我这里有一行简单的代码，可总是编译失败（失败的原因：（并非所有的代码路径都返回值）），求高手指点一下
using System;
class grades
{
    public double[] grad = new double[12];
    grades()
    {
        Console.WriteLine("请输入12个学生的分数");
        for (int i = 0; i <= 11; i++)
        {
            Console.WriteLine("grad[" + i + "]=");
            grad[i] = double.Parse(Console.ReadLine());   
        }

    }
   static  double  jiafa (double[] grad)
    {  
        string str="";

        for (int i = 0; i <= 11; i++)
        {
            grad[i] = grad[i] + 7;
            str = string.Format("grad[i]={0}", grad);
        }
        Console.WriteLine(str);
    }
    public static void Main()
    {
        grades g1 = new grades();
    }
}

啥意思

using System;
class grades
{
    public double[] grad = new double[12];
    grades()
    {
        Console.WriteLine("请输入12个学生的分数");
        for (int i = 0; i <= 11; i++)
        {
            Console.WriteLine("grad[" + i + "]=");
            grad[i] = double.Parse(Console.ReadLine());
        }

    }
    void jiafa()
    {
        string str = "";

        for (int i = 0; i <= 11; i++)
        {
            grad[i] = grad[i] + 7;
            str = string.Format("grad[" + i + "]={0}", grad[i]);
            Console.WriteLine(str);
        }
        
    }
    public static void Main()
    {
        grades g1 = new grades();
        g1.jiafa();
    }
}



我成功了！！！！！！！！！！！！！！！！！！！！！

那你觉得该怎样改咧！                           

谢了呀