FAQ: Interfaces - Testing Interfaces

This community-built FAQ covers the “Testing Interfaces” exercise from the lesson “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]
1 Like

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.

2 Likes

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

1 Like

Why would you go to all the trouble of implementing the interfaces and not teach actually using the interface?

      Sedan sedan1 = new Sedan(60);
      Sedan sedan2 = new Sedan(70);
      Truck truck = new Truck(45, 500);

      IAutomobile[] autos = { sedan1, sedan2, truck };

      foreach(IAutomobile auto in autos)
      {
        Console.WriteLine($"Auto with license plate: {auto.LicensePlate} with {auto.Wheels}wheels, currently going {auto.Speed} mph.");
      }

      sedan1.SpeedUp();
      sedan2.SpeedUp();
      truck.SpeedUp();

      foreach(IAutomobile auto in autos)
      {
        Console.WriteLine($"Auto with plate: {auto.LicensePlate} sped up to {auto.Speed} mph.");
      }

You could have added SpeedUp and SlowDown to the interace as well! This lesson fails to demonstrate why interfaces are actually useful.

So I’m pretty new to this but I thought that I’d try playing around a little bit. I wanted to place the instances in an object array and then use a foreach loop to iterate through that array calling the speedup method for each object instance. When I tried running it i got this error:

Program.cs(23,14): error CS1061: ‘object’ does not contain a definition for ‘SpeedUp’ and no accessible extension method ‘SpeedUp’ accepting a first argument of type ‘object’ could be found (are you missing a using directive or an assembly reference?) [/home/ccuser/workspace/csharp-interfaces-testing-interfaces/LearnInterfaces.csproj]

Heres the code:

using System;

namespace LearnInterfaces
{
  class Program
  {
    static void Main(string[] args)
    {
      Sedan s1 = new Sedan(60);
      Sedan s2 = new Sedan(70);
      Truck t1 = new Truck(45, 500);

      s1.SpeedUp();

      object[] arr = new object[3];

      arr[0] = s1;
      arr[1] = s2;
      arr[2] = t1;

      foreach(var item in arr)
      {
        item.SpeedUp();
      }

      Console.WriteLine(s1.Speed + " " + s1.Wheels + " " + s1.LicensePlate);
      Console.WriteLine(s2.Speed + " " + s2.Wheels + " " + s2.LicensePlate);
      Console.WriteLine(t1.Speed + " " + t1.Wheels + " " + t1.LicensePlate);

    }
  }
}

Im not sure what I did wrong but I would really like to figure out how to do this.