C# Online Compiler | .NET Fiddle

using System;
namespace Application {
    // Base class Cost
	public class Shape {
	    
		public void setSide(int s) {
			side = s;
		}
		protected int side;
	}
	//interface
	public interface Cost {
		int getCost(int area);
	}
	// Derived class, using Interface and Base class
	class square: Shape, Cost {
	    //methods in derived class
		public int getArea() {
			return (side * side);
		}
		public int getCost(int area) {
			return area * 10;
		}
	}
	public class SquareInheritance {
		public static void Main(string[] args) {
			square sq = new square();
			int area;
			sq.setSide(5);
			area = sq.getArea();

			// Print the area of the object.
			Console.WriteLine("The area is: {0}", sq.getArea());
			Console.WriteLine("The cost is: {0}", sq.getCost(area));
			
		}
	}
}

 
using System;
namespace Application {
    // Base class Cost
    public class Shape {
        
        public void setSide(int s) {
            side = s;
        }
        protected int side;
    }
    //interface
    public interface Cost {
        int getCost(int area);
    }
    // Derived class, using Interface and Base class
    class square: Shape, Cost {
        //methods in derived class
        public int getArea() {
            return (side * side);
        }
        public int getCost(int area) {
            return area * 10;
        }
    }
    public class SquareInheritance {
        public static void Main(string[] args) {
            square sq = new square();
            int area;
            sq.setSide(5);
            area = sq.getArea();
​
            // Print the area of the object.
            Console.WriteLine("The area is: {0}", sq.getArea());
            Console.WriteLine("The cost is: {0}", sq.getCost(area));
            
        }
    }
}

Compilation error (line 6, col 7): The type or namespace name 'FluentValidation' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 7, col 7): The type or namespace name 'FluentValidation' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 8, col 7): The type or namespace name 'FluentValidation' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 66, col 37): The type or namespace name 'AbstractValidator<>' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 79, col 29): The type or namespace name 'AbstractValidator<>' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 94, col 13): The type or namespace name 'PropertyValidatorContext' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 113, col 93): The type or namespace name 'PropertyValidatorContext' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 143, col 89): The type or namespace name 'IRuleBuilderInitial<,>' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 144, col 13): The type or namespace name 'AbstractValidator<>' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 143, col 23): The type or namespace name 'IRuleBuilderOptions<,>' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 150, col 18): The type or namespace name 'IRuleBuilderOptions<,>' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 151, col 13): The type or namespace name 'AbstractValidator<>' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 149, col 23): The type or namespace name 'IRuleBuilderOptions<,>' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 157, col 13): The type or namespace name 'AbstractValidator<>' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 159, col 13): The type or namespace name 'IRuleBuilder<,>' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 156, col 24): The type or namespace name 'IRuleBuilderOptions<,>' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 246, col 60): The type or namespace name 'Severity' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 196, col 26): The type or namespace name 'IPropertyValidator' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 200, col 33): The type or namespace name 'IPropertyValidator' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 70, col 6): The name 'RuleSet' does not exist in the current context
Compilation error (line 83, col 13): The name 'RuleFor' does not exist in the current context
Compilation error (line 84, col 26): The name 'CascadeMode' does not exist in the current context
Compilation error (line 100, col 76): The type or namespace name 'StaticStringSource' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 118, col 76): The type or namespace name 'StaticStringSource' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 162, col 38): The type or namespace name 'ChildValidatorAdaptor' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 208, col 57): The type or namespace name 'StaticStringSource' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 215, col 57): The type or namespace name 'LazyStringSource' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 222, col 57): The type or namespace name 'LazyStringSource' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 229, col 54): The type or namespace name 'StaticStringSource' could not be found (are you missing a using directive or an assembly reference?)