Question

我是开发新手，所以请原谅我这是一个愚蠢的问题。这是我当前项目的片段。我为＃34;最喜欢的＆＃34;而得到一个红色的波浪形在我的switch语句中，当我使用if语句来分配我的＆＃34;最喜欢的＆＃34;基于用户输入的变量，但如果我只是直接声明最喜欢的变量，我就不会得到红色的波浪形。错误表示当前上下文中不存在收藏夹。如果以前曾经问过这个问题，我很抱歉，但我对这个论坛也是全新的。提前谢谢您的任何光线消失。

 class Program
{
    enum Color  
    {
        Red, Orange, Green, Blue, Indigo, Violet
    }
    static void Main(string[] args)
    {
        string userChoice = Console.ReadLine();
        Console.WriteLine("Please enter the number of your favorite color out of the following choices. /n Where 1 = Red /n 2 = Orange /n 3 = Green /n 4 = Blue /n 5 = Indigo /n 6 = Violet");

        if (userChoice == "1")
        {
            Color favorite = Color.Red;
        }
        if (userChoice == "2")
        {
            Color favorite = Color.Orange;
        }
        if (userChoice == "3")
        {
            Color favorite = Color.Green;
        }
        if (userChoice == "4")
        {
            Color favorite = Color.Blue;
        }
        if (userChoice == "5")
        {
            Color favorite = Color.Indigo;
        }
        if (userChoice == "6")
        {
            Color favorite = Color.Violet;
        }

        switch (favorite)
        {
            case Color.Red:
                Console.WriteLine("You chose Red");
                break;
            case Color.Orange:
                Console.WriteLine("You chose Orange");
                break;
            case Color.Green:
                Console.WriteLine("You chose Green");
                break;
            case Color.Blue:
                Console.WriteLine("You chose Blue");
                break;
            case Color.Indigo:
                Console.WriteLine("You chose Indigo");
                break;
            case Color.Violet:
                Console.WriteLine("You chose Violet");
                break;
            default:
                Console.WriteLine("You didn't choose a color");
                break;
        }
    }
}

Answer 1

为什么我收到当前不存在的“收藏”错误我的交换机上下文？

因为您已在favorite语句的正文中声明if作为局部变量。这意味着变量favorite无法从这些实体中访问。

我们如何解决这个问题？

我会尝试以下（还有其他选项）：

// Here we make use of a nullable.
// since the user hasn't enter any color yet the value of color is null.  
Color? color = null;

// Here we ask the user to enter his choice.
string userChoice = Console.ReadLine();
Console.WriteLine("Please enter the number of your favorite color out of the following choices. /n Where 1 = Red /n 2 = Orange /n 3 = Green /n 4 = Blue /n 5 = Indigo /n 6 = Violet");

// According to users input, we set the corresponding color.
// If not a correct color entered, then the value of color would stay null.
if (userChoice == "1")
{
    color = Color.Red;
}
if (userChoice == "2")
{
    color = Color.Green;
}
if (userChoice == "3")
{
    color = Color.Blue;
}
if (userChoice == "4")
{
    color = Color.Blue;
}

switch (color)
{
    case Color.Red:
        Console.WriteLine("You chose Red");
        break;
    case Color.Green:
        Console.WriteLine("You chose Green");
        break;
    case Color.Blue:
        Console.WriteLine("You chose Blue");
        break;
    case Color.Orange:
        Console.WriteLine("You chose Orange");
        break;
    case null:
        Console.WriteLine("You didn't choose a color");
        break;
    default:
        Console.WriteLine("You didn't choose a color");
        break;
}

Answer 2

你定义＆＃34;最喜欢的＆＃34;变量＆＃34;如果＆＃34;身体，然后你正在使用＆＃34;切换＆＃34;在＆＃34;最喜欢＆＃34;变量。但是，如果没有人＆＃34;如果＆＃34;那就让我们想一想会发生什么？身体火灾？ - 正确，＃34;最爱＆＃34;变量将不会定义，但它在＆＃34;开关＆＃34;中使用？陈述 - 逻辑错误。这就是编译器阻止你犯这个错误的原因。只需定义＆＃34;最喜欢的＆＃34;变量出现＆＃34;如果＆＃34;正文然后在＆＃34;中指定必要的值;如果＆＃34;体

Answer 3

“最爱”仅在每个“if”的范围内创建，然后被“销毁”。

if (userChoice == "1")           // Here favorite does not exist yet
{
    Color favorite = Color.Red;  // favorite is created and assigned Color.Red
}                                // The scope of "if" ends, so all variables created within are destroyed, in this case, "favorite"

所以你真正想做的是

Color? favorite = null;
if (userChoice == "1")     // Here favorite already exists
{
    favorite = Color.Red;   // favorite is assigned Color.Red
}                           // Same as before, but favorite is not destroyed this time because it was not created(declared) within "if"'s scope

另一方面，您无法按原样使用开关：

switch语句的控制类型由switch表达式建立。


•如果switch表达式的类型是sbyte，byte，short，ushort，int，uint，long，ulong，bool，char，string或enum-type，或者它是与nulllable类型对应的类型这些类型，那就是switch语句的管理类型。

•否则，从switch表达式的类型到以下可能的控制类型之一，必须存在一个用户定义的隐式转换（第6.4节）：sbyte，byte，short，ushort，int，uint，long，ulong ，char，string或者，与这些类型之一对应的可空类型。

•否则，如果不存在此类隐式转换，或者存在多个此类隐式转换，则会发生编译时错误。

（从C＃5.0规范，8.7.2 switch语句 - 您可以在... \ Microsoft Visual Studio 12.0 \ VC＃\ Specifications \ 1033 \ CSharp语言规范.docx中找到它）

所以在这里，假设你只是对if和switch语句进行了一些操作，那么最终结果将更多地是

static void Main(string[] args)
{
    Console.WriteLine("Please enter the number of your favorite color out of the following choices. \n Where 1 = Red \n 2 = Orange \n 3 = Green \n 4 = Blue \n 5 = Indigo \n 6 = Violet");
    string userChoice = Console.ReadLine();

    Color? favorite = null;

    if (userChoice == "1")
    {
        favorite = Color.Red;
    }
    if (userChoice == "2")
    {
        favorite = Color.Orange;
    }
    if (userChoice == "3")
    {
        favorite = Color.Green;
    }
    if (userChoice == "4")
    {
        favorite = Color.Blue;
    }
    if (userChoice == "5")
    {
        favorite = Color.Indigo;
    }
    if (userChoice == "6")
    {
        favorite = Color.Violet;
    }

    switch (userChoice)
    {
        case "1":
            Console.WriteLine("You chose Red");
            break;
        case "2":
            Console.WriteLine("You chose Orange");
            break;
        case "3":
            Console.WriteLine("You chose Green");
            break;
        case "4":
            Console.WriteLine("You chose Blue");
            break;
        case "5":
            Console.WriteLine("You chose Indigo");
            break;
        case "6":
            Console.WriteLine("You chose Violet");
            break;
        default:
            Console.WriteLine("You didn't choose a color");
            break;
    }

    Console.ReadKey(true);
}

为什么我的交换机当前上下文中不存在“最喜欢”的错误？

3 个答案: