WPF中如何设置TextBox负数时MaxLength为4，正数时为3？

针对你遇到的问题，需要从实时输入拦截和绑定值验证两方面入手，彻底解决无效格式和超范围输入的问题：

1. 替换输入行为，实现精准输入拦截 #

放弃原有的TextBoxIntInputBehavior，改用一个更严谨的行为类，它会实时过滤无效输入（包括多减号、非数字字符、超范围值），同时处理粘贴、键盘输入等场景：

public class RestrictedIntInputBehavior : Behavior<TextBox>
{
    public int Min { get; set; } = -999;
    public int Max { get; set; } = 999;

    protected override void OnAttached()
    {
        base.OnAttached();
        AssociatedObject.PreviewTextInput += OnPreviewTextInput;
        AssociatedObject.PreviewKeyDown += OnPreviewKeyDown;
        DataObject.AddPastingHandler(AssociatedObject, OnPaste);
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();
        AssociatedObject.PreviewTextInput -= OnPreviewTextInput;
        AssociatedObject.PreviewKeyDown -= OnPreviewKeyDown;
        DataObject.RemovePastingHandler(AssociatedObject, OnPaste);
    }

    private void OnPreviewTextInput(object sender, TextCompositionEventArgs e)
    {
        var textBox = sender as TextBox;
        if (textBox == null) return;

        var newText = GetNewText(textBox, e.Text);
        e.Handled = !IsValidInput(newText);
    }

    private void OnPreviewKeyDown(object sender, KeyEventArgs e)
    {
        var textBox = sender as TextBox;
        if (textBox == null) return;

        // 允许导航、删除类按键
        if (e.Key is Key.Back or Key.Delete or Key.Left or Key.Right or Key.Up or Key.Down)
            return;

        // 减号仅允许在开头输入一次
        if (e.Key == Key.OemMinus || e.Key == Key.Subtract)
        {
            e.Handled = !string.IsNullOrEmpty(textBox.Text) || textBox.SelectionStart != 0;
        }
    }

    private void OnPaste(object sender, DataObjectPastingEventArgs e)
    {
        var textBox = sender as TextBox;
        if (textBox == null || !e.DataObject.GetDataPresent(DataFormats.Text)) return;

        var pastedText = e.DataObject.GetData(DataFormats.Text) as string;
        var newText = GetNewText(textBox, pastedText);
        if (!IsValidInput(newText))
            e.CancelCommand();
    }

    private string GetNewText(TextBox textBox, string inputText)
    {
        var selectionStart = textBox.SelectionStart;
        var selectionLength = textBox.SelectionLength;
        return textBox.Text.Remove(selectionStart, selectionLength).Insert(selectionStart, inputText);
    }

    private bool IsValidInput(string input)
    {
        // 空输入允许（对应TargetNullValue）
        if (string.IsNullOrEmpty(input)) return true;

        // 验证是否为有效整数，且在指定范围内
        return int.TryParse(input, out var value) && value >= Min && value <= Max;
    }
}

2. 调整XAML配置 #

• 移除原有的MaxLength="4"属性，避免固定长度限制冲突（负数是4位，正数是3位，行为会自动根据范围控制输入长度）
• 替换行为为新的RestrictedIntInputBehavior，并指定Min和Max值
• 保留原有的IntRangeRule作为兜底验证，确保最终绑定到ViewModel的值符合要求

修改后的XAML：

<TextBox Height="{Binding ElementName=SomeComboBox, Path=ActualHeight}"
         VerticalAlignment="Top"
         Tag="int"
         HorizontalAlignment="Stretch"
         Margin="2, 4, 0, 0"
         AutomationProperties.AutomationId="Something_cbSome">
    <i:Interaction.Behaviors>
        <behaviors:RestrictedIntInputBehavior Min="-999" Max="999" />
    </i:Interaction.Behaviors>

    <TextBox.Text>
        <Binding Path="Something"
                 UpdateSourceTrigger="PropertyChanged"
                 TargetNullValue="">
            <Binding.ValidationRules>
                <validation:IntRangeRule Min="-999" Max="999" />
            </Binding.ValidationRules>
        </Binding>
    </TextBox.Text>
</TextBox>

3. 效果说明 #

• 用户无法输入---、8--2这类无效格式的内容，减号仅允许在开头输入一次
• 无法输入1234这类超过999的正数，也无法输入-1000这类小于-999的负数
• 空输入会被转换为null（对应TargetNullValue=""）
• 即使通过特殊方式绕过前端输入拦截，绑定验证规则会确保不符合范围的值不会更新到ViewModel

内容的提问来源于stack exchange，提问作者aca