FAQ: Interfaces - Testing Interfaces

This community-built FAQ covers the “Testing Interfaces” exercise from the lesson “Interfaces”.

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This exercise can be found in the following Codecademy content:

Learn C#

FAQs on the exercise Testing Interfaces

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I know this may not be directly related to interfaces, but could someone explain why this code throws me an error:

using System;

namespace LearnInterfaces
{
  class Program
  {
    static void Main(string[] args)
    {
      Sedan sedan1 = new Sedan(60);
      Sedan sedan2 = new Sedan(70);
      Truck truck1 = new Truck(45, 500);
      
      CallAutomobiles();
      
      sedan1.SpeedUp();
      sedan2.SpeedUp();
      truck1.SpeedUp();
      
      CallAutomobiles();
    }
    
    public void CallAutomobiles(){
      Console.WriteLine($"{sedan1.Speed} {sedan1.Wheels} {sedan1.LicensePlate}");
      Console.WriteLine($"{sedan2.Speed} {sedan2.Wheels} {sedan2.LicensePlate}");
      Console.WriteLine($"{truck1.Speed} {truck1.Wheels} {truck1.LicensePlate}");
    }
  }
}

I wanted to refactor the Console.WriteLine() method calls, but got the error:

Program.cs(23,28): error CS0103: The name 'sedan1' does not exist in the current context [/home/ccuser/workspace/csharp-interfaces-testing-interfaces/LearnInterfaces.csproj]
Program.cs(23,43): error CS0103: The name 'sedan1' does not exist in the current context [/home/ccuser/workspace/csharp-interfaces-testing-interfaces/LearnInterfaces.csproj]
Program.cs(23,59): error CS0103: The name 'sedan1' does not exist in the current context [/home/ccuser/workspace/csharp-interfaces-testing-interfaces/LearnInterfaces.csproj]
Program.cs(24,28): error CS0103: The name 'sedan2' does not exist in the current context [/home/ccuser/workspace/csharp-interfaces-testing-interfaces/LearnInterfaces.csproj]
Program.cs(24,43): error CS0103: The name 'sedan2' does not exist in the current context [/home/ccuser/workspace/csharp-interfaces-testing-interfaces/LearnInterfaces.csproj]
Program.cs(24,59): error CS0103: The name 'sedan2' does not exist in the current context [/home/ccuser/workspace/csharp-interfaces-testing-interfaces/LearnInterfaces.csproj]
Program.cs(25,28): error CS0103: The name 'truck1' does not exist in the current context [/home/ccuser/workspace/csharp-interfaces-testing-interfaces/LearnInterfaces.csproj]

I’m rather new myself, but some digging brought me to this answer: Scope prevents you from using objects in a different method than they were defined in.

The two ways around it that I see are either:

  1. Define the objects as class fields above your “Main” or any other methods. You define it in the class but instantiate it within Main. In this case, if you were to call CallAutomobiles before instantiating it in Main, you would get an error.
    or
  2. Pass the objects as arguments to CallAutomobiles. If you let CallAutomobiles take an argument of type “Sedan” and call it “sedan1” within CallAutomobiles, it will work as you intend.

This_thread on stackoverflow discusses these exact things and is definitely worth reading. For what it’s worth, Codecademy hints at using inheritance in the coming lesson to resolve this issue too.

It is down to scope, and I now understand this. Thank you very much!