String Interpolation

String interpolation is a syntactic shorthand for the string.Format() introduced in C# 6.

var name = "World";
var oldWay = string.Format("Hello, {0}!", name);  // returns "Hello, World"
var newWay = $"Hello, {name}!";                   // returns "Hello, World"

Basics of string interpolation

var name = "World";
var str = $"Hello, {name}!";
//str now contains: "Hello, World!";

Behind the scenes

Internally this

$"Hello, {name}!"

Will be compiled to something like this:

string.Format("Hello, {0}!", name);

Format numbers in strings

You can use a colon and the standard numeric format syntax to control how numbers are formatted.

var decimalValue = 120.5;

var asCurrency = $"It costs {decimalValue:C}";
// String value is "It costs $120.50" (depending on your local currency settings)

var withThreeDecimalPlaces = $"Exactly {decimalValue:F3}";
// String value is "Exactly 120.500"

var integerValue = 57;

var prefixedIfNecessary = $"{integerValue:D5}";
// String value is "00057"

Live Demo on .NET Fiddle

Format dates in strings

var date = new DateTime(2015, 11, 11);
var str = $"It's {date:MMMM d, yyyy}, make a wish!";
System.Console.WriteLine(str);

You can also use the DateTime.ToString method to format the DateTime object. This will produce the same output as the code above.

var date = new DateTime(2015, 11, 11);
var str = date.ToString("MMMM d, yyyy");
str = "It's " + str + ", make a wish!";
Console.WriteLine(str);

Output:

It’s November 11, 2015, make a wish!

Live Demo on .NET Fiddle

Live Demo using DateTime.ToString

Note: MM stands for months and mm for minutes. Be very careful when using these as mistakes can introduce bugs that may be difficult to discover.

Padding the output

String can be formatted to accept a padding parameter that will specify how many character positions the inserted string will use :

${value, padding}

NOTE: Positive padding values indicate left padding and negative padding values indicate right padding.

Left Padding

A left padding of 5 (adds 3 spaces before the value of number, so it takes up a total of 5 character positions in the resulting string.)

var number = 42;
var str = $"The answer to life, the universe and everything is {number, 5}.";
//str is "The answer to life, the universe and everything is    42.";
//                                                           ^^^^^
System.Console.WriteLine(str);

Output:

The answer to life, the universe and everything is    42.

Live Demo on .NET Fiddle

Right Padding

Right padding, which uses a negative padding value, will add spaces to the end of the current value.

var number = 42;
var str = $"The answer to life, the universe and everything is ${number, -5}.";
//str is "The answer to life, the universe and everything is 42   .";
//                                                           ^^^^^
System.Console.WriteLine(str);

Output:

The answer to life, the universe and everything is 42   .

Live Demo on .NET Fiddle

Padding with Format Specifiers

You can also use existing formatting specifiers in conjunction with padding.

var number = 42;
var str = $"The answer to life, the universe and everything is ${number, 5:f1}";
//str is "The answer to life, the universe and everything is 42.1 ";
//                                                           ^^^^^

Live Demo on .NET Fiddle

Expressions

Full expressions can also be used in interpolated strings.

var StrWithMathExpression = $"1 + 2 = {1 + 2}"; // -> "1 + 2 = 3"

string world = "world";
var StrWithFunctionCall = $"Hello, {world.ToUpper()}!"; // -> "Hello, WORLD!"