扩展静态方法和非扩展静态方法之间的区别是什么

Question

In class A I have a method:

  public static String GetTimestamp(this DateTime value) {
            return value.ToString("yyyyMMddHHmmssffff");
        }

I call it by:

        String timeStamp = GetTimestamp(new DateTime());

What is the difference in use if that method looked like:

  public static String GetTimestamp( DateTime value) {
                return value.ToString("yyyyMMddHHmmssffff");
            }

Answer 1

The point of an extension method is that you could have called it like:

String timeStamp = new DateTime().GetTimestamp();

to the same effect. That's the only change.

Answer 2

Extension Methods are intended to be used as "if they were" regular instance methods of their target types:

var timeStamp = new DateTime().GetTimestamp();

other than that, there is no difference.

---

They were introduced in C# 3.0 to create the beautiful syntax we normally see in LINQ today, such as:

  var soldProducts = customers.Where(x => x.IsActive)
                              .SelectMany(x => x.Orders)
                              .Where(x => x.Status == OrderStatus.Completed)
                              .Select(x => x.Product)
                              .ToList;

without extension methods, this syntax would not be possible and would require many regular method calls:

var activecustomers = Enumerable.Where(customers, x => x.IsActive);
var orders = Enumerable.SelectMany(activecustomers, x => x.Orders);
var completed = Enumerable.Where(orders, x => x.Status == OrderStatus.Completed);
var products = Enumerable.Select(completed, x => x.Product);
var soldProducts = Enumerable.ToList(products);

Not to mention that type inference is also playing a big part here, since most of these methods in System.Linq.Enumerable are actually generic methods with one or several type parameters, but the C# compiler is smart enough to infer the generic type parameters and remove the need to explicitly specify them.

Answer 3

With the extension method version, you can also call it with

dateTime.GetTimeStamp()

if you've includeded the namespace it's declared in.

Answer 4

Extension methods allow you to treat them as though they were a method inside of the type in question. So in your first example, that method can be treated as though it is a method inside of DateTime. so you could do something like this:

var foo = new DateTime();
var timestamp = foo.GetTimestamp();