为什么在此C＃.NET控制器方法中，我具有此签名？

Question

I am pretty new to C# .NET and I have the following doubt.

On a page on which I am working on I found the following link:

<a class="ui-btn-inline ui-btn ui-icon-delete ui-btn-icon-notext ui-corner-all" href="@Url.Action("Delete", "Groups", new { id = item.gruppoId })">Delete</a>

This link call a Delete() method on a GroupsController class.

Ok, this is this method:

public ActionResult Delete(int id = 0)
{
    .......................
    .......................
    .......................
    DO SOME STUFF
    .......................
    .......................
    .......................
    return View(model);
}

My doubt is related to the signature of this method: why is the parameter int id = 0 ?

What does the = 0 mean? To begin with I thought that this was a simple initialization and that it change the it value to 0 but using the debbugger I discovered that it don't change the id value. So what it exactly do?

Answer 1

It's called an optional parameter. It means you can call the method with no argument, like this:

Delete();

If you do that, the value of the id parameter in the function will be 0 .

Answer 2

You're right in saying that the = 0 sets the value of the id parameter.

But it's important to note that it only does so when you do not pass that parameter .

Take for example:

public void SaySomething( var something = "Hello" )
{
    Console.WriteLine( something );
}

//...

SaySomething();
SaySomething("I am sleeping.");

The first call to the function does not pass a parameter. Therefore, the default value "Hello" is used to write to the console.

The second call already sets a value for the parameter, so it does not get overwritten by the default value you set up. "I am sleeping." will be printed in this case.