c# - 无法在WP 8.1中将void分配给隐式类型的局部变量

Question

我正在开发WP 8.1中的音乐播放器应用，并且试图在其中实现跳转列表功能。
为了实现跳转列表，我遵循给出的示例here。

我正在对可用代码进行一些必要的更改，这就是我实现所需功能的方式，因为我对C#，MVVM和WP 8.1的了解很少。

但是我在ViewModel中遇到错误

var items = ContactModel.CreateSampleData();

错误是:

Cannot assign void to an implicitly-typed local variable

可能是什么原因？？？谁能帮助我了解我哪里出了问题。

using System.Collections;
using Windows.UI.Xaml.Data;
using JumpListSample.Common.JumpList;
using System.Collections.Generic;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Media.Imaging;
using System;
using Windows.Storage;
using Windows.Storage.Search;
using System.Collections.ObjectModel;

namespace JumpListSample.ViewModels
{
    public class ContactsViewModel
    {
        private IList data;
        public IList Data
        {
            get
            {
                if (data == null)
                {
                    var items = ContactModel.CreateSampleData();
                    data = items.ToAlphaGroups(x => x.Name);
                }
                return data;
            }
        }
        private CollectionViewSource collection;

        public CollectionViewSource Collection
        {
            get
            {
                if (collection == null)
                {
                    collection = new CollectionViewSource();
                    collection.Source = Data;
                    collection.IsSourceGrouped = true;
                }
                return collection;
            }
        }
    }

    public class ContactModel
    {
        // constructor
        public ContactModel()
        {
            Name = "name";
            Albumart = new BitmapImage(new Uri("ms-appx:///Assets/Logo.scale-240.png"));
        }

        public async  static void CreateSampleData()
        {
            ObservableCollection<ContactModel> data = new ObservableCollection<ContactModel>();

            try
            {
                IReadOnlyList<IStorageItem> MusicLibrary = await KnownFolders.MusicLibrary.GetFoldersAsync(CommonFolderQuery.GroupByAlbum);
                foreach (IStorageItem item in MusicLibrary)
                {
                    ContactModel obj = new ContactModel();
                    IStorageItem musicItem = item;
                    obj.Name = musicItem.Name;
                    obj.Albumart = new BitmapImage(new Uri("ms-appx:///Assets/Logo.scale-240.png"));
                    data.Add(obj);
                }
            }
            catch
            {
            }
            finally
            {
            } 
        }

        public string Name { get; set; }
        public ImageSource Albumart { get; set; }
    }
}

演示代码可以从 here下载。

Answer 1

我同意评论员克林特·古德的建议。您的CreateSampleData()方法似乎负责初始化ContactModel实例的集合。大概是从Data属性调用此函数，因为这些是您要用来初始化data字段的项。

就是说，您遇到了一个严重的问题:C#属性不支持async/await模式。解决此问题的一个惯用法是异步初始化后备字段，然后在初始化完成后引发属性更改的事件。不幸的是，您的ContactsViewModel类没有实现INotifyPropertyChanged或继承DependencyObject(基于XAML的程序处理属性更改通知的两种方式)。

综合所有这些并对代码进行必要的更改，您将得到如下结果:

ContactsViewModel:

public class ContactsViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    private void OnPropertyChanged(string name)
    {
        PropertyChangedEventHandler handler = PropertyChanged;

        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(name));
        }
    }

    private IList data;
    public IList Data
    {
        get
        {
            if (data == null)
            {
                // Ignore the returned Task...we're not going to do anything with it
                var _ = InitData();
            }

            // Caller should handle null. If not, need to return "new Data[0]" here instead
            return data;
        }
    }

    private async Task InitData()
    {
        // Catch _specific_ exceptions here, if necessary

        var items = await ContactModel.CreateSampleData();
        data = items.ToAlphaGroups(x => x.Name);
        OnPropertyChanged("Data");
    }

    private CollectionViewSource collection;

    public CollectionViewSource Collection
    {
        get
        {
            if (collection == null)
            {
                collection = new CollectionViewSource();
                collection.Source = Data;
                collection.IsSourceGrouped = true;
            }
            return collection;
        }
    }
}

联系人模型:

public class ContactModel
{
    // constructor
    public ContactModel()
    {
        Name = "name";
        Albumart = new BitmapImage(new Uri("ms-appx:///Assets/Logo.scale-240.png"));
    }

    public async static Task<ObservableCollection<ContactModel>> CreateSampleData()
    {
        ObservableCollection<ContactModel> data = new ObservableCollection<ContactModel>();

        IReadOnlyList<IStorageItem> MusicLibrary = await KnownFolders.MusicLibrary.GetFoldersAsync(CommonFolderQuery.GroupByAlbum);
        foreach (IStorageItem item in MusicLibrary)
        {
            ContactModel obj = new ContactModel();
            IStorageItem musicItem = item;
            obj.Name = musicItem.Name;
            obj.Albumart = new BitmapImage(new Uri("ms-appx:///Assets/Logo.scale-240.png"));
            data.Add(obj);
        }

        return data;
    }

    public string Name { get; set; }
    public ImageSource Albumart { get; set; }
}

请注意，我还完全删除了您的try/catch。简单地吞下异常非常不好。如果您认为需要处理任何异常，请在调用方(即 ContactsViewModel.InitData())中处理它们，在该处您可以做一些有用的事情，并在必要时通知用户(但仅处理特定的异常，您知道如何响应)。