FAQ: Basic Classes and Objects - Overloading Constructors

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#1

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This community-built FAQ covers the “Overloading Constructors” exercise from the lesson “Basic Classes and Objects”.

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#2

I’m using the overload from the first example.


public Forest(int area, string country = "Unknown")
{
  this.Area = area;
  this.Country = country;
}

Here’s how.

using System;

public class Program
{
	static void Main(string[] args)
	{
		Forest f = new Forest(800, "Africa");
		Console.WriteLine(f.Area);
		Console.WriteLine(f.Country);

		Forest f2 = new Forest(400);
		Console.WriteLine(f2.Area);
		Console.WriteLine(f2.Country);
	}

	class Forest
  {

    public int Area { get; set; }
    public string Country { get; set; }

    public Forest(int area, string country)
    { 
      this.Area = area;
      this.Country = country;
    }

    // constructors is default values ---> It doesn't work
    public Forest(int area, string country = "Unknown")
    {
      this.Area = area;
      this.Country = country;
    }

  }

}

I have as an answer

main.cs(29,12): error CS0111: A member `Program.Forest.Forest(int, string)'is already defined. Rename this member or use different parameter types
main.cs(22,12): (Location of the symbol related to previous error)
Compilation failed: 1 error(s), 0 warnings
compiler exit status 1

Unlike when I use the second method, which works.


using System;

public class Program
{
	static void Main(string[] args)
	{
		Forest f = new Forest(800, "Africa");
		Console.WriteLine(f.Area);
		Console.WriteLine(f.Country);

		Forest f2 = new Forest(400);
		Console.WriteLine(f2.Area);
		Console.WriteLine(f2.Country);
	}

	class Forest
  {

    public int Area { get; set; }
    public string Country { get; set; }

    public Forest(int area, string country)
    { 
      this.Area = area;
      this.Country = country;
    }

    // It's working
    public Forest(int area) : this(area, "Unknown")
    { 
      Console.WriteLine("Country property not specified. Value defaulted to 'Unknown'.");
    }

  }

}

800
Africa
Country property not specified. Value defaulted to 'Unknown'.
400
Unknown
1 Like
#3

On point 3: When you run the code, you should see the warning message and “Unknown” printed to the console. Why are these two things printed?

This is exactly what happens. However, WHY ARE these two things printed out? Left for me to figure out? Don’t leave me hanging like this Codecademy :frowning:

7 Likes
#4

I believe the issue here is that you don’t actually need to have 2 separate constructors when using the default argument method. Try removing the first constructor (the one where you do not use a default value for the country field). You are trying to use the same parameters for 2 different constructor methods so the program doesn’t know which to choose. “Rename this member or use different parameter types” from the error message lead me to this conclusion.

1 Like
#5

@tag0472869507 is right, this is the issue.

You have to understand how the C# picks the correct overload (for any method not just the constructor).
It looks at the method’s signature. The signature is the number of parameters and the types of the parameters. There are a few other things that matter too, such as the order of the parameters and any keywords (such as ref). It will compare the arguments passed in and see if it can find a signature that matches.

So the signatures for your two Forest constructors are int, string and int, string. Obviously, it cannot know if you intend to use the default value constructor or not unless you omit the country parameter, but if you do pass in and int and a string it has not way to know which one you want it to use.

What you are after is constructor chaining:

public class Forest
{
    public int Area { get; set; }
    public string Country { get; set; }
    private const string _unknownCountry = "Unknown";
						 
    // this(area, "Unknown") is calling the other constructor that matches this signature and runs first.
    public Forest(int area): this(area, _unknownCountry)
    {
       // This is ran after the other constructor
        Console.WriteLine($"Country property not specified. Value defaulted to '{_unknownCountry}'.");
    }

    public Forest(int area, string country)
    {
        this.Area = area;
        this.Country = country;
    }
}
1 Like
#6

This is the example provided in the introduction:

public Forest(int area, string country)
{ 
  this.Area = area;
  this.Country = country;
}

public Forest(int area) : this(country, "Unknown")
{ 
  Console.WriteLine("Country property not specified. Value defaulted to 'Unknown'.");
}

I believe it should be

public Forest(int area) : this(area, "Unknown")

Instead of “country” - as country is the value being called as “unknown”

This really confused me for ages while trying to follow this example. Suggest that is corrected unless there’s something I’m missing?

8 Likes
#7

I just want to elaborate on the answer given here. This lesson caused quite a bit of confusion, but I think I understand it now and think it could use more explanation. Here’s the code with my explanation:

public class Forest
{
    /* omitted code */
    
    /* if a new instance of Forest is called with one parameter, C# uses this constructor.
    (int area) refers to the parameters passed into this particular overload.
    this(area, _unknownCountry) calls another Forest constructor that has the parameters area (an int), and _unknownCountry (a string).
    essentially, this constructor passes area and _unknownCountry into the constructor with two parameters, hence the name "constructor chaining". */
    public Forest(int area): this(area, _unknownCountry)
    {
        // after the second constructor is called, this code runs.
        Console.WriteLine($"Country property not specified. Value defaulted to '{_unknownCountry}'.");
    }

    // the above code is basically this:
    public Forest(int area)
    {
      Forest(area, _unknownCountry);
    }

    //which would call this constructor
    public Forest(int area, string country)
    {
        this.Area = area;
        this.Country = country;
    }
}

Theoretically, you could chain these indefinitely, building recursively. For example:

public class Forest
{
  /* fields, parameters, etc. */

  private const string _unknown = "unknown";

  public Forest() : this(0)
  {
    Console.WriteLine("no properties specified, defaulting to 0");
  }

  public Forest(int area) : this (area, unknown)
  {
    if (area != 0) {
      Console.WriteLine("only area specified, defaulting others to \"unknown\"");
    }
  }

  /* etc etc for other parameters */

  public Forest(int area, string country, int trees, string name)
  {
    this.Area = area;
    this.Country = country;
    this.Trees = trees;
    this.Name = name;
  }
}

tl;dr: Using : this() is equivalent to using Forest(area, _unknownCountry), because this loops back to the Forest class. Feel free to make any additions/corrections/edits as needed. Hope this helps!

4 Likes
#8

The reason for the chaining syntax - you can’t actually call another constructor from inside a constructor, like you can with regular methods. It’s the same general idea, though.

#9

This is the best explanation of how this() works and that developers made a mistake in material explanation

#10

Yeah this completely threw me too! I think you are correct. I have reported bug on the content.

#11

I did it, it works, but the site is not letting me progress. Here is my code:

  public Forest(string name, string biome)
    {
      this.Name = name;
      this.Biome = biome;
      Age = 0;
    }
    
    public Forest(string name) : this (name, "Unknown")
    {
      Console.WriteLine("Biome defaulted to 'Unknown'");
    }

And here is the reason it’s giving for me not progressing:

Define a new constructor with one argument and add : this(...) to the end.

Not sure what to do here.

#12

After many hours i think i understand how this() is used
Here is my code.

 class Forest
  {
    public int A {get; private set;}
    public string B {get; private set;}
    //First constructor and is parameterless
    public Forest ()
    {
      Console.WriteLine("I am the first CONSTRUCTOR!");
    }
  //second constructor 
   public Forest(int a , string b, double c)
        : this()//thats how we call the parameterless constructor
   {
       this.A = a;
       Console.WriteLine(a);
       Console.WriteLine(b);
       Console.WriteLine(c);

   }
// third constructor
   public Forest (int a, string b)
        : this (a, "John" , 20.20)
//here "a" from third constructor is equal to "a" from second constructor wich is equal A proprety
   {    
      this.B = b;
     Console.WriteLine(a);
     Console.WriteLine(b);

   }



}

And now we call the third Constructor

lass Program
  {
    static void Main(string[] args)
    {
      Forest f = new Forest(200, "Michael");// here we instantiate the third constructor and notice how all constructors are called with it
      Console.WriteLine(f.A);// notice that A is 200 same as the parameter 
      Console.WriteLine(f.B);
    }
  }

Console

I am the first CONSTRUCTOR!
200
John
20.2
200
Michael
200
Michael
#13

Why should I use this() which generated so much problems instead of just using

Method(var parameter1, var parameter2 = "Predetermined value")
{
    ...
}

That even prevented me from having the “it prints twice” error.

2 Likes
#14

What is the point of constructor overloading if I can just assign default values? Why would I ever need to overload a constructor when I can just give it a default value

#15

I don’t understand this:

public Forest(string name) : this(name, "Unknown")

.
I like this better since it makes more sense and is more readable:

public Forest(int area, string country = "Unknown")

.
I don’t understand the first example. Why is there (string name) or even the this(name, "Unknown"), what does that really assign things to. Does it declare the int area parameter and then looks for constructors of the same class and sets name to unknown? This is less better.