FAQ: Basic Classes and Objects - Overloading Constructors

#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
#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:

#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;
    }
}