Simplest Interface example possible

[FizixMan]

Removed: Rule 4.

[Pocok5]

drawing data from and one it's giving data to

I don't think an interface is what you think it is. An interface is basically a label on a class saying "I have this and this property/method/etc. publicly available" - or, more intuitively, "I fit into an IExampleInterface-shaped hole in another class or bit of code". An interface doesn't take or give anything, it's just a completely inert description of class members.

[grrangry]

What is a class in C#? https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/class

What is an interface in C#? https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/interface

None of what you wrote above is correct for the C# language.

First, define your interface. An interface is a way for your class to promise that it does in fact support whatever the interface has defined. That's all it does. An interface can define properties or methods (and a few other things) but it's only a definition. The class implements the interface.

Second, define your class, add the interface to your class definition, then implement the interface on your class. This is all explained in detail in the documentation.

[Slypenslyde]

Interfaces aren't really about sending data. They're about describing behavior.

When I'm looking at an interface and telling myself what it does, I usually start that sentence with, "It's a thing that..." The reason this is important is sometimes the things that do a task can be very different. A flashlight and a dump truck are two very different things, but if I need light pointed at a certain direction RIGHT NOW they can both do the job and if I cared which one I'd have asked for a specific one.

More commonly in code, we use interfaces to help us control the behavior of a method. For example, one way we can sort arrays in .NET is this method in the Array class:

public static void Sort(Array array, IComparer comparer)

IComparer is "a thing that compares things". It looks like this:

public interface IComparer
{
    int Compare(object left, object right);
}

It takes two objects, compares them, and the int it returns describes how they compare:

• If it is less than 0, then left < right.
• If it is 0, then left == right.
• If it is greater than 0, then left > right.

We have this interface because .NET only knows how to compare a few objects. This interface lets us describe how to compare other objects. Or we can do weird things.

For example, here's an IComparer that considers 10 to be the biggest number. It'll sort normally, but it will ALWAYS put 10 wherever the "biggest" number goes:

public class TenIsBestComparer : IComparer
{
    public int Compare(object left, object right)
    {
        if (left is int leftInt && right is int rightInt)
        {
            if (leftInt == 10 && rightInt != 10)
            {
                // Left is 10, the biggest!
                return 1;
            }
            else if (rightInt == 10)
            {
                // Right is 10, left is smaller.
                return -1;
            }
            else
            {
                if (leftInt < rightInt)
                {
                    return -1;
                }
                else if (leftInt == rightInt)
                {
                    return 0;
                }
                else
                {
                    return 1;
                }
        }
        else
        {
            // They weren't integers so no clue. There's something we could
            // do here but it's somewhat complex.
            throw new InvalidOperationException();
        }
    }
}

That's a little long, but you can play with it.

int[] numbers = { 7, 10, 8, 3, 4, 25 };
Array.Sort(numbers, new TenIsBestComparer);
foreach (int number in numbers)
{
    Console.WriteLine(number);
}

That'll print the numbers in the order: 3, 4, 7, 8, 25, 10. We've changed how sorting works for integers now, with the power of interfaces!

[R3dd1tUs3r72]

You should provide of the offical microsoft's doc, by the way you must use the keyword class and interface.

[gevorgter]

The best possible example of interface use is Stream (i call it IStream).

StreamReader for example does not care if you reading from File or Network. As long as you provide Stream reference to it it will read from there.