为什么对象引用未设置为 LINQ 中的对象实例 Where for Two Column in ComboBox

Question

我正在使用 C# WPF 连接到 SQL Server

中的数据库

我有一个组合框，其中有两列要显示一个 SelectedValuePath

问题是我无法将 LINQ 用于其中一列 (属性)

我的 XAML :

 <Grid Background="#FFCFCFCF">
    <ComboBox x:Name="HESNAGHD" IsEditable="True" IsTextSearchEnabled="True" FlowDirection="RightToLeft" Margin="148,73,149,314" RenderTransformOrigin="0.5,0.5" StaysOpenOnEdit="True" >
        <ComboBox.ItemsPanel  >
            <ItemsPanelTemplate>
                <VirtualizingStackPanel VirtualizingPanel.IsVirtualizing="True" VirtualizingPanel.VirtualizationMode="Recycling"/>
            </ItemsPanelTemplate>
        </ComboBox.ItemsPanel>
        <ComboBox.ItemTemplate>
            <DataTemplate>
                <StackPanel Orientation="Horizontal">
                    <TextBlock Text="{Binding nam}" TextAlignment="Justify" Width="500"  ></TextBlock>
                    <TextBlock Text="{Binding Expr1}" TextAlignment="Justify" Width="100"  ></TextBlock>
                </StackPanel>
            </DataTemplate>
        </ComboBox.ItemTemplate>
    </ComboBox>
    <Label Content="Mine ComboBox : " HorizontalAlignment="Left" Height="28" Margin="34,77,0,0" VerticalAlignment="Top" Width="109"/>
    <Label Content="Search : " HorizontalAlignment="Left" Height="28" Margin="30,10,0,0" VerticalAlignment="Top" Width="72"/>
    <TextBox x:Name="OneSearch" HorizontalAlignment="Left" Height="32" Margin="148,10,0,0" TextWrapping="Wrap" Text="" VerticalAlignment="Top" Width="359" TextChanged="OneSearch_TextChanged"/>
</Grid>

我的客服：

 public partial class CUST_HESAB
{
    public string nam { get; set; }
    public string Expr1 { get; set; }
    public override string ToString() => nam;
}
public partial class MainWindow : Window
{
    bool IsReadyAll = false;
    List<CUST_HESAB> PBLIST = new List<CUST_HESAB>();

    CorrectWPFEntities dbms = new CorrectWPFEntities();
    public MainWindow()
    {
        InitializeComponent();
        var ITT = dbms.Database.SqlQuery<CUST_HESAB>("SELECT hes, NAME AS nam, hes AS Expr1 FROM CUST_HESAB").ToList();
        foreach (var item in ITT)
        {
            PBLIST.Add(item);
        }
        HESNAGHD.ItemsSource = PBLIST.ToList();
        HESNAGHD.SelectedValuePath = "Expr1";
        HESNAGHD.SelectedIndex = 0;
        HESNAGHD.IsDropDownOpen = true;
        HESNAGHD.IsDropDownOpen = false;

    }

    private void OneSearch_TextChanged(object sender, TextChangedEventArgs e)
    {
        if (IsReadyAll == true)
        {

            if (!string.IsNullOrEmpty(OneSearch.Text.Trim().ToLower()))
            {
                HESNAGHD.IsDropDownOpen = true;
                var myfilter0 = from u in PBLIST where u.nam.Contains(OneSearch.Text) select u.nam.ToList();
                var myfilter = PBLIST.Where(x => x.nam.Contains(OneSearch.Text)).ToList();

                if (!ReferenceEquals(myfilter, null))
                {
                    HESNAGHD.ItemsSource = myfilter;
                }
                else
                {
                    HESNAGHD.ItemsSource = dbms.Database.SqlQuery<CUST_HESAB>("SELECT hes, NAME AS nam, hes AS Expr1 FROM CUST_HESAB").ToList();
                }
            }
            else
            {
                HESNAGHD.ItemsSource = dbms.Database.SqlQuery<CUST_HESAB>("SELECT hes, NAME AS nam, hes AS Expr1 FROM CUST_HESAB").ToList();
            }
        }

    }

    private void Window_ContentRendered(object sender, EventArgs e)
    {
        IsReadyAll = true;
    }
}

我的问题：在 OneSearch_TextChanged Where 不起作用它说 null 但它不是 !

enter image description here

请帮忙

Answer 1

更改此行

var myfilter = PBLIST.Where(x => x.nam.Contains(OneSearch.Text)).ToList();

至

var myfilter = PBLIST.Where(x => x.nam != null && x.nam.Contains(OneSearch.Text)).ToList();

为什么对象引用未设置为 LINQ 中的对象实例 Where for Two Column in ComboBox

Why Object reference not set to an instance of an object in LINQ Where for Two Column in ComboBox

c#

linq

wpf

combobox

entity-framework