Catalog / Primitives / convert-concatenation-to-interpolation

Convert String Concatenation to Interpolation

Turns a chain of + string concatenations into one interpolated string: "Hello, " + name + "!" becomes $"Hello, {name}!".

Target

The concatenation, by a caret anywhere in it. The whole chain the caret is in is converted, through parentheses around nested concatenations.

Precondition

The expression is a + that concatenates strings.
No operand of the chain is itself a numeric addition, as in a + b + " total", where a + b adds numbers before the concatenation.
There are no comments between the operands.

Transformation

String literals become text, keeping their escapes as written. Braces are doubled so they stay text.
The result is a verbatim $@"..." when every string literal is verbatim. Otherwise it is a regular $"...", and verbatim literals are escaped.
Character literals become text.
An interpolated string operand is merged in.
Any other operand becomes an interpolation {operand}. Parentheses around it are dropped, except around a conditional, whose colon would start a format.
value.ToString("F2") on a formattable value type becomes {value:F2}.
The result is written on one line, in place of the chain.

Preserved

The resulting string: interpolation formats each value with the current culture, as concatenation does.
Comments before and after the expression.

Limitations

A value of a type whose IFormattable.ToString(null, provider) differs from its ToString() is formatted by the former after conversion.
Raw string literals are interpolated as values rather than merged as text.
An expression split over several lines becomes one line, however long.

Error codes

Code	Meaning
`not-a-concatenation`	the expression at the caret is not a string concatenation
`numeric-addition`	an operand of the chain adds numbers before the concatenation
`contains-comments`	there are comments between the operands, which the string could not keep

Cases

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

#char-operand

A character literal becomes text

success

TargetSample.cs, the /*^*/ caret

Sample.csmodified

View fixture on GitHub

#comments-rejected

Refuses a concatenation with comments between its operands, which the string could not keep

refusal

TargetSample.cs, the /*^*/ caret

Refusescontains-comments; every file is left unchanged

Sample.csinput

View fixture on GitHub

#conditional-operand

A conditional operand keeps its parentheses, whose colon would otherwise start a format

success

TargetSample.cs, the /*^*/ caret

Sample.csmodified

View fixture on GitHub

#escapes-braces

Braces in the literals are doubled so they stay text

success

TargetSample.cs, the /*^*/ caret

Sample.csmodified

View fixture on GitHub

#format-string

A value formatted with ToString and a format string becomes a format specifier

success

TargetSample.cs, the /*^*/ caret

Sample.csmodified

View fixture on GitHub

#generic

A value of a type parameter is interpolated

success

TargetSample.cs, the /*^*/ caret

Sample.csmodified

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#interpolated-operand

An interpolated string operand is merged into the result

success

TargetSample.cs, the /*^*/ caret

Sample.csmodified

View fixture on GitHub

#keeps-escape-sequences

Escape sequences in the literals are kept as written

success

TargetSample.cs, the /*^*/ caret

Sample.csmodified

View fixture on GitHub

#mixed-verbatim

A verbatim literal among regular ones has its text escaped for a regular interpolated string

success

TargetSample.cs, the /*^*/ caret

Sample.csmodified

View fixture on GitHub

#multi-line

A concatenation split over several lines becomes one line, keeping the statement's comments

success

TargetSample.cs, the /*^*/ caret

Sample.csmodified

View fixture on GitHub

#non-string-operand

A number is interpolated as concatenation would format it

success

TargetSample.cs, the /*^*/ caret

Sample.csmodified

View fixture on GitHub

#not-a-concatenation-rejected

Refuses an addition that does not produce a string

refusal

TargetSample.cs, the /*^*/ caret

Refusesnot-a-concatenation; every file is left unchanged

Sample.csinput

View fixture on GitHub

#nullable-context

In a nullable context a nullable operand is interpolated as it was concatenated

success

TargetSample.cs, the /*^*/ caret

Projectnullable enable

Sample.csmodified

View fixture on GitHub

#numeric-addition-rejected

Refuses a chain whose leading operands are added as numbers before the concatenation

refusal

TargetSample.cs, the /*^*/ caret

Refusesnumeric-addition; every file is left unchanged

Sample.csinput

View fixture on GitHub

#parenthesised-addition

A parenthesised numeric addition is interpolated without its parentheses

success

TargetSample.cs, the /*^*/ caret

Sample.csmodified

View fixture on GitHub

#simple

String literals and a string operand become one interpolated string

success

TargetSample.cs, the /*^*/ caret

Sample.csmodified

View fixture on GitHub

#verbatim

Verbatim literals make a verbatim interpolated string

success

TargetSample.cs, the /*^*/ caret

Sample.csmodified

View fixture on GitHub

1	1		public class Sample
2	2		{
3	3		public string Describe(string name, int count, bool ok, decimal price, int a, int b)
4	4		{
5		−	return name + /^/':' + ' ' + count;
	5	+	return $"{name}: {count}";
6	6		}
7	7		}

1		public class Sample
2		{
3		public string Describe(string name, int count)
4		{
5		return /^/"Name: " + name + // who
6		", count: " + count;
7		}
8		}

1	1		public class Sample
2	2		{
3	3		public string Describe(string name, int count, bool ok, decimal price, int a, int b)
4	4		{
5		−	return "Status: " + /^/(ok ? "on" : "off");
	5	+	return $"Status: {(ok ? "on" : "off")}";
6	6		}
7	7		}

1	1		public class Sample
2	2		{
3	3		public string Describe(string name, int count, bool ok, decimal price, int a, int b)
4	4		{
5		−	return /^/"{" + name + "}";
	5	+	return $"{{{name}}}";
6	6		}
7	7		}

1	1		public class Sample
2	2		{
3	3		public string Describe(string name, int count, bool ok, decimal price, int a, int b)
4	4		{
5		−	return /^/"Price: " + price.ToString("F2") + " each";
	5	+	return $"Price: {price:F2} each";
6	6		}
7	7		}

1	1		public class Sample
2	2		{
3	3		public string Describe<T>(T value)
4	4		{
5		−	return /^/"Value: " + value;
	5	+	return $"Value: {value}";
6	6		}
7	7		}

1	1		public class Sample
2	2		{
3	3		public string Describe(string name, int count, bool ok, decimal price, int a, int b)
4	4		{
5		−	return $"{count} x " + /^/name;
	5	+	return $"{count} x {name}";
6	6		}
7	7		}

1	1		public class Sample
2	2		{
3	3		public string Describe(string name, int count, bool ok, decimal price, int a, int b)
4	4		{
5		−	return /^/"Name:\t\"" + name + "\"\n";
	5	+	return $"Name:\t\"{name}\"\n";
6	6		}
7	7		}

1	1		public class Sample
2	2		{
3	3		public string Describe(string name, int count, bool ok, decimal price, int a, int b)
4	4		{
5		−	return /^/@"C:\Temp\" + name + ".txt\n";
	5	+	return $"C:\\Temp\\{name}.txt\n";
6	6		}
7	7		}

1	1		public class Sample
2	2		{
3	3		public string Describe(string name, int count)
4	4		{
5	5		// The summary line.
6		−	var text = /^/"Name: " +
7		−	name +
8		−	", count: " +
9		−	count; // for the log
	6	+	var text = $"Name: {name}, count: {count}"; // for the log
10	7		return text;
11	8		}
12	9		}

1	1		public class Sample
2	2		{
3	3		public string Describe(string name, int count, bool ok, decimal price, int a, int b)
4	4		{
5		−	return "Total: " /^/+ count + " items";
	5	+	return $"Total: {count} items";
6	6		}
7	7		}

1		public class Sample
2		{
3		public string Describe(string name, int count, bool ok, decimal price, int a, int b)
4		{
5		return (a /^/+ b).ToString();
6		}
7		}

1	1		public class Sample
2	2		{
3	3		public string Greet(string? name)
4	4		{
5		−	return /^/"Hello " + name;
	5	+	return $"Hello {name}";
6	6		}
7	7		}

1		public class Sample
2		{
3		public string Describe(string name, int count, bool ok, decimal price, int a, int b)
4		{
5		return a + b + /^/" total";
6		}
7		}

1	1		public class Sample
2	2		{
3	3		public string Describe(string name, int count, bool ok, decimal price, int a, int b)
4	4		{
5		−	return "Sum: " + /^/(a + b);
	5	+	return $"Sum: {a + b}";
6	6		}
7	7		}

1	1		public class Sample
2	2		{
3	3		public string Describe(string name, int count, bool ok, decimal price, int a, int b)
4	4		{
5		−	return "Hello, " + /^/name + "!";
	5	+	return $"Hello, {name}!";
6	6		}
7	7		}

1	1		public class Sample
2	2		{
3	3		public string Describe(string name, int count, bool ok, decimal price, int a, int b)
4	4		{
5		−	return /^/@"C:\Temp\" + name + @".txt";
	5	+	return $@"C:\Temp\{name}.txt";
6	6		}
7	7		}