I've run into this exact problem before when working with custom template selectors, so let's break down the most likely causes and fixes for your issue where SelectedIndex stays stuck at 0 and SelectedItem always points to the first row.

1. Check IsSynchronizedWithCurrentItem #

Your ListBox has IsSynchronizedWithCurrentItem="True" enabled. This binds the selected item to the CurrentItem of the underlying collection view. If the collection view's CurrentItem is stuck on the first element (e.g., your data source doesn't properly implement INotifyCollectionChanged, or you're accidentally resetting the view elsewhere), it will force the selection to always lock onto the first item.

Fix:
Remove or set IsSynchronizedWithCurrentItem="False" temporarily to test if selection returns to normal:

<ListBox x:Name="listBox" 
         ItemsSource="{Binding Objects}" 
         ItemTemplateSelector="{StaticResource studentDataTemplateSelector}" 
         IsSynchronizedWithCurrentItem="False" <!-- Adjust this setting -->
         ItemContainerStyle="{DynamicResource _ListBoxItemStyle}" 
         SelectionChanged="listBox_SelectionChanged" 
         MouseDoubleClick="listBox_MouseDoubleClick">
</ListBox>

2. Audit Your _ListBoxItemStyle #

Custom item styles often accidentally break selection behavior. If your style overrides the IsSelected property or uses triggers that interfere with selection state, it can cause the ListBox to misreport selected items.

What to look for:

• Avoid hardcoding IsSelected values in setters (this forces all items to stay unselected):

  <!-- BAD: This overrides normal selection logic -->
  <Setter Property="IsSelected" Value="False"/>
  

• Check for triggers that incorrectly set IsSelected (e.g., forcing selection on mouse over instead of just highlighting):

  <!-- BAD: This bypasses standard click-based selection -->
  <Style.Triggers>
      <Trigger Property="IsMouseOver" Value="True">
          <Setter Property="IsSelected" Value="True"/>
      </Trigger>
  </Style.Triggers>
  

3. Fix Object Equality in Your File Class #

ListBox uses object equality to track selected items. If your File class doesn't override Equals and GetHashCode, it relies on reference equality by default. If your Objects collection has duplicate data objects with different references, the ListBox can't correctly map the clicked item to the selected item, leading it to fall back to the first element.

Fix:
Override Equals and GetHashCode in your File class using unique identifiers (like filename + extension + creation time):

public class File
{
    public string NameFile { get; set; }
    public string Size { get; set; }
    public DateTime CreateTime { get; set; }
    public string Extension { get; set; }

    public override bool Equals(object obj)
    {
        if (obj is not File otherFile) return false;
        
        // Compare unique properties to determine equality
        return NameFile == otherFile.NameFile 
               && Extension == otherFile.Extension 
               && CreateTime == otherFile.CreateTime;
    }

    public override int GetHashCode()
    {
        // Combine properties to generate a unique hash code
        return HashCode.Combine(NameFile, Extension, CreateTime);
    }
}

4. Check Your Custom FileUi Control #

If your FileUi control is intercepting mouse events (e.g., marking them as handled), the ListBoxItem won't receive the click input needed to update selection state.

Fixes:

• Add IsHitTestVisible="False" to your FileUi control temporarily to test if selection works (this lets clicks pass through to the ListBoxItem):

  <ccontrols:FileUi ContainStr="1Item" 
                    TitleStr="{Binding NameFile}" 
                    SizeStr="{Binding Size}" 
                    CreatedTime="{Binding CreateTime}" 
                    ImageUri="Assets/Music.png"
                    IsHitTestVisible="False"/>
  

• Inspect the FileUi code-behind for event handlers that set e.Handled = true (especially PreviewMouseDown, MouseDown, or MouseDoubleClick). Remove or adjust these so events bubble up to the ListBoxItem.

5. Validate Your TemplateSelector Logic #

Double-check that your StudentDataTemplateSelector is returning the correct template for each item without modifying the item itself. If it's accidentally returning a null template or altering the item reference, this can cause selection issues.

Quick Debug Tip:
Add a breakpoint in SelectTemplate to confirm each item is the correct File instance, and that the returned template matches what you expect.

内容的提问来源于stack exchange，提问作者Diego Kabir