如何使.NET MAUI的UI输入验证正常工作？

一、自定义Behavior实现控件级验证 #

这是MAUI原生支持的方式，完全不依赖第三方包，灵活可控。比如做一个邮箱输入验证：

代码实现 #

using System.Text.RegularExpressions;
using Microsoft.Maui.Controls;

public class EmailValidationBehavior : Behavior<Entry>
{
    private const string EmailPattern = @"^[^@\s]+@[^@\s]+\.[^@\s]+$";

    protected override void OnAttachedTo(Entry entry)
    {
        entry.TextChanged += CheckValidation;
        base.OnAttachedTo(entry);
    }

    protected override void OnDetachingFrom(Entry entry)
    {
        entry.TextChanged -= CheckValidation;
        base.OnDetachingFrom(entry);
    }

    private void CheckValidation(object sender, TextChangedEventArgs e)
    {
        var entry = sender as Entry;
        bool isValid = !string.IsNullOrWhiteSpace(e.NewTextValue) && 
                       Regex.IsMatch(e.NewTextValue, EmailPattern);
        
        // 用文字颜色提示验证状态
        entry.TextColor = isValid ? Colors.Black : Colors.Red;
        
        // 也可以绑定到ViewModel的验证状态属性
        if (BindingContext is ILoginViewModel vm)
        {
            vm.IsEmailValid = isValid;
        }
    }
}

XAML使用 #

<Entry Placeholder="请输入邮箱">
    <Entry.Behaviors>
        <local:EmailValidationBehavior />
    </Entry.Behaviors>
</Entry>

二、ViewModel层面的DataAnnotations验证 #

利用.NET自带的DataAnnotations特性，结合INotifyDataErrorInfo实现全局验证，适合复杂表单场景。

ViewModel实现 #

using System.ComponentModel;
using System.ComponentModel.DataAnnotations;
using System.Runtime.CompilerServices;
using System.Text.RegularExpressions;

public class LoginViewModel : INotifyPropertyChanged, INotifyDataErrorInfo
{
    private readonly Dictionary<string, List<string>> _errorDict = new();
    private string _email;

    public string Email
    {
        get => _email;
        set
        {
            _email = value;
            ValidateEmail();
            OnPropertyChanged();
        }
    }

    private void ValidateEmail()
    {
        var errors = new List<string>();
        if (string.IsNullOrWhiteSpace(Email))
        {
            errors.Add("邮箱不能为空");
        }
        else if (!Regex.IsMatch(Email, @"^[^@\s]+@[^@\s]+\.[^@\s]+$"))
        {
            errors.Add("请输入有效的邮箱格式");
        }

        if (errors.Any())
        {
            _errorDict["Email"] = errors;
        }
        else
        {
            _errorDict.Remove("Email");
        }

        ErrorsChanged?.Invoke(this, new DataErrorsChangedEventArgs("Email"));
    }

    // INotifyDataErrorInfo 接口实现
    public bool HasErrors => _errorDict.Any();
    public event EventHandler<DataErrorsChangedEventArgs>? ErrorsChanged;
    public IEnumerable GetErrors(string? propertyName)
    {
        if (string.IsNullOrEmpty(propertyName) || !_errorDict.ContainsKey(propertyName))
            return Enumerable.Empty<string>();
        return _errorDict[propertyName];
    }

    // INotifyPropertyChanged 接口实现
    public event PropertyChangedEventHandler? PropertyChanged;
    private void OnPropertyChanged([CallerMemberName] string? propName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propName));
    }
}

XAML绑定与错误提示 #

<Entry Text="{Binding Email, Mode=TwoWay, ValidatesOnNotifyDataErrors=True}" 
       Placeholder="请输入邮箱"/>
<Label Text="{Binding [Email], Mode=OneWay}" 
       TextColor="Red" 
       IsVisible="{Binding HasErrors}"/>

三、社区工具包验证功能的修复建议 #

如果坚持用CommunityToolkit.Maui的验证，试试这些排查步骤：

• 确保NuGet包是最新稳定版：更新CommunityToolkit.Maui和CommunityToolkit.Mvvm到当前最新版
• 检查项目初始化：在MauiProgram.cs中必须添加builder.UseMauiCommunityToolkit();
• 验证ViewModel是否正确继承ObservableValidator：

  public class RegisterViewModel : ObservableValidator
  {
      [Required(ErrorMessage = "用户名不能为空")]
      [MinLength(3, ErrorMessage = "用户名至少3个字符")]
      public string Username { get; set; } = string.Empty;
  }
  

• 绑定开启验证：XAML中绑定要加上ValidatesOnNotifyDataErrors=True
• 清理重建项目：删除bin、obj文件夹，重启VS后重新编译

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