C# - $ - String Interpolation

The $ special character identifies a string literal as an interpolated string. An interpolated string is a string literal that might contain interpolation expressions. When an interpolated string is resolved to a result string, items with interpolation expressions are replaced by the string representations of the expression results. This feature is available starting with C# 6. String interpolation provides a more readable and convenient syntax to create formatted strings than a string composite formatting feature. The following example uses both features to produce the same output:

string name = "Mark";

var date = DateTime.Now;

// Composite formatting:

Console.WriteLine("Hello, {0}! Today is {1}, it's {2:HH:mm} now.", name, date.DayOfWeek, date);

// String interpolation:

Console.WriteLine($"Hello, {name}! Today is {date.DayOfWeek}, it's {date:HH:mm} now.");

// Both calls produce the same output that is similar to:

// Hello, Mark! Today is Wednesday, it's 19:40 now.

Console.WriteLine($"|{"Left",-7}|{"Right",7}|");

const int FieldWidthRightAligned = 20;

Console.WriteLine($"{Math.PI,FieldWidthRightAligned} - default formatting of the pi number");

Console.WriteLine($"{Math.PI,FieldWidthRightAligned:F3} - display only three decimal digits of the pi number");

// Expected output is:

// |Left   |  Right|

//     3.14159265358979 - default formatting of the pi number

//                3.142 - display only three decimal digits of the pi number

string MyName = "Horace";

int age = 34;

Console.WriteLine($"He asked, \"Is your name {MyName}?\", but didn't wait for a reply :-{{");

Console.WriteLine($"{name} is {age} year{(age == 1 ? "" : "s")} old.");

// Expected output is:

// He asked, "Is your name Horace?", but didn't wait for a reply :-{

// Horace is 34 years old.

double speedOfLight = 299792.458;

FormattableString message = $"The speed of light is {speedOfLight:N3} km/s.";

System.Globalization.CultureInfo.CurrentCulture = System.Globalization.CultureInfo.GetCultureInfo("nl-NL");

string messageInCurrentCulture = message.ToString();

var specificCulture = System.Globalization.CultureInfo.GetCultureInfo("en-IN");

string messageInSpecificCulture = message.ToString(specificCulture);

string messageInInvariantCulture = FormattableString.Invariant(message);

Console.WriteLine($"{System.Globalization.CultureInfo.CurrentCulture,-10} {messageInCurrentCulture}");

Console.WriteLine($"{specificCulture,-10} {messageInSpecificCulture}");

Console.WriteLine($"{"Invariant",-10} {messageInInvariantCulture}");

// Expected output is:

// nl-NL      The speed of light is 299.792,458 km/s.

// en-IN      The speed of light is 2,99,792.458 km/s.

// Invariant  The speed of light is 299,792.458 km/s.