C# Error: The contextual keyword 'var' may only appear within a local variable declaration

Question

我编写了以下函数来创建 axWindowsMediaPlayer 播放列表：

WMPLib.IWMPPlaylist p2 = axWindowsMediaPlayer.playlistCollection.newPlaylist("Playlist 1");

private void CreatePlaylist(string _currentId)
{
  string selectedElementPageTypeValue = MainContentAreaBl.GetSelectedElementPageTypeValue(_currentId);
  var selectedElementJumpToValue = MainContentAreaBl.GetSelectedElementValue(_currentId, "jumpTo");
  if (selectedElementJumpToValue != null)
  {
     _currentId = selectedElementJumpToValue;                
     if (_currentId != null && _currentId != "menu" && selectedElementPageTypeValue == "video")
     {
        var playerFile = Path.Combine(Common.ContentFolderPath, MainContentAreaBl.GetSelectedElementDataPathValue(_currentId));
        p2.appendItem(axWindowsMediaPlayer.newMedia(playerFile));
        axWindowsMediaPlayer.currentPlaylist = p2;
        CreatePlaylist(_currentId);
     }                
     axWindowsMediaPlayer.Ctlcontrols.play();
  }
}

此处 var p2 声明为 class 级别。当我编译我的应用程序时，我收到以下错误消息：

The contextual keyword 'var' may only appear within a local variable declaration

但是，我不能将 var p2 = axWindowsMediaPlayer.playlistCollection.newPlaylist("Playlist 1"); 放入递归函数中，因为它会在每次迭代时创建新的播放列表。

如何在我的函数中访问 p2？

编辑 1： 我在输出中看到了这个 Window

COM Reference 'WMPLib' is the interop assembly for ActiveX control 'AxWMPLib' but was marked to be linked by the compiler with the /link flag. This COM reference will be treated as a reference and will not be linked.

此外，现在它在 axWindowsMediaplayer 上显示以下错误：

A field initializer cannot reference the non-static field, method or property

此信息是否与我看到的错误有关？如何解决这个问题？

Answer 1

您必须使用正确的类型声明它，而不是使用 var:

AxWMPLib.IWMPPlaylist p2 = axWindowsMediaPlayer.playlistCollection.newPlaylist("Playlist 1");

var 只允许在局部变量上使用，而不能在字段上使用，这就是错误消息告诉您的内容。错误消息并不意味着字段声明在错误的地方，您只是对字段类型使用了错误的语法。

Answer 2

MSDN 说：

To correct this error

If the variable belongs at class scope, give it an explicit type. Otherwise move it inside the method where it will be used.

所以你可以在声明变量类型的同时给出正确的类型，比如

IWMPPlaylist p2 = axWindowsMediaPlayer.playlistCollection.newPlaylist("Playlist 1");

否则您可以将变量移动到将要使用它的方法中。在您的情况下，您可以将其移动到 CreatePlaylist 方法中。

Answer 3

其他人已经提到了这个问题。您想要的特定类型是 IWMPPlaylist，因此整行将如下所示。

IWMPPlaylist p2 = axWindowsMediaPlayer.playlistCollection.newPlaylist("Playlist 1");

Answer 4

在 Constructor 中初始化播放列表并将播放列表作为参数传递，正如@thumbnumkeys（现已删除）所回答的那样。这是对我有用的代码：

namespace ABC
{
public partial class MainContentArea : Form
{
    private string _currentId;        

    public MainContentArea(string topicId, Menu menu)
    {
        InitializeComponent();
        _currentId = topicId;
        _menu = menu;
        WMPLib.IWMPPlaylist p2 = axWindowsMediaPlayer.playlistCollection.newPlaylist("Playlist 1");
        CreatePlaylist(_currentId, p2);
    }
private void CreatePlaylist(string _currentId, WMPLib.IWMPPlaylist p2)
{    
   var selectedElementJumpToValue = MainContentAreaBl.GetSelectedElementValue(_currentId, "jumpTo");
   string selectedElementPageTypeValue = MainContentAreaBl.GetSelectedElementPageTypeValue(selectedElementJumpToValue);            
   if (selectedElementJumpToValue != null)
   {
      _currentId = selectedElementJumpToValue;                
      if (_currentId != null && _currentId != "menu" && selectedElementPageTypeValue == "video")
      {
         var playerFile = Path.Combine(Common.ContentFolderPath, MainContentAreaBl.GetSelectedElementDataPathValue(_currentId));                    
         p2.appendItem(axWindowsMediaPlayer.newMedia(playerFile));
         axWindowsMediaPlayer.currentPlaylist = p2;
                CreatePlaylist(_currentId, p2);
      }
            //axWindowsMediaPlayer.BringToFront();
   }
        axWindowsMediaPlayer.Ctlcontrols.play();
}
}
}

C# Error: The contextual keyword 'var' may only appear within a local variable declaration

C# Error: The contextual keyword 'var' may only appear within a local variable declaration

c#

var

compiler-errors

winforms