C# MVVM模式下的数据验证方法

在C#中使用MVVM模式进行数据验证可以通过以下方法实现：

1. 使用INotifyDataErrorInfo接口：在ViewModel中实现INotifyDataErrorInfo接口，并在属性的setter中进行数据验证，然后通过ErrorsChanged事件通知View。

public class ViewModel : INotifyDataErrorInfo
{
    private string _username;
    public string Username
    {
        get { return _username; }
        set
        {
            if (string.IsNullOrEmpty(value))
            {
                AddError("Username", "Username cannot be empty");
            }
            else
            {
                RemoveError("Username");
            }
            _username = value;
            OnPropertyChanged("Username");
        }
    }

    private Dictionary<string, List<string>> _errors = new Dictionary<string, List<string>>();

    public event EventHandler<DataErrorsChangedEventArgs> ErrorsChanged;

    public IEnumerable GetErrors(string propertyName)
    {
        if (_errors.ContainsKey(propertyName))
            return _errors[propertyName];
        return null;
    }

    public bool HasErrors => _errors.Any();

    private void AddError(string propertyName, string error)
    {
        if (!_errors.ContainsKey(propertyName))
            _errors[propertyName] = new List<string>();
        if (!_errors[propertyName].Contains(error))
        {
            _errors[propertyName].Add(error);
            ErrorsChanged?.Invoke(this, new DataErrorsChangedEventArgs(propertyName));
        }
    }

    private void RemoveError(string propertyName)
    {
        if (_errors.ContainsKey(propertyName))
        {
            _errors.Remove(propertyName);
            ErrorsChanged?.Invoke(this, new DataErrorsChangedEventArgs(propertyName));
        }
    }

    private void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

2. 使用IDataErrorInfo接口：ViewModel类实现IDataErrorInfo接口，对属性进行数据验证，并在Error属性中返回错误信息。

public class ViewModel : IDataErrorInfo
{
    private string _username;
    public string Username
    {
        get { return _username; }
        set
        {
            if (string.IsNullOrEmpty(value))
            {
                Error = "Username cannot be empty";
            }
            else
            {
                Error = null;
            }
            _username = value;
            OnPropertyChanged("Username");
        }
    }

    public string Error { get; private set; }

    public string this[string columnName]
    {
        get
        {
            switch (columnName)
            {
                case "Username":
                    if (string.IsNullOrEmpty(Username))
                        return "Username cannot be empty";
                    break;
            }
            return null;
        }
    }

    private void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

这样在View中绑定ViewModel的属性时，会实时进行数据验证，并显示错误信息。