在 xamarin 项目 System.InvalidCastException 中：“指定的强制转换无效”

问题描述

这是我在 xamarin 内容页面中的代码 此元素的名称是 LandingPage

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
            xmlns:d="http://xamarin.com/schemas/2014/forms/design"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
           mc:Ignorable="d"
         xmlns:vm="clr-namespace:Music.viewmodels"
         NavigationPage.HasNavigationBar="False"
         FlowDirection="RightToLeft"
         x:Class="Music.View.LandingPage">

<ContentPage.BindingContext>
    <vm:Landingviewmodel/>
</ContentPage.BindingContext>
<Frame HasShadow="True" HorizontalOptions="FillAndExpand"  VerticalOptions="FillAndExpand" BackgroundColor="{StaticResource mygradiant}">
    <Grid RowSpacing="30" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand">
        <Grid.RowDeFinitions>
            <RowDeFinition Height="*"/>
            <RowDeFinition Height="2*"/>
        </Grid.RowDeFinitions>
        <StackLayout>
            <Frame CornerRadius="50" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand">
                <Grid RowSpacing="30" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand">
                    <Grid.RowDeFinitions>
                        <RowDeFinition Height="*"/>
                        <RowDeFinition Height="Auto"/>
                    </Grid.RowDeFinitions>
                    <Image Aspect="AspectFill"  Grid.RowSpan="2" Source="{Binding RecentMusic.CoverImage}" />
                    <Grid  Grid.Row="1" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand" HeightRequest="80">
                        <BoxView BackgroundColor="Black"  Opacity="0.7" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand"/>
                        <StackLayout Margin="40,0" VerticalOptions="Center" HorizontalOptions="FillAndExpand">
                            <Label Text="{Binding RecentMusic.Title}"  TextColor="White" FontSize="17"/>
                            <Label Text="{Binding RecentMusic.Artist}"  TextColor="White" FontSize="12" Opacity="0.8"/>
                        </StackLayout>


                    </Grid>
                </Grid>
            </Frame>
            <Frame HasShadow="True" HeightRequest="40" WidthRequest="40" 
                   CornerRadius="20" Margin="0,-25,40,0" HorizontalOptions="End" VerticalOptions="Start" 
                   BackgroundColor="{StaticResource spacegradiant}">
                <Image Source="play.png" HeightRequest="15" WidthRequest="15" VerticalOptions="Center"/>

            </Frame>
        </StackLayout>
    </Grid>
</Frame>

这是我的视图模型

using Music.View;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Input;
using Xamarin.Forms;

  namespace Music.viewmodels
{
public class Landingviewmodel : Baseviewmodel
{ 
    public Landingviewmodel()
    {
        musicList = GetMusicList();
        recentMusic = musicList.Where(x => x.IsRecent == true).FirstOrDefault();
    }
    ObservableCollection<Model.Music> musicList;
    public ObservableCollection<Model.Music> MusicList
    {
        get { return musicList; }
        set
        {
            musicList = value;
            OnPropertyChanged();
        }
    }

    private Model.Music recentMusic;
    public Model.Music RecentMusic
    {
        get { return recentMusic; }
        set
        {
            recentMusic = value;
            OnPropertyChanged();
        }
    }

    private Model.Music selectedMusic;
    public Model.Music SelectedMusic
    {
        get { return selectedMusic; }
        set
        {
            selectedMusic = value;
            OnPropertyChanged();
        }
    }

    public ICommand SelectionCommand => new Command(PlayMusic);
    private void PlayMusic()
    {
        if (selectedMusic != null)
        {
            var viewmodel = new Playerviewmodel(selectedMusic,musicList);
            var playerPage = new PlayerPage { BindingContext = viewmodel };
            var navigation = Application.Current.MainPage as NavigationPage;
            navigation.PushAsync(playerPage,true);
        }
    }
    private ObservableCollection<Model.Music> GetMusicList()
    {
        return new ObservableCollection<Model.Music>
        {
            new Model.Music { Title = "music",Artist = "artist ",Url = "http://ir.vavmusic.com/download/M/Mohsen%20Ebrahimzadeh/Single/320/Mohsen%20Ebrahimzadeh%20-%20Tabe%20Eshgh%20%28Dooneh%20Dooneh%202%29.mp3",CoverImage = "https://encrypted-tbn0.gstatic.com/images?q=tbn%3AANd9GcRU6FVly4jMTD3AKB_sHxqPofJVQwqqUj5peEvgA1H4XegM3uJ7&usqp=CAU",IsRecent = true},new Model.Music { Title = "music",Artist = "artist",Url = "http://ir.vavmusic.com/download/M/Mohsen%20Ebrahimzadeh/Single/320/Mohsen%20Ebrahimzadeh%20-%20Tabe%20Eshgh%20%28Dooneh%20Dooneh%202%29.mp33",CoverImage = "https://encrypted-tbn0.gstatic.com/images?q=tbn%3AANd9GcRm-su97lHFGZrbR6BkgL32qbzZBj2f3gKGrFR0Pn66ih01SyGj&usqp=CAU"},};
    }
}

}

当我运行项目时，在 LandingPage.xaml.g.cd 中出现此错误， 如果我删除与 viewmodel 的任何关系，它的工作正常，

system.invalidCastException Message=指定的演员表无效。

作为评论资源字典=>

    <Application.Resources>
    <ResourceDictionary>
        <Color x:Key="GradiantStart">#858585</Color>
        <Color x:Key="GradiantEnd">#575757</Color>
        <Color x:Key="IconBackGroundColor">#d6d7d7</Color>
        <Color x:Key="IconBorderColor">#e62e2d</Color>
        <Color x:Key="IconLableColor">#fff</Color>

        <LinearGradientBrush x:Key="mygradiant" EndPoint="1,0">
            <GradientStop Color="#E7EDF8"
                      Offset="0.1" />
            <GradientStop Color="#E3E7EE"
                      Offset="1.0" />
        </LinearGradientBrush>
        <LinearGradientBrush x:Key="spacegradiant" EndPoint="1,0">
            <GradientStop Color="#D4420c"
                      Offset="0.1" />
            <GradientStop Color="#f3a283"
                      Offset="1.0" />
        </LinearGradientBrush>
        <LinearGradientBrush x:Key="PlayListGradiant" EndPoint="1,0">
            <GradientStop Color="#E3E7EE"
                      Offset="0.1" />
            <GradientStop Color="#FBFBFB"
                      Offset="1.0" />
        </LinearGradientBrush>
    </ResourceDictionary>
</Application.Resources>

解决方法

谢谢@Cfun和@ColeX - MSFT

我的错误是，在 Background 和 BackgroundColor 之间不同，我尝试使用渐变作为背景色，