` System.InvalidCastException: '指定的强制转换无效。'答案

【问题标题】：` System.InvalidCastException: 'Specified cast is not valid.'` System.InvalidCastException: '指定的强制转换无效。'
【发布时间】：2021-08-26 21:55:54
【问题描述】：

NavigatePage.xaml：

    <Grid Grid.Row="2">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
    </Grid>
    <Button x:Name="AddToCartButton" BackgroundColor="#FB9B63" CornerRadius="10" HeightRequest="60" FontSize="20" HorizontalOptions="Center"
            Text="Entrer" TextColor="White" VerticalOptions="Center" Clicked="AddToCartButton_Clicked" />
</Grid>

NavigatePage.xaml.cs：

 private void AddToCartButton_Clicked(object sender, EventArgs e)
        {
            var btn = (Button)sender;
            var item = (ProductViewModel)btn.BindingContext;

            ContentPage page=null;

            switch (item.Name)
            {
                case "QR_Manager":
                   page = new Views.Report();
                    break;

            }
            Navigation.PushAsync(page);

MainViewModel.cs：

namespace UpManager.Dashboard.ViewModels
{
    public class MainViewModel : BaseViewModel
    {
        public IList<ProductViewModel> Products { get; set; }

        private ProductViewModel _selectedProduct;

        public ProductViewModel SelectedProduct
        {
            get { return _selectedProduct; }
            set { SetProperty(ref _selectedProduct, value); }
        }

        public ShoppingCartViewModel ShoppingCart { get; set; }

        public ICommand RemoveItemCommand { private set; get; }


        public MainViewModel()
        {
            Products = new ObservableRangeCollection<ProductViewModel>()
            {
                new ProductViewModel()
                {
                    HeroColor = "#95C9F7",
                    Name="QR_Manager",
                    ImageUrl = "QRM",
                    IsFeatured = true,
                    Description = "Contained in a glass polygonal florarium",
                },
                new ProductViewModel()
                {
                    HeroColor = "#FFCA81",
                    Name="Yellow Sun",
                    ImageUrl = "yellow_moss",
                    IsFeatured = true,
                    Description = "Contained in a yellow glass polygonal florarium",
                    

                },

                new ProductViewModel()
                {
                    HeroColor = "#A2BAD3",
                    Name="Grey Blue",
                    ImageUrl = "grey_moss",
                    IsFeatured = true,
                    Description = "Contained in a glass polygonal florarium",
                },

                new ProductViewModel()
                {
                    HeroColor = "#F796DD",
                    Name="Pink",
                    ImageUrl = "pink_moss",
                    IsFeatured = false,
                    Description = "Contained in a glass polygonal florarium",
                },

                 new ProductViewModel()
                {
                    HeroColor = "#95C9F7",
                    Name="Sky Blue",
                    ImageUrl = "blue_moss",
                    IsFeatured = false,
                    Description = "Contained in a glass polygonal florarium",
                },

                new ProductViewModel()
                {
                    HeroColor = "#D69EFC",
                    Name="Lavender",
                    ImageUrl = "lavender_moss",
                    IsFeatured = false,
                    Description = "Contained in a glass polygonal florarium",

                },
                new ProductViewModel()
                {
                    HeroColor = "#74D69E",
                    Name="Green Life",
                    ImageUrl = "green_moss",
                    IsFeatured = true,
                    Description = "Contained in a glass polygonal florarium",

                },
                new ProductViewModel()
                {
                    HeroColor = "#FB8183",
                    Name="Red",
                    ImageUrl = "red_moss",
                    IsFeatured = false,
                    Description = "Contained in a glass polygonal florarium",

                },
                new ProductViewModel()
                {
                    HeroColor = "#FB9B64",
                    Name="Orange",
                    ImageUrl = "orange_moss",
                    IsFeatured = false,
                    Description = "Contained in a glass polygonal florarium",

                },
                new ProductViewModel()
                {
                    HeroColor = "#D69EFC",
                    Name="Lavender",
                    ImageUrl = "lavender_moss",
                    IsFeatured = false,
                    Description = "Contained in a glass polygonal florarium",

                },

            };

我有这个异常 `System.InvalidCastException: 'Specified cast is not valid.'换行：

var item = (ProductViewModel)btn.BindingContext;

我希望 MainViewModel() 中的每个新 ProductViewModel() 都有一个按钮让我转到页面，例如名称为 QR_Manager 的新 ProductViewModel() 我想转到 ManagerPage 和名称为 Yellow Sun 的新 ProductViewModel()想去页面YellowSunPage。

【问题讨论】：

@Jason 这是一般情况
这是在 ListView 内部吗？这是我几个小时前问你的问题，你没有回答。
@Json 是在列表中
ItemsSource 是 ObservableRangeCollection<ProductViewModel> 吗？调试器说BindingContext的类型是什么？
异常：值：{System.InvalidCastException：指定的转换无效。在 C:\Users\DELL\source\repos\project\UpManager\UpManager\UpManager\Dashboard\Controls\NavigatePage 中的 UpManager.Dashboard.Controls.NavigatePage.AddToCartButton_Clicked (System.Object sender, System.EventArgs e) [0x00008]。 xaml.cs:49 atXamarin.Forms.Button.Xamarin.Forms.Internals.IButtonElement.PropagateUpClicked () [0x00000] 在 D:\a\1\s\Xamarin.Forms.Core\Button.cs:187

标签： c# xaml xamarin xamarin.forms xamarin.android

【解决方案1】：

这个错误是因为你在Button上绑定的BindingContext的类型不对应，也就是说绑定路径不正确。

解决问题

将按钮放在ListView 中。

<ListView >
     <ListView.ItemTemplate>
         <DataTemplate>
             <ViewCell>
                 <Button x:Name="AddToCartButton" 
                         BackgroundColor="#FB9B63" CornerRadius="10" HeightRequest="60" FontSize="20" HorizontalOptions="Center"
                         Text="Entrer" TextColor="White" VerticalOptions="Center" Clicked="AddToCartButton_Clicked" />
             </ViewCell>
         </DataTemplate>
     </ListView.ItemTemplate>
</ListView>

在视图模型中将ItemsSource 与Products 绑定。
```
<ListView ItemsSource="{Binding Products}" >
```

在page.xaml.cs中将视图模型设置为BindingContext。

public NavigatePage()
{

   InitializeComponent();

   MainViewModel model = new MainViewModel();
   this.BindingContext = model;
}

【讨论】：