7		−	return Measure(/[/new List<T> { item, item }/]/);
	7	+	List<T> items = new List<T> { item, item };
	8	+	return Measure(items);
8	9		}
9	10
10	11		private int Measure<T>(List<T> items) => items.Count;
11	12		}

1	1		public class Sample
2	2		{
3	3		public decimal Net(decimal price, decimal rate)
4	4		{
5	5		var total = price;
6	6
7	7		// Apply the discount.
8		−	return total - /[/price * rate/]/; // before tax
	8	+	decimal discount = price * rate;
	9	+	return total - discount; // before tax
9	10		}
10	11		}

1		public class Sample
2		{
3		public int Last(int[] items)
4		{
5		var i = 0;
6		while (i < /[/items.Length - 1/]/)
7		{
8		i++;
9		}
10
11		return i;
12		}
13		}

1		public class Sample
2		{
3		public int Sum(int a, int b)
4		{
5		return /[/a + b/]/;
6		}
7		}

1		public class Sample
2		{
3		public int /[/Calc/]/(int a, int b)
4		{
5		return a + b;
6		}
7		}

1		public class Sample
2		{
3		private int _limit = /[/10 * 2/]/;
4
5		public int Limit() => _limit;
6		}

1	1		public class Sample
2	2		{
3	3		public string Describe(int id)
4	4		{
5		−	return /[/Find(id)?.Trim()/]/ ?? "none";
	5	+	string? trimmed = Find(id)?.Trim();
	6	+	return trimmed ?? "none";
6	7		}
7	8
8	9		private string? Find(int id) => id > 0 ? " found " : null;
9	10		}

1	1		public class Sample
2	2		{
3	3		public int Square(int a, int b)
4	4		{
5		−	return (/[/a + b/]/) * (a + b);
	5	+	int sum = a + b;
	6	+	return sum * sum;
6	7		}
7	8
8	9		public int Sum(int a, int b)
9	10		{
10	11		return a + b;
11	12		}
12	13		}

1	1		public class Sample
2	2		{
3	3		private int _counter;
4	4
5	5		public int Pair()
6	6		{
7		−	return /[/Next()/]/ * 10 + Next();
	7	+	int first = Next();
	8	+	return first * 10 + Next();
8	9		}
9	10
10	11		private int Next() => ++_counter;
11	12		}

1	1		public class Sample
2	2		{
3	3		public decimal Discounted(decimal price, decimal rate)
4	4		{
5		−	return price * (/[/1 - rate/]/);
	5	+	decimal factor = 1 - rate;
	6	+	return price * factor;
6	7		}
7	8		}

1		using System;
2
3		public class Sample
4		{
5		public void Log(string text)
6		{
7		/[/Console.WriteLine(text)/]/;
8		}
9		}

Extract Local Variable

Target

Precondition

Transformation

Preserved

Limitations

Error codes

Cases

#conditionally-evaluated-rejected

#declared-in-statement-rejected

#embedded-statement

#expression-bodied-rejected

#generic-type

#keeps-comments

#loop-condition-rejected

#name-conflict-rejected

#not-an-expression-rejected

#not-in-statement-rejected

#nullable-context

#occurrences-in-statement

#side-effects-selection-only

#simple-expression

#void-expression-rejected