In C#, we can declare members in a few different ways.
public int Area => length * breadth;
and
public int Area = length * breadth;
and 
public int Area { get; set; } = 100;
Though they look similar, they are very different. Let's have a deeper look.

Expression Bodied Member
public int Area => length * breadth;
The first property has a getter under the hood that will be called each time you access the property. It is called an expression-bodied member, and it is different from a lambda expression. 

C# compiler converts an expression-bodied member to following code. 
public int Area
{
    get
    {
        return length * breadth;
    }
}

Field Initializer
public int Area = length * breadth;
This is a simple field with a field initializer, which will be evaluated only once, when the containing class or type is instantiated. This doesn't provide the benefits of encapsulation, like properties do.

Auto-Property Initializer
public int Area { get; set; } = 100;
This initializes the property to 100. It allows us to declare the initial value of the auto-property as part of the property declaration. This makes it easier to perform the initialization exactly once. 

Properties with an initializer can be read-only. They can also be assigned in the type's constructor. They are useful in creating immutable types. 
public int Area { get; } = 100;