Catalog / Primitives / convert-switch-expression-to-statement

Convert Switch Expression to Switch Statement

Turns a switch expression that is the whole value of a return, an assignment or a local's initialiser into a switch statement.

Precondition

The caret is on the switch expression, on its governing expression or its switch keyword.
The switch expression is exactly one of:
the value of a return statement;
the right-hand side of a simple assignment used as a statement;
the initialiser of a local declared on its own, without using or const, whose type can be named.

Transformation

Each arm becomes a section, in order:
a constant pattern c becomes case c:;
the alternatives of an or pattern become separate labels, unless the arm has a when clause or declares a variable;
any other pattern, with its when clause, becomes a pattern label;
_ becomes default:.
A section returns the arm's value, or assigns it and breaks. A throw expression becomes a throw statement.
A local initialised by the switch expression is declared first, with its type written out in place of var, and assigned in each section.
A switch expression with no discard arm throws SwitchExpressionException for an unmatched value, so a default section throwing it is added, with the value when it is a simple name. This keeps a switch that is exhaustive without a discard, such as one on bool, from falling off the end.

Preserved

Behaviour, including the exception for an unmatched value.
Comments above an arm move above its section.

Limitations

Switch expressions nested in larger expressions, passed as arguments, or used as expression bodies are refused; extract them to a local first.
The added SwitchExpressionException carries the unmatched value only when the governing expression is a simple name or member access, to avoid evaluating it twice.

Error codes

Code	Meaning
`not-a-switch`	the caret is not on a `switch` expression
`unsupported-context`	the switch expression is not the whole value of a return, an assignment statement or a single local's initialiser
`anonymous-type`	the local it initialises has an anonymous type, which cannot be declared separately

Cases

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

#anonymous-type-rejected

Refuses a local of anonymous type, which cannot be declared before the switch

refusal

TargetSample.cs, the /*^*/ caret

Refusesanonymous-type; every file is left unchanged

Sample.csinput

View fixture on GitHub

#assigned

An assigned switch expression becomes sections that assign and break, and a throw arm becomes a throw statement

success

TargetPricing.cs, the /*^*/ caret

Pricing.csmodified

View fixture on GitHub

#comments-kept

Comments above an arm move above its section, and comments before the statement stay before the switch

success

TargetParser.cs, the /*^*/ caret

Parser.csmodified

View fixture on GitHub

#declaration

A local initialised by a switch expression is declared with its type first, and when clauses are kept

success

TargetShapes.cs, the /*^*/ caret

Shapes.csmodified

View fixture on GitHub

#nested-expression-rejected

Refuses a switch expression that is part of a larger expression

refusal

TargetSample.cs, the /*^*/ caret

Refusesunsupported-context; every file is left unchanged

Sample.csinput

View fixture on GitHub

#no-discard-arm

Without a discard arm, the default section throws the exception the switch expression would have thrown

success

TargetSign.cs, the /*^*/ caret

Sign.csmodified

View fixture on GitHub

#not-a-switch-rejected

Refuses a caret that is not on a switch expression

refusal

TargetSample.cs, the /*^*/ caret

Refusesnot-a-switch; every file is left unchanged

Sample.csinput

View fixture on GitHub

#returned

A returned switch expression becomes a switch statement whose sections return, or patterns becoming separate labels

success

TargetShipping.cs, the /*^*/ caret

Shipping.csmodified

View fixture on GitHub

1		using System;
2
3		namespace Shop
4		{
5		public class Sample
6		{
7		public void Print(int code)
8		{
9		var item = code /^/switch
10		{
11		1 => new { Name = "one" },
12		_ => new { Name = "other" },
13		};
14		Console.WriteLine(item.Name);
15		}
16		}
17		}

1	1		using System;
2	2
3	3		namespace Shop
4	4		{
5	5		public class Pricing
6	6		{
7	7		private decimal _rate;
8	8
9	9		public decimal Rate => _rate;
10	10
11	11		public void Apply(string tier)
12	12		{
13		−	_rate = tier /^/switch
	13	+	switch (tier)
14	14		{
15		−	"gold" => 0.2m,
16		−	"silver" => 0.1m,
17		−	_ => throw new ArgumentException("unknown tier"),
18		−	};
	15	+	case "gold":
	16	+	_rate = 0.2m;
	17	+	break;
	18	+	case "silver":
	19	+	_rate = 0.1m;
	20	+	break;
	21	+	default:
	22	+	throw new ArgumentException("unknown tier");
	23	+	}
19	24		}
20	25		}
21	26		}

1	1		namespace Shop
2	2		{
3	3		public class Parser
4	4		{
5	5		public int Digit(char c)
6	6		{
7	7		// Only binary digits are expected.
8		−	return c /^/switch
	8	+	switch (c)
9	9		{
10	10		// The usual case.
11		−	'0' => 0,
12		−	'1' => 1,
13		−	_ => -1,
14		−	};
	11	+	case '0':
	12	+	return 0;
	13	+	case '1':
	14	+	return 1;
	15	+	default:
	16	+	return -1;
	17	+	}
15	18		}
16	19		}
17	20		}

1	1		using System;
2	2
3	3		namespace Shop
4	4		{
5	5		public class Circle
6	6		{
7	7		public double Radius;
8	8		}
9	9
10	10		public class Shapes
11	11		{
12	12		public void Print(object shape)
13	13		{
14		−	var name = shape /^/switch
	14	+	string name;
	15	+	switch (shape)
15	16		{
16		−	Circle circle when circle.Radius > 10 => "big circle",
17		−	Circle => "circle",
18		−	_ => "unknown",
19		−	};
	17	+	case Circle circle when circle.Radius > 10:
	18	+	name = "big circle";
	19	+	break;
	20	+	case Circle:
	21	+	name = "circle";
	22	+	break;
	23	+	default:
	24	+	name = "unknown";
	25	+	break;
	26	+	}
20	27		Console.WriteLine(name);
21	28		}
22	29		}
23	30		}

1		namespace Shop
2		{
3		public class Sample
4		{
5		public string Name(int code)
6		{
7		return "code " + (code /^/switch
8		{
9		1 => "one",
10		_ => "other",
11		});
12		}
13		}
14		}

	1	+	using System.Runtime.CompilerServices;
	2	+
1	3		namespace Shop
2	4		{
3	5		public class Sign
4	6		{
5	7		public string Describe(bool positive)
6	8		{
7		−	return positive /^/switch
	9	+	switch (positive)
8	10		{
9		−	true => "positive",
10		−	false => "not positive",
11		−	};
	11	+	case true:
	12	+	return "positive";
	13	+	case false:
	14	+	return "not positive";
	15	+	default:
	16	+	throw new SwitchExpressionException(positive);
	17	+	}
12	18		}
13	19		}
14	20		}

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		namespace Shop
2	2		{
3	3		public class Shipping
4	4		{
5	5		public string Zone(int code)
6	6		{
7		−	return code /^/switch
	7	+	switch (code)
8	8		{
9		−	1 => "local",
10		−	2 or 3 => "national",
11		−	_ => "international",
12		−	};
	9	+	case 1:
	10	+	return "local";
	11	+	case 2:
	12	+	case 3:
	13	+	return "national";
	14	+	default:
	15	+	return "international";
	16	+	}
13	17		}
14	18		}
15	19		}