String interpolation is used to concatenate, format, and manipulate strings in C#. It provides more readable and convenient syntax to create formatted strings than a native string composite format feature.

Interpolated string structure:

String interpolation syntax starts with a ‘$’ symbol and you can't leave any white space between $ and the ", also expressions will be defined within a bracket {} as shown below, 

{<interpolationExpression>[,<alignment>][:<formatString>]}

Elements in square brackets are optional,

 The code snippet concatenates the name and website variables with a string using string interpolation.

string name = "Sabari";
string website = "Techiclues.com";

// Composite formatting
Console.WriteLine("Welcome {0}!, {1} is a developer community website.", name, website);
// String interpolation
Console.WriteLine($"Welcome {name}!, {website} is a developer community website.");

Output:

Welcome Sabari!, Techiclues.com is a developer community website.
Welcome Sabari!, Techiclues.com is a developer community website.

The below code snippet concatenates the car, model, and price variables with a string using string interpolation.

string car = "Audi A1";
string model = "Sportback";
string price = "45,000";

Console.WriteLine($"Prices for an {car} {model} start around ${price}");

Output:

Prices for an Audi A1 Sportback start around $45,000

The following example constructs a string with spacing and appends 10 characters after the first string. 

string car = "Audi A1";
string model = "Sportback";            

// {car, 10} - adds 10 characters space from the first object.
Console.WriteLine($"{car}{model,15} is a good premium car");  

Output:

Audi A1      Sportback is a good premium car

String interpolation and expression is used in the below code example,

string name = "John";
int age = 25;

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

Output:

John is 25 years old.