在 WPF ComboBox 中设置默认值

Set default value in WPF ComboBox

提问by Nilesh Barai

I am using ComboBox ItemsSource property binding to display items from a List to combo box.

我正在使用 ComboBox ItemsSource 属性绑定来显示从列表到组合框的项目。

Following is the code:

以下是代码：

<ComboBox x:Name="Cmb_Tax" ItemsSource="{Binding TaxList}" 
            DisplayMemberPath="ChargeName" SelectedItem="{Binding 
            SelectedTax,UpdateSourceTrigger=PropertyChanged}" IsEditable="True" 
            IsTextSearchEnabled="True" SelectionChanged="Cmb_Tax_SelectionChanged"/>

Classes.Charges _selected_tax = new Classes.Charges();
public Classes.Charges SelectedTax
{
    get
    {
        return _selected_tax;
    }
    set
    {
        _selected_tax = value;
    }
}

List<Classes.Charges> _taxlist = new List<Classes.Charges>();
public List<Classes.Charges> TaxList
{
    get
    {
        return _taxlist;
    }
    set
    {
        _taxlist = value;
        OnPropertyChanged("TaxList");
    }
}

It displays the items in the combo box correctly.

它正确显示组合框中的项目。

There is a particular item in TaxList "No Tax"which I want to be selected by default in the combo box. This item can be present at any index in the list (Not necessary first or last item of the list).

TaxList 中有一个特定项目"No Tax"，我希望在组合框中默认选择它。该项目可以出现在列表中的任何索引处（不需要列表的第一项或最后一项）。

I am trying to use the following code to set the selected index property of combo box, but sadly its not working.

我正在尝试使用以下代码来设置组合框的选定索引属性，但遗憾的是它不起作用。

TaxList = Classes.Charges.GetChargeList("Tax");
Cmb_Tax.DataContext = this;            
int i = TaxList.FindIndex(x => x.ChargeName == tax_name);
Cmb_Tax.SelectedIndex = i;

The Method FindIndex() returns the index of the "No Tax"correctly but when I try assigning it to SelectedIndexof combo the SelectedIndexdoesn't change. It stays at -1.

方法 FindIndex() 返回"No Tax"正确的索引，但是当我尝试将它分配给SelectedIndex组合时SelectedIndex并没有改变。它保持在-1。

Update1

更新1

private void Cmb_Tax_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    MessageBox.Show(SelectedTax.ChargeName);
}

Update2Updated the code as per suggested by @ElectricRouge

Update2根据@ElectricRouge 的建议更新了代码

<ComboBox x:Name="Cmb_Tax" ItemsSource="{Binding TaxList, Mode=TwoWay}" 
                      DisplayMemberPath="ChargeName" SelectedItem="{Binding SelectedTax,UpdateSourceTrigger=PropertyChanged,Mode=TwoWay}"
                      IsEditable="True" IsTextSearchEnabled="True" 
                      SelectionChanged="Cmb_Tax_SelectionChanged"/>


Classes.Charges _selected_tax = new Classes.Charges();
        public Classes.Charges SelectedTax
        {
            get
            {
                return _selected_tax;
            }
            set
            {
                _selected_tax = value;
                OnPropertyChanged("SelectedTax");
            }
        }

        ObservableCollection<Classes.Charges> _taxlist = new ObservableCollection<Classes.Charges>();
        public ObservableCollection<Classes.Charges> TaxList
        {
            get
            {
                return _taxlist;
            }
            set
            {
                _taxlist = value;
                OnPropertyChanged("TaxList");
            }
        }

public void Load_Tax(string tax_name = null, Classes.Charges selected_tax = null)
        {
            TaxList = Classes.Charges.GetParticularChargeList("Tax");
            Cmb_Tax.DataContext = this;
            //Cmb_Tax.SelectedValue = tax_name;
            SelectedTax = selected_tax;
            //int i = TaxList.FindIndex(x => x.ChargeName == tax_name);
            //Cmb_Tax.SelectedIndex = i;
        }

Any idea why this must be happening? Also please suggest any other approach to display default in combo box.

知道为什么这一定会发生吗？还请建议在组合框中显示默认值的任何其他方法。

采纳答案by Lorentz Vedeler

Here's a working sample:

这是一个工作示例：

Viewmodel:

视图模型：

 public MainWindow()
    {
        InitializeComponent();

        var vm = new ViewModel();
        this.DataContext = vm;
        this.Loaded += (o,e) => vm.LoadData();
    }

    public class ViewModel : INotifyPropertyChanged
    {
        private IList<Charges> taxList;
        public ICollectionView TaxList { get; private set; }

        public void LoadData()
        {                
            taxList = Charges.GetChargeList("taxes");

            TaxList = CollectionViewSource.GetDefaultView(taxList);
            RaisePropertyChanged("TaxList");

            TaxList.CurrentChanged += TaxList_CurrentChanged;              

            var noTax = taxList.FirstOrDefault(c => c.ChargeName == "No Tax");
            TaxList.MoveCurrentTo(noTax);
        }

        void TaxList_CurrentChanged(object sender, EventArgs e)
        {
            var currentCharge = TaxList.CurrentItem as Charges;
            if(currentCharge != null)
                MessageBox.Show(currentCharge.ChargeName);
        }



        public event PropertyChangedEventHandler PropertyChanged;
        public void RaisePropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }
}

View:

看法：

<ComboBox x:Name="cboTaxList" 
              ItemsSource="{Binding TaxList}" 
              DisplayMemberPath="ChargeName"
              IsSynchronizedWithCurrentItem="True" />

回答by ElectricRouge

List does not implement INotifyCollectionChangedmake it ObservableCollection

列表没有实现INotifyCollectionChanged使它ObservableCollection

ObservableCollection<Classes.Charges> _taxlist = new ObservableCollection<Classes.Charges>();
public ObservableCollection<Classes.Charges> TaxList
{
    get
    {
        return _taxlist;
    }
    set
    {
        _taxlist = value;
        OnPropertyChanged("TaxList");
    }
}

And try setting the Mode=TwoWay

并尝试设置 Mode=TwoWay

SelectedItem="{Binding SelectedTax,UpdateSourceTrigger=PropertyChanged,Mode=TwoWay}"