设计模式：简单工厂

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<br>namespace SimleFactory {     public class Operation     {         public double NumberA { get; set; }         public double NumberB { get; set; }         public virtual double GetResult()         {             double result = 0;             return result;         }     }     public class OperationAdd:Operation     {         public override double GetResult()         {             return NumberA + NumberB;         }     }     public class OperationSub : Operation     {         public override double GetResult()         {             return NumberA - NumberB;         }     }     public class OperationMul : Operation     {         public override double GetResult()         {             return NumberA * NumberB;         }     }     public class OperationDiv : Operation     {         public override double GetResult()         {             if (NumberB == 0)                 throw new Exception("除数不能为0.");             return NumberA / NumberB;         }     }     public class OperationFactory     {         public static Operation CreateOperation(string operate)         {             Operation oper = null;             switch(operate)             {                 case "+":                     oper = new OperationAdd();                     break;                 case "-":                     oper = new OperationAdd();                     break;                 case "*":                     oper = new OperationAdd();                     break;                 case "/":                     oper = new OperationAdd();                     break;             }             return oper;         }     } }

　　测试代码：

            Operation oper = OperationFactory.CreateOperation("+");
            oper.NumberA = 1;
            oper.NumberB = 2;
            double result= oper.GetResult();
            Assert.AreEqual(result, 3);