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String Interpolation

Remarks:

String interpolation is a shorthand for the string.Format() method that makes it easier to build strings with variable and expression values inside of them.

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

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!"

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.

Simple Usage

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);

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

Formatting 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