如何从列表中选择一个随机字符串并将其分配给一个变量C＃

Question

我想制作一个程序，用户输入一个字符串并将该字符串存储在名为word1的列表中。我想从列表中选择一个随机字符串，并将其显示在标签中。我正在尝试使用多种方法和类作为练习。这里是我的代码：

这是班上的Class1.cs：

main main = new main(); 

    public string flashCards1() 
    { 
     List<string> word1 = main.GetList1(); 

     Random rnd = new Random(); 

     int rtn = rnd.Next(word1.Count - 1); 

     string word = word1[rtn]; 

     string rtnWord = word.ToString(); 

     return rtnWord; 
    } 

这一个是在main.cs（非主）和它谈论到Class。就像我说的这部分可能是不必要的，但我试图用多种方法练习。

private List<string> word2 = new List<string>(); 
    private List<string> word1 = new List<string>(); 

    public List<string> GetList1() 
    { 
     return word1; 
    } 

    public void SetList1(List<string> updatedList) 
    { 
     word1 = updatedList; 
    } 

这一个是在Form1.cs和设置标签flashCards1的返回值：

private void go_Click(object sender, EventArgs e) 
    { 
     menu.Visible = false; 
     cards.Visible = true; 

     label1.Text = class1.flashCards1(); 

    } 

这也是Form1.cs和保存文本列表：

private void enter_Click(object sender, EventArgs e) 
    { 
     List<string> word1 = main.GetList1(); 

     word1.Add(textBox1.Text); 

     main.SetList1(word1); 
    } 

当我运行这段代码它给我的错误：

System.ArgumentOutOfRangeException was unhandled HResult=-2146233086 Message='maxValue' must be greater than zero. Parameter name: maxValue ParamName=maxValue Source=mscorlib StackTrace: at System.Random.Next(Int32 maxValue) at WindowsFormsApplication1.Class1.flashCards1() in C:\Users\lottes\Source\Workspaces\Workspace\WindowsFormsApplication1\WindowsFormsApplication1\Class1.cs:line 19 at WindowsFormsApplication1.Form1.go_Click(Object sender, EventArgs e) in C:\Users\lottes\Source\Workspaces\Workspace\WindowsFormsApplication1\WindowsFormsApplication1\Form1.cs:line 62 at System.Windows.Forms.Control.OnClick(EventArgs e) at System.Windows.Forms.Button.OnClick(EventArgs e) at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent) at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks) at System.Windows.Forms.Control.WndProc(Message& m) at System.Windows.Forms.ButtonBase.WndProc(Message& m) at System.Windows.Forms.Button.WndProc(Message& m) at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m) at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m) at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam) at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg) at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData) at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context) at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context) at System.Windows.Forms.Application.Run(Form mainForm) at WindowsFormsApplication1.Program.Main() in C:\Users\lottes\Source\Workspaces\Workspace\WindowsFormsApplication1\WindowsFormsApplication1\Program.cs:line 19 at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args) at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args) at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly() at System.Threading.ThreadHelper.ThreadStart_Context(Object state) at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) at System.Threading.ThreadHelper.ThreadStart() InnerException:

我试图输入很多值，但它似乎不工作。我也看了这个帖子： How could I get a random string from a list and assign it to a variable

但是他们的解决方案给了我同样的错误。 任何帮助将不胜感激。对不起，如果这是太多/很少的信息。我试着说得通。

Answer 1

您的问题与Random.Next(int)方法。当您尝试使用负值作为最大参数时，会出现此异常。这是因为它返回的值在>= 0 and < maxValue之间。它获取由负值搞砸了（-1你的情况，当列表中有0项）

解决方法是简单的，只是改变：

 int rtn = rnd.Next(word1.Count - 1); 

到：

 int rtn = rnd.Next(word1.Count); 

这应该因为随机总是返回一个值小于的最大值，所以应该从0到N-1访问，其中N是你列表的大小。

之后你会遇到更多的问题（与这个直接问题相关的问题更少），因为核心问题是你在flashcards1()函数中使用它之前没有初始化你的列表并添加了值。在向我们展示的示例中如何解决这个问题并不清楚，但我建议您查看一下函数的执行顺序。