C＃-数组新知识，需要澄清

Question

namespace Program
{
    class Path
    {
      private readonly MapLocation[] _path; 

      public Path(MapLocation[] path)
       {
          _path = path;
       } 
    }
}

I am having trouble understanding _path = path; How does one go about understanding how this works and why it's needed?

Answer 1

class Path
{
  private readonly MapLocation[] B; 

  public Path(MapLocation[] A)
   {
      B = A;
   } 
}

Basically you are assigning the array path that is passed in by the constructor to the variable _path, in the example above I have renamed them A and B , so it is assigning the array of maplocations from the constructor to the object variable ( B ) for storage.

Answer 2

There are a few things you need to understand here so let's take it one step at a time.

Value Types vs Reference Types

Value Types:

In C#, some things are passed by value while others are passed by reference. Here is an example of something passed by value:

class Path
{
   private int _number; 

   public Path(int number)
   {
      _number = number;
   } 
}

Here is how we would use the above class:

int number = 5;
Path p1 = new Path(number);

The variable number is passed by value. What that means is: A copy of the value of what is stored in number is passed to the p1 object. Therefore, if you change the value of it within the Path class, the value of number which we pass will not change. It will only change within p1 object. Please read more on C# value types .

Reference Types:

Arrays are one of the things which are passed by reference in C#. Here is an example of that. I will explain readonly later on:

class Path
{
   private readonly MapLocation[] _path; 

   public Path(MapLocation[] path)
   {
      _path = path;
   } 
}

Here is how we would use the above class:

// 2 is max number of items for this array
MapLocation[] locations = new MapLoations[2];  
locations[0] = new MapLocation();
Path p1 = new Path(locations);

The variable locations is passed by reference. What that means is: A reference to the memory location of where locations is stored, is passed to the p1 object. Therefore, if you change the value of it within the Path class, for example, if you add another item to the array, you will notice the change in locations . Why? Because you passed a reference to the memory location of locations and thus whoever (for example Path ) has access to it, they can change things at that location. Please read more on C# Reference types . So what is happening is we pass the reference of locations to the Path constructor, and within the constructor it is assigned to the _path variable.

The easiest way to understand reference types is to imagine you are passing a rope which is tied to the memory location. Anyone who has the rope, can find their way to the memory location and change it . So in our example we create a variable named locations and then we tie a rope to it. We pass the other end of the rope to the Path constructor, and within the constructor the rope is passed to the _path variable. Now from within the class you can use the rope called _path and find the memory location all the way to locations . This rope can span many classes and methods and so on. Anyhow enough with the rope.

So what is the readonly for? This means that assignment to the variable can only happen from within the constructor and nowhere else. So in your example, _path can only be assigned to from within the constructor of the class where it appears which is Path in this case. But you ask why would someone want to do this? Well sometime you want to create a class, assign something to the variable, and then you never want to allow anyone to assign to it again.

Hope that helps.

Answer 3

Path is a class. Outside of the class, you can create one or more objects of type Path .

Each Path object has a private reference to an array of MapLocation objects. This array reference is called _path .

When a Path object is constructed, another reference to an array of MapLocation objects is passed into the Path() constructor as an argument called path . Probably the expectation is that the referenced array already contains some MapLocation objects.

The purpose of the line of code you're asking about is to make _path reference the same MapLocation array as path does. So that the new instance of Path that's getting created will have be able to access/use that array.