//创建一个委托
public delegate decimal MathOperation(decimal left, decimal right);//创建方法参数和返回结果符合委托的定义
public static decimal Add(decimal left, decimal right)
{return left + right;
}public static decimal Subtract(decimal left, decimal right)
{return left - right;
}public static decimal Multiply(decimal left, decimal right)
{return left * right;
}public static decimal Divide(decimal left, decimal right)
{return left / right;
}//返回委托类型
private static MathOperation GetOperation(char oper)
{switch(oper){case '+': return Add;case '-': return Subtract;case '*': return Multiply;case '/': return Divide;}throw new NotSupportedException("");
}//封装一个方法用来把操作数和符号考虑进来,返回委托类型
private static decimal Eval(string expr)
{var elements = expr.Split(new []{' '}, 3);var left = Decimal.Parse(elements[0]);var right = Decimal.Parse(elements[1]);var ope = elements[2][0];return GetOperation(op)(left, right);
}void Main()
{Console.WriteLine(Eval("1 3 +"));
}

public interface IMathOperation
{decimal Compute(decimal left, decimal right);
}public class AddOperation : IMathOperation
{decimal Compute(decimal left, decimal right){return left + right;}
}public class SubtractOperation : IMathOperation
{decimal Compute(decimal left,, decimal right){return left - right;}
}public class MultiplyOperation : IMathOperation
{decimal Compute(decimal left, decimal right){return left * right;}
}public class DivideOperation : IMathOperation
{decimal Compute(decimal left, decimal right){return left/right;}
}//获取接口类型
private static IMathOperation GetOperation(char oper)
{switch(oper){case '+': return new AddOperation();case '-': return new SubtractOperation();case '*': return new MultiplyOperation();case '/': return new DivideOperation();}throw new NotSupportedException("");
}...

public delegate decimal MathOperation(decimal left, decimal right);private static MathOperation GetOperation(char oper)
{switch(oper){case '+': return delgate(decimal left, decimal right) {return left + right;};case '-': return delgate(decimal left, decimal right) {return left - right;};case '*': return delgate(decimal left, decimal right) {return left * right;};case '/': return delgate(decimal left, decimal right) {return left / right;};}throw new NotSupportedException("");
}

private static Func<decimal, decimal, decimal> GetOperation(char oper)
{switch(oper){case '+': return (left, right) => left + right;case '-': return (left, right) => left - right;case '*': return (left, right) => left * right;case '/': return (left, right) => left / right;}throw new NotSupportedExcepton("");
}