如何取消点击Combobox下拉按钮时的文本全选，有无MVVM绑定实现方案？

以下两种方案均完全符合MVVM规范，无需在业务代码中直接操作ComboBox实例，也不会影响原有Text属性的绑定逻辑：

方案1：使用附加属性实现（无外部依赖） #

通过自定义附加属性封装逻辑，所有ComboBox可直接复用该能力：

public static class ComboBoxAttachedProperties
{
    public static readonly DependencyProperty DisableAutoSelectOnDropDownOpenProperty = DependencyProperty.RegisterAttached(
        "DisableAutoSelectOnDropDownOpen", typeof(bool), typeof(ComboBoxAttachedProperties), new PropertyMetadata(false, OnDisableAutoSelectChanged));

    public static void SetDisableAutoSelectOnDropDownOpen(DependencyObject element, bool value)
    {
        element.SetValue(DisableAutoSelectOnDropDownOpenProperty, value);
    }

    public static bool GetDisableAutoSelectOnDropDownOpen(DependencyObject element)
    {
        return (bool)element.GetValue(DisableAutoSelectOnDropDownOpenProperty);
    }

    private static void OnDisableAutoSelectChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (d is not ComboBox comboBox) return;
        if ((bool)e.NewValue)
        {
            comboBox.DropDownOpened += ComboBox_DropDownOpened;
        }
        else
        {
            comboBox.DropDownOpened -= ComboBox_DropDownOpened;
        }
    }

    private static void ComboBox_DropDownOpened(object sender, EventArgs e)
    {
        if (sender is not ComboBox { IsEditable: true } comboBox) return;
        if (comboBox.Template.FindName("PART_EditableTextBox", comboBox) is TextBox textBox)
        {
            textBox.SelectionStart = comboBox.Text.Length;
            textBox.SelectionLength = 0;
        }
    }
}

XAML中直接声明使用即可，local替换为你定义附加属性的命名空间：

<ComboBox 
    IsEditable="True"
    Text="{Binding YourViewModelTextProperty}"
    local:ComboBoxAttachedProperties.DisableAutoSelectOnDropDownOpen="True"/>

方案2：使用Blend Behavior实现（代码更简洁） #

如果项目已引用Microsoft.Xaml.Behaviors.Wpf包，可以用行为实现，逻辑更内聚：

public class DisableComboBoxAutoSelectBehavior : Behavior<ComboBox>
{
    protected override void OnAttached()
    {
        base.OnAttached();
        AssociatedObject.DropDownOpened += OnDropDownOpened;
    }

    protected override void OnDetaching()
    {
        AssociatedObject.DropDownOpened -= OnDropDownOpened;
        base.OnDetaching();
    }

    private void OnDropDownOpened(object sender, EventArgs e)
    {
        if (!AssociatedObject.IsEditable) return;
        if (AssociatedObject.Template.FindName("PART_EditableTextBox", AssociatedObject) is TextBox textBox)
        {
            textBox.CaretIndex = AssociatedObject.Text.Length;
        }
    }
}

XAML中使用方式如下，i为Behaviors命名空间xmlns:i="http://schemas.microsoft.com/xaml/behaviors"：

<ComboBox 
    IsEditable="True"
    Text="{Binding YourViewModelTextProperty}">
    <i:Interaction.Behaviors>
        <local:DisableComboBoxAutoSelectBehavior/>
    </i:Interaction.Behaviors>
</ComboBox>

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