Interfaces are a type of contract between interface and class or struct. Interface is defined by interface keyword and the name of interface should usually start with capital letter I to make it more recognizable.
When we say Interface is a contract, what do you really mean?
Every class or struct that uses some interface must implement all of its functionalities.
Why would you use Interface at all?
There are multiple reasons actually:
Firstly, a class or classes that use Interface must use all of Interfaces functionalities. Secondly, names of all properties, methods… must be same in all classes as they are in that specific Interface. Furthermore Interfaces are obligatory, imagine situation where you must in a few classes use 3 methods Get, Insert and Delete, when you use interface with your classes there is no possibility of forgetting to implement either of these methods. Lastly, because C# does not allow us to inherit multiple classes but allows us to implement multiple Interfaces, we can create multiple interfaces where each interface inherits a class and use these interface as workaround.
What can we define inside Interface?
Methods (but without implementation)
What are Interface limitations?
Interfaces cannot define:
We cannot make interface or its members static
What about access modifiers?
By default all interface members are public and we cannot change it.
Can we instantiate Interface? Yes we can, limitation is that you must instantiate them using a class or struct that implements that interface.
Is there a limit to how many Interfaces can we implement at the same time? Yes and no. Although language specification state that you can implement as many Interfaces as you like, there are few technical limitations: heap size and metadata tables size, but worry not, you will never exceed that limit.
Please, drop by tomorrow to see a few Interface examples .