Catalog / Primitives / invert-if

Invert If

Negates the condition of an if statement and swaps its branches.

Precondition

The caret is on an if statement, between if and the closing parenthesis of its condition.
When the if has no else, it must be in a block, and one of these holds:
Nothing follows it, and the block is the body of a method, function or accessor that returns nothing, or of a loop, so that running off its end is an implicit return or continue.
Code follows it, and its branch always jumps away (returns, throws, breaks or continues), so that the code after it only runs when the condition is false.

Transformation

The condition is replaced by its logical negation, in its most direct form: == becomes !=, < becomes >=, !x becomes x, x is T becomes x is not T, x is not null becomes x is null, and && and || follow De Morgan's laws, with parentheses added where precedence needs them. Anything else is negated with !.
With an else, the branches swap. An else if becomes the body of the new then branch, in braces, so that the new else cannot attach to it.
Without an else:
When nothing follows, the implicit exit becomes the new branch and the old branch follows the if, which introduces an early-return guard clause: if (c) { A(); } becomes if (!c) { return; } A();.
When the branch is exactly the implicit exit, such as return; at the end of a method returning nothing, the code after the if becomes its branch and the jump is dropped, which removes a guard clause.
Otherwise, when the code after the if also always jumps away, it becomes the branch and the old branch follows the if.

Preserved

The behaviour of every path through the statement.
Comments inside each branch move with that branch; comments before the if stay before it.

Limitations

Flipping an ordering comparison is only behaviour-preserving when the values are totally ordered. Comparisons of floating-point values (which may be NaN), nullable values (which may be null) and user-defined orderings are negated with !( ... ) instead. Equality is always flipped, assuming a user-defined != is the negation of its ==, as C# requires them in pairs.
An if without an else in a switch section, or ending a block that is not a method or loop body, is refused rather than rewritten with a goto or a break.

Error codes

Code	Meaning
`not-an-if`	the caret is not on an `if` statement
`code-follows`	the `if` has no `else` and the code after it cannot become its branch, because the branch or that code can run on past the end
`no-implicit-exit`	the `if` has no `else`, nothing follows it, and its block does not end a method or loop body

Cases

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

#both-paths-jump

When the branch and the code after the if both leave the method, they swap places

success

TargetNames.cs, the /*^*/ caret

Projectnullable enable

Names.csmodified

View fixture on GitHub

#code-follows-rejected

Refuses an if with no else whose branch can fall through to the code after it

refusal

TargetSample.cs, the /*^*/ caret

Refusescode-follows; every file is left unchanged

Sample.csinput

View fixture on GitHub

#comments-move-with-branches

Comments inside each branch move with it, and a negated call gains !

success

TargetStock.cs, the /*^*/ caret

Stock.csmodified

View fixture on GitHub

#de-morgan

Negating && and || follows De Morgan's laws, parenthesising where precedence needs it

success

TargetDiscount.cs, the /*^*/ caret

Discount.csmodified

View fixture on GitHub

#else-if-chain

An else if becomes the body of the new then branch, in braces so the else cannot attach to it

success

TargetGrade.cs, the /*^*/ caret

Grade.csmodified

View fixture on GitHub

#embedded-statements

Branches without braces are swapped as they are, and a double negation is removed

success

TargetSign.cs, the /*^*/ caret

Sign.csmodified

View fixture on GitHub

#floating-point-comparison

A comparison of values that may be NaN or null is negated with ! rather than flipped

success

TargetGauge.cs, the /*^*/ caret

Gauge.csmodified

View fixture on GitHub

#guard-clause-continue

An if with no else that ends a loop body becomes an early continue

success

TargetBatch.cs, the /*^*/ caret

Batch.csmodified

View fixture on GitHub

#guard-clause-return

An if with no else that ends a void method becomes an early return

success

TargetShipping.cs, the /*^*/ caret

Shipping.csmodified

View fixture on GitHub

#no-implicit-exit-rejected

Refuses an if with no else that ends a block which does not end the method or a loop body

refusal

TargetSample.cs, the /*^*/ caret

Refusesno-implicit-exit; every file is left unchanged

Sample.csinput

View fixture on GitHub

#not-an-if-rejected

Refuses a caret that is not on an if statement

refusal

TargetSample.cs, the /*^*/ caret

Refusesnot-an-if; every file is left unchanged

Sample.csinput

View fixture on GitHub

#remove-guard-clause

An early return that ends a void method is inverted into an if around the code after it

success

TargetShipping.cs, the /*^*/ caret

Shipping.csmodified

View fixture on GitHub

#simple

Negates the condition of an if with an else and swaps the branches

success

TargetSample.cs, the /*^*/ caret

Sample.csmodified

View fixture on GitHub

#type-pattern

A type test becomes a negated pattern, keeping its designation

success

TargetShapes.cs, the /*^*/ caret

Shapes.csmodified

View fixture on GitHub

1	1		using System;
2	2
3	3		namespace Shop
4	4		{
5	5		public class Names
6	6		{
7	7		public int Length(string? name)
8	8		{
9		−	/^/if (name is null)
	9	+	if (name is not null)
10	10		{
11		−	throw new ArgumentNullException(nameof(name));
	11	+	return name.Length;
12	12		}
13	13
14		−	return name.Length;
	14	+	throw new ArgumentNullException(nameof(name));
15	15		}
16	16		}
17	17		}

1		using System;
2
3		namespace Shop
4		{
5		public class Sample
6		{
7		public void Log(int value)
8		{
9		/^/if (value > 0)
10		{
11		Console.WriteLine("positive");
12		}
13
14		Console.WriteLine(value);
15		}
16		}
17		}

1	1		namespace Shop
2	2		{
3	3		public class Stock
4	4		{
5	5		public string Status(int level)
6	6		{
7	7		// Decide what to show.
8		−	/^/if (IsLow(level))
	8	+	if (!IsLow(level))
	9	+	{
	10	+	// Nothing to do.
	11	+	return "ok";
	12	+	}
	13	+	else
9	14		{
10	15		// Ask for more.
11	16		return "reorder";
12		−	}
13		−	else
14		−	{
15		−	// Nothing to do.
16		−	return "ok";
17	17		}
18	18		}
19	19
20	20		private bool IsLow(int level) => level < 5;
21	21		}
22	22		}

1	1		namespace Shop
2	2		{
3	3		public class Discount
4	4		{
5	5		public decimal Rate(int quantity, bool member, string code)
6	6		{
7		−	/^/if (quantity >= 10 && !member \|\| code == null)
	7	+	if ((quantity < 10 \|\| member) && code != null)
8	8		{
9		−	return 0.1m;
	9	+	return 0m;
10	10		}
11	11		else
12	12		{
13		−	return 0m;
	13	+	return 0.1m;
14	14		}
15	15		}
16	16		}
17	17		}

1	1		namespace Shop
2	2		{
3	3		public class Grade
4	4		{
5	5		public string Of(int score)
6	6		{
7		−	/^/if (score < 50)
	7	+	if (score >= 50)
8	8		{
9		−	return "fail";
10		−	}
11		−	else if (score < 80)
12		−	{
13		−	return "pass";
	9	+	if (score < 80)
	10	+	{
	11	+	return "pass";
	12	+	}
	13	+	else
	14	+	{
	15	+	return "merit";
	16	+	}
14	17		}
15	18		else
16	19		{
17		−	return "merit";
	20	+	return "fail";
18	21		}
19	22		}
20	23		}
21	24		}

1	1		namespace Shop
2	2		{
3	3		public class Sign
4	4		{
5	5		public int Of(bool negative)
6	6		{
7		−	/^/if (!negative)
	7	+	if (negative)
	8	+	return -1;
	9	+	else
8	10		return 1;
9		−	else
10		−	return -1;
11	11		}
12	12		}
13	13		}

1	1		namespace Shop
2	2		{
3	3		public class Gauge
4	4		{
5	5		public string Describe(double ratio, int? count)
6	6		{
7		−	/^/if (ratio > 0.5 && count < 3)
	7	+	if (!(ratio > 0.5) \|\| !(count < 3))
8	8		{
9		−	return "high";
	9	+	return "low";
10	10		}
11	11		else
12	12		{
13		−	return "low";
	13	+	return "high";
14	14		}
15	15		}
16	16		}
17	17		}

1	1		using System;
2	2		using System.Collections.Generic;
3	3
4	4		namespace Shop
5	5		{
6	6		public class Batch
7	7		{
8	8		public void Process(List<int> quantities)
9	9		{
10	10		foreach (var quantity in quantities)
11	11		{
12		−	/^/if (quantity > 0)
	12	+	if (quantity <= 0)
13	13		{
14		−	Console.WriteLine(quantity);
	14	+	continue;
15	15		}
	16	+
	17	+	Console.WriteLine(quantity);
16	18		}
17	19		}
18	20		}
19	21		}

1	1		using System;
2	2
3	3		namespace Shop
4	4		{
5	5		public class Shipping
6	6		{
7	7		public void Ship(string order)
8	8		{
9	9		Console.WriteLine("checking");
10		−	/^/if (order != null)
	10	+	if (order == null)
11	11		{
12		−	Console.WriteLine("packing");
13		−	Console.WriteLine(order);
	12	+	return;
14	13		}
	14	+
	15	+	Console.WriteLine("packing");
	16	+	Console.WriteLine(order);
15	17		}
16	18		}
17	19		}

1		namespace Shop
2		{
3		public class Sample
4		{
5		public int Twice(int value)
6		{
7		/^/return value * 2;
8		}
9		}
10		}

1	1		public class Sample
2	2		{
3	3		public string Describe(int value)
4	4		{
5		−	/^/if (value > 0)
	5	+	if (value <= 0)
	6	+	{
	7	+	return "not positive";
	8	+	}
	9	+	else
6	10		{
7	11		return "positive";
8	12		}
9		−	else
10		−	{
11		−	return "not positive";
12		−	}
13	13		}
14	14		}

1	1		namespace Shop
2	2		{
3	3		public class Circle
4	4		{
5	5		public double Radius;
6	6		}
7	7
8	8		public class Shapes
9	9		{
10	10		public double Area(object shape)
11	11		{
12		−	/^/if (shape is Circle circle)
	12	+	if (shape is not Circle circle)
13	13		{
14		−	return 3.14 * circle.Radius * circle.Radius;
	14	+	return 0;
15	15		}
16	16		else
17	17		{
18		−	return 0;
	18	+	return 3.14 * circle.Radius * circle.Radius;
19	19		}
20	20		}
21	21		}
22	22		}