如何解决DelegateCommand RefreshCommand 刷新 ObservableCollection 绑定的 DataGrid

当我向数据库添加新行时，我正在尝试更新我的 DataGrid（名称：dgLicenseholder，绑定到 ObservableCollection。我正在使用 MVVMLight）。为什么这不起作用？

在我的 ViewModel 中，我有一个 public DelegateCommand RefreshCommand {get; private set; } 和一个方法：

private void Refresh() {

    licenseHolders.Add(new LicenseHolders());
}

licenseHolders 是一个 ObservableCollection 列表，如下所示：

public ObservableCollection<LicenseHolders> licenseHolders { get; }
    = new ObservableCollection<LicenseHolders>();

LicenseHolders 是我的 Model 中的一个类，用于保存数据：

public class LicenseHolders {
    public int ID { get; set; }
    public string Foretaksnavn { get; set; }
    // more like the above...
}

在 XAML 中，我已将命令绑定到这样的按钮；

Command="{Binding RefreshCommand}"
CommandParameter="{Binding ElementName=dgLicenseHolder}"

我更新数据库的方法放在 ViewModel 中，并由包含 DataGrid 的窗口的代码隐藏中的点击事件调用。

AddToDB()

NewLicenseHolder nlh = Application.Current.Windows.OfType<NewLicenseHolder>().FirstOrDefault();

try {

   using (SqlConnection sqlCon = new(connectionString))
   using (SqlCommand sqlCmd = new(sqlString,sqlCon)) {

        sqlCon.Open();
        
        sqlCmd.Parameters.AddWithValue("@Foretaksnavn,nlh.txtForetaksnavn.Text.ToString());
        // more of the above...
        sqlCmd.ExecuteNonQuery();
    }
}

问题 1：

我绑定 RefreshCommand 的方式有什么问题？我添加到数据库的方法工作正常，但我必须重新打开 DataGrid 的窗口才能“捕获”更改。

问题 2：

如何绑定用于更新数据库的方法而不将其放入点击事件中？

更新代码（尝试实施用户 ChrisBD 建议的解决方案）

LicenseHolder.cs - 持有者类（模型）

namespace Ridel.Hub.Model {

    public class LicenseHolder {

        public int ID { get; set; }
        public string Foretaksnavn { get; set; }
        public string Foretaksnummer { get; set; }
        public string Adresse { get; set; }
        public int Postnummer { get; set; }
        public string Poststed { get; set; }
        public string BIC { get; set; }
        public string IBAN { get; set; }
        public string Kontakt { get; set; }
        public string Epost { get; set; }
        public string Tlf { get; set; }
        public string Kontonummer { get; set; }
    }
}

RidelHubMainViewModel.cs (ViewModel)

using System;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Windows;
using System.Windows.Input;
using GalaSoft.MvvmLight.CommandWpf;
using Ridel.Hub.Model;

namespace Ridel.Hub.ViewModel {

    public class RidelHubMainViewModel : INotifyPropertyChanged {

        public event PropertyChangedEventHandler PropertyChanged;

        public ICommand RefreshCommand { get; private set; }

        public ObservableCollection<LicenseHolder> LicenseHolders { get; set; }

        public RidelHubMainViewModel() {

            RefreshCommand = new RelayCommand(this.ExecuteRefreshCommand);
            LicenseHolders = new ObservableCollection<LicenseHolder>();
            FillDataGridLicenseHolders();
        }

        private void ExecuteRefreshCommand() {

            NewLicenseHolder nlh = Application.Current.Windows.OfType<NewLicenseHolder>().FirstOrDefault();

            if (LicenseHolders == null) {

                LicenseHolders = new ObservableCollection<LicenseHolder>();

            } else {

                AddToDB();
                LicenseHolders.Clear();
                LicenseHolders.Add(new LicenseHolder() { Foretaksnavn = nlh.txtForetaksnavn.Text.ToString() });
                LicenseHolders.Add(new LicenseHolder() { Foretaksnummer = nlh.txtForetaksnr.Text.ToString() });
                LicenseHolders.Add(new LicenseHolder() { Adresse = nlh.txtAdresse.Text.ToString() });
                LicenseHolders.Add(new LicenseHolder() { Postnummer = Convert.ToInt32(nlh.txtPostnummer.Text.ToString()) });
                LicenseHolders.Add(new LicenseHolder() { Poststed = nlh.txtPoststed.Text.ToString() });
                LicenseHolders.Add(new LicenseHolder() { BIC = nlh.txtBIC.Text.ToString() });
                LicenseHolders.Add(new LicenseHolder() { IBAN = nlh.txtIBAN.Text.ToString() });
                LicenseHolders.Add(new LicenseHolder() { Kontakt = nlh.txtKontakt.Text.ToString() });
                LicenseHolders.Add(new LicenseHolder() { Epost = nlh.txtEpost.Text.ToString() });
                LicenseHolders.Add(new LicenseHolder() { Tlf = nlh.txtTlf.Text.ToString() });
                LicenseHolders.Add(new LicenseHolder() { Kontonummer = nlh.txtKontonr.Text.ToString() });
            }
        }

        public void FillDataGridLicenseHolders() {

            // Initially populates my DataGrid with the Sql server table
            try {

                using (SqlConnection sqlCon = new(ConnectionString.connectionString))
                using (SqlCommand sqlCmd = new("select * from tblLicenseHolder",sqlCon))
                using (SqlDataAdapter sqlDaAd = new(sqlCmd))
                using (DataSet ds = new()) {

                    sqlCon.Open();
                    sqlDaAd.Fill(ds,"tblLicenseHolder");

                    foreach (DataRow dr in ds.Tables[0].Rows) {

                        LicenseHolders.Add(new LicenseHolder {

                            ID = Convert.ToInt32(dr[0].ToString()),Foretaksnavn = dr[1].ToString(),Foretaksnummer = dr[2].ToString(),Adresse = dr[3].ToString(),Postnummer = (int)dr[4],Poststed = dr[5].ToString(),BIC = dr[6].ToString(),IBAN = dr[7].ToString(),Kontakt = dr[8].ToString(),Epost = dr[9].ToString(),Tlf = dr[10].ToString(),Kontonummer = dr[11].ToString()
                        });
                    }
                }

            } catch (Exception ex) {

                MessageBox.Show(ex.Message,"Message",MessageBoxButton.OK,MessageBoxImage.Information);
            }
        }

        private void OnPropertyChanged(string myLicenseHolder) {

            PropertyChanged?.Invoke(this,new PropertyChangedEventArgs(myLicenseHolder));       
        }

        private void OnLicenseHoldersPropertyChanged() {

            PropertyChanged?.Invoke(this,new PropertyChangedEventArgs("LicenseHolders"));
        }

        #region Slett løyvehaver
// How I delete rows from the DataGrid
        private static bool RemoveFromDB(LicenseHolder myLicenseHolder) {

            string sqlString = $"Delete from tblLicenseHolder where ID = '{myLicenseHolder.ID}'";
            
            if (MessageBox.Show("Er du sikker på at du ønsker å slette valgt løyvehaver?","Slett løyvehaver",MessageBoxButton.YesNo,MessageBoxImage.Warning) == MessageBoxResult.Yes) {
                
                try {

                    using (SqlConnection sqlCon = new(ConnectionString.connectionString))
                    using (SqlCommand sqlCmd = new(sqlString,sqlCon)) {

                        sqlCon.Open();
                        sqlCmd.ExecuteNonQuery();
                        return true;
                    }

                } catch {

                    return false;
                }

            } else {

                return false;
            }
        }

        private void RemoveLicenseHolderExecute(LicenseHolder myLicenseHolder) {
            
            bool result = RemoveFromDB(myLicenseHolder);
            if (result)
                LicenseHolders.Remove(myLicenseHolder);
        }

        private RelayCommand<LicenseHolder> _removeLicenseHoldersCommand;

        public RelayCommand<LicenseHolder> RemoveLicenseHoldersCommand => _removeLicenseHoldersCommand
            ??= new RelayCommand<LicenseHolder>(RemoveLicenseHolderExecute,RemoveLicenseHolderCanExecute);

        private bool RemoveLicenseHolderCanExecute(LicenseHolder myLicenseHolder) {

            return LicenseHolders.Contains(myLicenseHolder);
        }

        #endregion

        public void AddToDB() {

            string sqlString = "insert into tblLicenseHolder (Foretaksnavn,Foretaksnummer,Adresse,Postnummer,Poststed,BIC,IBAN,Kontakt,Epost,Tlf,Kontonummer) " +
                "values (@Foretaksnavn,@Foretaksnummer,@Adresse,@Postnummer,@Poststed,@BIC,@IBAN,@Kontakt,@Epost,@Tlf,@Kontonummer)";

            NewLicenseHolder nlh = Application.Current.Windows.OfType<NewLicenseHolder>().FirstOrDefault();

            try {

                using (SqlConnection sqlCon = new(ConnectionString.connectionString))
                using (SqlCommand sqlCmd = new(sqlString,sqlCon)) {

                    sqlCon.Open();

                    if (string.IsNullOrEmpty(nlh.txtForetaksnavn.Text) || string.IsNullOrEmpty(nlh.txtForetaksnr.Text)
                        || string.IsNullOrEmpty(nlh.txtAdresse.Text) || string.IsNullOrEmpty(nlh.txtPostnummer.Text)
                        || string.IsNullOrEmpty(nlh.txtPoststed.Text) || string.IsNullOrEmpty(nlh.txtBIC.Text)
                        || string.IsNullOrEmpty(nlh.txtIBAN.Text) || string.IsNullOrEmpty(nlh.txtKontakt.Text)
                        || string.IsNullOrEmpty(nlh.txtEpost.Text) || string.IsNullOrEmpty(nlh.txtTlf.Text)
                        || string.IsNullOrEmpty(nlh.txtKontonr.Text)) {

                        MessageBox.Show("Vennligst fyll ut alle tekstboksene.");
                        sqlCon.Close();

                    } else {                       

                        sqlCmd.Parameters.AddWithValue("@Foretaksnavn",nlh.txtForetaksnavn.Text.ToString());
                        sqlCmd.Parameters.AddWithValue("@Foretaksnummer",nlh.txtForetaksnr.Text.ToString());
                        sqlCmd.Parameters.AddWithValue("@Adresse",nlh.txtAdresse.Text.ToString());
                        sqlCmd.Parameters.AddWithValue("@Postnummer",nlh.txtPostnummer.Text);
                        sqlCmd.Parameters.AddWithValue("@Poststed",nlh.txtPoststed.Text.ToString());
                        sqlCmd.Parameters.AddWithValue("@BIC",nlh.txtBIC.Text.ToString());
                        sqlCmd.Parameters.AddWithValue("@IBAN",nlh.txtIBAN.Text.ToString());
                        sqlCmd.Parameters.AddWithValue("@Kontakt",nlh.txtKontakt.Text.ToString());
                        sqlCmd.Parameters.AddWithValue("@Epost",nlh.txtEpost.Text.ToString());
                        sqlCmd.Parameters.AddWithValue("@Tlf",nlh.txtTlf.Text.ToString());
                        sqlCmd.Parameters.AddWithValue("@Kontonummer",nlh.txtKontonr.Text.ToString());
                        sqlCmd.ExecuteNonQuery();
                                                                  
                        MessageBox.Show("Ny løyvehaver lagret. Husk å oppdatere listen.");
                    }
                }

            } catch (Exception ex) {

                MessageBox.Show(ex.Message,MessageBoxImage.Information);
            }
        }
    }
}

查看，代码隐藏 - 父窗口（LicenseHoldersWindow）

using System.Windows;
using System.Windows.Controls;
using Ridel.Hub.ViewModel;

namespace Ridel.Hub {

    public partial class LicenseHoldersWindow : Window {

        public LicenseHoldersWindow() {
            
             InitializeComponent();
             DataContext = new RidelHubMainViewModel();
        }

        private btnNew_Click(object sender,RoutedEventArgs e) {

             NewLicenseHolder newLicenseHolder = new();
             newLicenseHolder.ShowDialog();
        }
    }
}

视图，XAML - 父窗口（LicenseHoldersWindow）


<Window
    x:Class="Ridel.Hub.LicenseHoldersWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:local="clr-namespace:Ridel.Hub"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:viewmodel="clr-namespace:Ridel.Hub.ViewModel"
    Title="License Holders"
    d:DataContext="{d:DesignInstance Type=viewmodel:RidelHubMainViewModel}"
    mc:Ignorable="d">

<!-- GUI stuff -->
<!-- also button to open new child-window "NewLicenseHolder" -->

<DataGrid
        x:Name="dgLicenseHolder"       
        AutoGenerateColumns="False"       
        IsReadOnly="True"
        ItemsSource="{Binding LicenseHolders,Mode=OneWay}"
        SelectionChanged="dgLicenseHolder_SelectionChanged"
        SelectionMode="Single">

查看，代码隐藏子窗口（NewLicenseHolder）

using System.ComponentModel;
using System.Windows;
using Prism.Commands;
using Ridel.Hub.ViewModel;

namespace Ridel.Hub {

    public partial class NewLicenseHolder : Window,INotifyPropertyChanged {

        public NewLicenseHolder() {
            
            InitializeComponent();
            btnLogOut.Content = UserInfo.UserName;
            DataContext = new RidelHubMainViewModel();
        }

        public event PropertyChangedEventHandler PropertyChanged;

        private void btnLagre_Click(object sender,RoutedEventArgs e) {

        // Button: Save new LicenseHolder
           ((RidelHubMainViewModel)DataContext).RefreshCommand.Execute(null);
           this.Close();
        }
    }
}

视图，XAML-子窗口（NewLicenseHolder）

<Window
    x:Class="Ridel.Hub.NewLicenseHolder"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" xmlns:viewmodel="clr-namespace:Ridel.Hub.ViewModel"
    xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
    d:DataContext="{d:DesignInstance Type=viewmodel:RidelHubMainViewModel}"
    mc:Ignorable="d">

<Button
      Name="btnLagre"
      Foreground="White"
      HorizontalContentAlignment="Center"
      Click="btnLagre_Click"
      Command="{Binding RefreshCommand}"
      Style="{DynamicResource ButtonWithRoundCornersGreen}" >
</Button>

好的，所以，如果你通读了所有这些：上帝保佑。如您所见，我在子表单中添加了新的行信息，单击保存，理想情况下，我希望我的 DataGrid 自动更新（无需单击任何额外的“刷新”或“更新”按钮。

现在发生的情况是，当我单击保存时，出现错误： System.NullReferenceException: 'Object reference not set to an instance of an object.' Local1 was null. 在我的 AddToDB() 方法中，在 if-else statement 处，我检查没有任何文本框为空。所以我在 NewLicenseHolder class 中引用 ViewModel 的方式显然有问题：

NewLicenseHolder nlh = Application.Current.Window.OfType<NewLicenseHolder>().FirstOrDefault();

如何正确引用它，并确保在离开子窗体并返回父窗体时更新 DataGrid？

非常感谢，非常感谢任何指导纠正我的想法！

解决方法

好吧，关于数据库代码如何链接到您的代码，我在这里没有看到很多内容，但这可能会有所帮助：

public class ViewModel
{
    public ViewModel() 
    {
        RefreshCommand = new Command(async()=> await ExecuteRefreshCommand());
        LicenceHolders = new ObservableCollection<LicenceHolder>();
    }

    public ICommand RefreshCommand { get; private set; }
    public ObservableCollection<LicenceHolder> LicenceHolders { get; set; }

    private async Task ExecuteRefreshCommand()
    {
        if (LicenceHolders == null)
        {
            LicenceHolders = new ObservableCollection<LicenceHolder>();
        }
        LicenceHolders.Clear();

        //Code here for async retrieval of licence holder records from database 
        //and addition of new LicenceHolder instances to the LicenceHolders collection
    }
}

在视图 XAML 中

<Button Text="Refresh" Command="{Binding RefreshCommand}"/> 这将触发 RefreshCommand

使用 ViewModel 注入查看背后的代码

public partial class SomeView : ContentPage
{
    public SomeView(ViewModel viewModel) 
    {
        InitializeComponent();
        BindingContext = viewModel;
    }
}

否则

public partial class SomeView : ContentPage
    {
        public SomeView() 
        {
            InitializeComponent();
            BindingContext = new ViewModel();
        }
    }

理想情况下，您需要使用命令来触发 ViewModel 中的事件或处理。这可以通过在上面的视图中绑定或像这样从后面的代码中触发

private void Button_OnClick(object sender,OnClickEventArgs e)
{
   ((ViewModel)BindingContext).RefreshCommand.Execute();
}

严格来说，我们应该使用一个标志来指示正在进行的异步数据库访问 - 这可以通过在 ViewModel 中使用公共布尔值（标志）来实现，该标志用作 Command 实例中的 CanExecute 条件创建，这将阻止对同一代码的多次调用，直到它完成为止。例如

视图模型： bool IsBusy {get;set;} 在调用异步数据访问例程之前设置为真，完成后设置为假

在构造函数中，命令实例化现在变成 RefreshCommand = new Command(async()=> await ExecuteRefreshCommand(),()=>!IsBusy);

XAML 绑定会自动选择这一点，但如果您使用代码背后的代码来触发命令，这将变为：

if (((ViewModel)BindingContext).RefreshCommand.CanExecute())
{
   ((ViewModel)BindingContext).RefreshCommand.Execute();
}

注意： 如果您在运行时用新实例替换 LicenceHolders ObservableCollection，而不是更改其内容，那么您将需要通过调用 OnPropertyChanged 处理程序手动引发/调用 PropertyChanged。例如

protected void OnLicenHoldersPropertyChanged()
{
   PropertyChanged?.Invoke(this,new PropertyChangedEventArgs("LicenceHolders"));
}

看来我可能混淆了 MVVM 框架 - 无论如何，这里有一些 MVVMLite 的工作代码。

   public class LicenseHolder
{
    public string Name { get; set; }
}

public class ViewModel
{
    public ViewModel()
    {
        RefreshCommand = new GalaSoft.MvvmLight.CommandWpf.RelayCommand(this.ExecuteRefreshCommand);
        LicenseHolders = new ObservableCollection<LicenseHolder>();
    }
    public ICommand RefreshCommand { get; private set; }
    public ObservableCollection<LicenseHolder> LicenseHolders { get; set; }
    private void ExecuteRefreshCommand(object o)
    {
        if (LicenseHolders == null)
        {
            LicenseHolders = new ObservableCollection<LicenseHolder>();
        }
        LicenseHolders.Clear();
        //Code here for async retrieval of licence holder records from database 
        //and addition of new LicenceHolder instances to the LicenceHolders collection
        LicenseHolders.Add(new LicenseHolder(){Name ="Ted"});
        LicenseHolders.Add(new LicenseHolder(){Name = "Bill"});

    }
}

查看：

<Window x:Class="WpfApp1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:WpfApp1"
    mc:Ignorable="d"
    Title="MainWindow" Height="450" Width="800">
<Window.Resources>
    <local:ViewModel x:Key="ViewModelAsDataSource" />
</Window.Resources>

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition />
        <RowDefinition />
        <RowDefinition />
        <RowDefinition />
    </Grid.RowDefinitions>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="40"/>
        <ColumnDefinition/>
        <ColumnDefinition Width="40"/>
    </Grid.ColumnDefinitions>
    <DataGrid Grid.Column="1" Grid.Row="1" ItemsSource="{Binding LicenseHolders }" AutoGenerateColumns="false">
        <DataGrid.Columns>
            <DataGridTextColumn Header="License Holder Name" Width="Auto" MinWidth="18" Binding="{Binding Name }" />
        </DataGrid.Columns>

    </DataGrid>

    <Button Grid.Column="1" Grid.Row="2" Content="Refresh" Command="{Binding RefreshCommand}" Width="50" Height="30"/>
     <Button Grid.Column="1" Grid.Row="3" Content="Refresh" Click="ButtonBase_OnClick" Width="50" Height="30"/>

背后的代码：

    public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        DataContext = new ViewModel();
    }
    private void ButtonBase_OnClick(object sender,RoutedEventArgs e)
    {
        ((ViewModel)DataContext).RefreshCommand.Execute(null);
    }

}

DelegateCommand RefreshCommand 刷新 ObservableCollection 绑定的 DataGrid

如何解决DelegateCommand RefreshCommand 刷新 ObservableCollection 绑定的 DataGrid

问题 1：

问题 2：

更新代码（尝试实施用户 ChrisBD 建议的解决方案）

解决方法

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