学习 C#

理解 C# If-Else 语句：新手指南

Layla Porter

7m 37s

在 C# 编程中，基于条件的决策是创建动态响应应用程序的基本方面。这时就需要用到 if-else 语句了。 C# 中的 if-else 语句允许程序根据指定条件是真还是假来执行不同的代码块。

本指南的灵感来源于LaylaCodesIt的视频" C# 入门教程第 2 部分 - 学习如何编写 'if-else' 语句和 'while' 循环"，它将向您介绍 C# 中 if-else 语句的基础知识，以及在编写布尔条件时使用条件运算符和逻辑运算符，并演示如何使您的程序能够智能地与用户输入进行交互。

引入条件逻辑

条件逻辑允许程序根据某些条件做出决策。 Layla 的视频介绍了 if-else 语句的概念和语法，这是一个条件逻辑的例子，它会根据指定条件是真还是假来执行不同的代码块。

在以下代码示例中，Layla 编写了一条问候消息，询问用户是否喜欢奶酪：

// Asking the user a question and storing their response
Console.WriteLine($"Hello {name}, do you like cheese?");
string doTheyLikeCheese = Console.ReadLine();

// Asking the user a question and storing their response
Console.WriteLine($"Hello {name}, do you like cheese?");
string doTheyLikeCheese = Console.ReadLine();

' Asking the user a question and storing their response
Console.WriteLine($"Hello {name}, do you like cheese?")
Dim doTheyLikeCheese As String = Console.ReadLine()

' Asking the user a question and storing their response
Console.WriteLine($"Hello {name}, do you like cheese?")
Dim doTheyLikeCheese As String = Console.ReadLine()

Simple If-Else Statement in C#

然后，这个 if-else 条件语句会检查用户的回答，并根据用户的回答改变其输出响应。该条件由条件运算符构成，并根据其结果执行 if-else 块中的语句。以下是一个包含一行语句的简单示例：

// Checking the user's response and providing appropriate output
if (doTheyLikeCheese == "yes")
{
    Console.WriteLine("Here is a lump of cheese.");
}
else
{
    Console.WriteLine("Here is a lump of coal.");
}

// Checking the user's response and providing appropriate output
if (doTheyLikeCheese == "yes")
{
    Console.WriteLine("Here is a lump of cheese.");
}
else
{
    Console.WriteLine("Here is a lump of coal.");
}

' Checking the user's response and providing appropriate output
If doTheyLikeCheese = "yes" Then
    Console.WriteLine("Here is a lump of cheese.")
Else
    Console.WriteLine("Here is a lump of coal.")
End If

' Checking the user's response and providing appropriate output
If doTheyLikeCheese = "yes" Then
    Console.WriteLine("Here is a lump of cheese.")
Else
    Console.WriteLine("Here is a lump of coal.")
End If

如果用户回答"是"，则 if 语句中的代码将显示一条消息，提供一块奶酪。如果用户回复的不是"是"，则 else 代码块将输出"这里有一块煤"。

区分大小写问题

使用基本的 if-else 语句时，可能会出现一个常见问题：区分大小写。如果用户输入"Yes"而不是"yes"，程序将无法识别为有效输入。为了解决这个问题，Layla 使用 ToLower() 方法，该方法会自动将用户的输入转换为小写，从而避免大小写敏感性问题。

// Using ToLower() to handle case sensitivity in user input
if (doTheyLikeCheese.ToLower() == "yes")
{
    Console.WriteLine("Here is a lump of cheese.");
}
else
{
    Console.WriteLine("Here is a lump of coal.");
}

// Using ToLower() to handle case sensitivity in user input
if (doTheyLikeCheese.ToLower() == "yes")
{
    Console.WriteLine("Here is a lump of cheese.");
}
else
{
    Console.WriteLine("Here is a lump of coal.");
}

' Using ToLower() to handle case sensitivity in user input
If doTheyLikeCheese.ToLower() = "yes" Then
    Console.WriteLine("Here is a lump of cheese.")
Else
    Console.WriteLine("Here is a lump of coal.")
End If

' Using ToLower() to handle case sensitivity in user input
If doTheyLikeCheese.ToLower() = "yes" Then
    Console.WriteLine("Here is a lump of cheese.")
Else
    Console.WriteLine("Here is a lump of coal.")
End If

处理多种条件

Layla 接着解释了如何使用 else-if 语句处理两个以上可能的响应。在这里，程序会检查多个条件语句——无论用户回答"是"、"否"还是其他任何内容。

// Checking multiple conditions using else-if
if (doTheyLikeCheese.ToLower() == "yes")
{
    Console.WriteLine("Here is a lump of cheese.");
}
else if (doTheyLikeCheese.ToLower() == "no")
{
    Console.WriteLine("Here is a lump of coal.");
}
else
{
    Console.WriteLine("Please give a yes or no answer.");
}

// Checking multiple conditions using else-if
if (doTheyLikeCheese.ToLower() == "yes")
{
    Console.WriteLine("Here is a lump of cheese.");
}
else if (doTheyLikeCheese.ToLower() == "no")
{
    Console.WriteLine("Here is a lump of coal.");
}
else
{
    Console.WriteLine("Please give a yes or no answer.");
}

' Checking multiple conditions using else-if
If doTheyLikeCheese.ToLower() = "yes" Then
    Console.WriteLine("Here is a lump of cheese.")
ElseIf doTheyLikeCheese.ToLower() = "no" Then
    Console.WriteLine("Here is a lump of coal.")
Else
    Console.WriteLine("Please give a yes or no answer.")
End If

' Checking multiple conditions using else-if
If doTheyLikeCheese.ToLower() = "yes" Then
    Console.WriteLine("Here is a lump of cheese.")
ElseIf doTheyLikeCheese.ToLower() = "no" Then
    Console.WriteLine("Here is a lump of coal.")
Else
    Console.WriteLine("Please give a yes or no answer.")
End If

和以前一样，如果 if 语句中的布尔表达式为真，则当用户回答"是"时，程序会提供一块奶酪。然而，在 else-if 语句中，它不再对任何其他回复都提供一块煤炭，而是只在用户回复"否"时才提供。如果用户回答的不是"是"或"否"，则程序的 else 部分会要求用户提供"是"或"否"的答案。让我们看看上述代码的实际运行结果：（图片可以改进）

使用循环

为了确保用户提供有效响应，引入了循环。循环可以反复要求用户输入，直到用户提供正确答案为止。这叫做"while"循环，下面的例子展示了它的工作原理：

// Using a while loop to repeatedly ask for valid input
bool hasAnswered = false;

while (!hasAnswered)
{
    // Checking the user's response and setting hasAnswered to true for valid responses
    if (doTheyLikeCheese.ToLower() == "yes")
    {
        Console.WriteLine("Here is a lump of cheese.");
        hasAnswered = true;
    }
    else if (doTheyLikeCheese.ToLower() == "no")
    {
        Console.WriteLine("Here is a lump of coal.");
        hasAnswered = true;
    }
    else
    {
        Console.WriteLine("Please give a yes or no answer.");
        doTheyLikeCheese = Console.ReadLine();
    }
}

// Using a while loop to repeatedly ask for valid input
bool hasAnswered = false;

while (!hasAnswered)
{
    // Checking the user's response and setting hasAnswered to true for valid responses
    if (doTheyLikeCheese.ToLower() == "yes")
    {
        Console.WriteLine("Here is a lump of cheese.");
        hasAnswered = true;
    }
    else if (doTheyLikeCheese.ToLower() == "no")
    {
        Console.WriteLine("Here is a lump of coal.");
        hasAnswered = true;
    }
    else
    {
        Console.WriteLine("Please give a yes or no answer.");
        doTheyLikeCheese = Console.ReadLine();
    }
}

' Using a while loop to repeatedly ask for valid input
Dim hasAnswered As Boolean = False

While Not hasAnswered
    ' Checking the user's response and setting hasAnswered to true for valid responses
    If doTheyLikeCheese.ToLower() = "yes" Then
        Console.WriteLine("Here is a lump of cheese.")
        hasAnswered = True
    ElseIf doTheyLikeCheese.ToLower() = "no" Then
        Console.WriteLine("Here is a lump of coal.")
        hasAnswered = True
    Else
        Console.WriteLine("Please give a yes or no answer.")
        doTheyLikeCheese = Console.ReadLine()
    End If
End While

' Using a while loop to repeatedly ask for valid input
Dim hasAnswered As Boolean = False

While Not hasAnswered
    ' Checking the user's response and setting hasAnswered to true for valid responses
    If doTheyLikeCheese.ToLower() = "yes" Then
        Console.WriteLine("Here is a lump of cheese.")
        hasAnswered = True
    ElseIf doTheyLikeCheese.ToLower() = "no" Then
        Console.WriteLine("Here is a lump of coal.")
        hasAnswered = True
    Else
        Console.WriteLine("Please give a yes or no answer.")
        doTheyLikeCheese = Console.ReadLine()
    End If
End While

在上面的例子中，循环会一直提示用户，直到他们回答"是"或"否"为止。在莱拉的视频中，你可以看到代码执行时的实际效果：