What are extension methods?

Extension methods in C# are used to extend the functionality of a class which is not provided by the class itself, without altering the definition of the original type. An extension method is a static method of a static class, where the this modifier is applied to the first parameter. The type of the first parameter will be the type that is extended.

Rules to Use Extension Methods

We need to import the namespace in which the static class is defined which contains the extension methods. Suppose I have defined my method in a class which is again defined under namespace. In that case, we need to include the namespace using the keyword using wherever we want to use the method.
C# only supports extension methods, it does not support extension properties, extension events, extension operators.
Extension methods must be declared in non-generic and static classes.
C# compiler looks only for the extension methods defined in static classes that are themselves defined at the file scope. In other words, if we define a static class in some class and then define our extension methods, in that case, we will get a compile time error as shown below.

1- Auto-properties, and their improvements

Auto-properties have been available in C# for a long time by now, the thing is that Microsoft now introduce a new feature. “Initializers” can be used to initialize these properties to their defaults. A few of the ways that they can be used is like this:

private string Name { get; set; } = “C# Learners”;

// Without a setter, a readonly
private string Name { get; } = “C# Learners”;

// Simply read-only
private readonly string Name = “C# Learners”;

2- Null-conditional operator

Nullable types are instances of the System.Nullable<T> struct. A nullable type can represent the correct range of values for its underlying value type, plus an additional null value. For example, a Nullable<Int32>, pronounced “Nullable of Int32,” can be assigned any value from -2147483648 to 2147483647, or it can be assigned the null value. A Nullable<bool> can be assigned the values true false, or null. The ability to assign null to numeric and Boolean types is especially useful when you are dealing with databases and other data types that contain elements that may not be assigned a value. For example, a Boolean field in a database can store the values true or false, or it may be undefined.

Nullable types have the following characteristics:

Nullable types represent value-type variables that can be assigned the value of null. You cannot create a nullable type based on a reference type. (Reference types already support the null value.)

The syntax T? is shorthand for Nullable<T>, where T is a value type. The two forms are interchangeable.
Assign a value to a nullable type just as you would for an ordinary value type, for example int? x = 10; or double? d = 4.108. A nullable type can also be assigned the value null: int? x = null.

Use the Nullable<T>.GetValueOrDefault method to return either the assigned value, or the default value for the underlying type if the value is null, for example int j = x.GetValueOrDefault();

Use the HasValue and Value read-only properties to test for null and retrieve the value, as shown in the following example: if(x.HasValue) j = x.Value;

The HasValue property returns true if the variable contains a value, or false if it is null.

The Value property returns a value if one is assigned. Otherwise, a System.InvalidOperationException is thrown.

The default value for HasValue is false. The Value property has no default value.

You can also use the == and != operators with a nullable type, as shown in the following example: if (x != null) y = x;

Use the ?? operator to assign a default value that will be applied when a nullable type whose current value is null is assigned to a non-nullable type, for example int? x = null; int y = x ?? -1;

Nested nullable types are not allowed. The following line will not compile: Nullable<Nullable<int>> n;