行为绑定属性

我创建了一个验证电子邮件输入的行为。下面是网络上的许多例子。正如你所看到的，我的行为有两个可绑定的属性，一个是IsValid，第二个是ErrorMessage。

在文本更改时，应用程序运行正则表达式验证，并没有任何问题地将值true/false分配给IsValid属性。但是当它试图给ErrorMessage属性赋值时，它会触发异常:

BindableProperty "ErrorMessage"是readonly.

有人知道为什么会这样吗?提前感谢!

public class EmailValidatorBehavior : Behavior<Entry>
{
    const string emailRegex = @"^(?("")("".+?(?<!'')""@)|(([0-9a-z](('.(?!'.))|[-!#'$%&''*'+/='?'^`'{'}'|~'w])*)(?<=[0-9a-z])@))" +
    @"(?('[)('[('d{1,3}'.){3}'d{1,3}'])|(([0-9a-z][-'w]*[0-9a-z]*'.)+[a-z0-9]['-a-z0-9]{0,22}[a-z0-9]))$";
    static readonly BindablePropertyKey IsValidPropertyKey = BindableProperty.CreateReadOnly("IsValid", typeof(bool), typeof(EmailValidatorBehavior), false);
    public static readonly BindableProperty IsValidProperty = IsValidPropertyKey.BindableProperty;
    static readonly BindablePropertyKey ErrorMessagePropertyKey = BindableProperty.CreateReadOnly("ErrorMessage", typeof(String), typeof(EmailValidatorBehavior), "");
    public static BindableProperty ErrorMessageProperty = ErrorMessagePropertyKey.BindableProperty;

    public bool IsValid
    {
        get { return (bool)base.GetValue(IsValidProperty); }
        private set { base.SetValue(IsValidPropertyKey, value); }
    }
    public string ErrorMessage
    {
        get { return (string)base.GetValue(ErrorMessageProperty); }
        private set { base.SetValue(ErrorMessageProperty, value); }
    }

    void HandleTextChanged(object sender, TextChangedEventArgs e)
    {

        IsValid = (Regex.IsMatch(e.NewTextValue, emailRegex,        
RegexOptions.IgnoreCase, TimeSpan.FromMilliseconds(250)));
        ((Entry)sender).TextColor = IsValid ? Color.Default : Color.Red;
        if (String.IsNullOrEmpty(e.OldTextValue))
            ErrorMessage = "";
        else
        {
            ErrorMessage = "Please enter email address";
            return;
        }
        if (!IsValid)
            ErrorMessage = "Please enter valid email address";
        else
            ErrorMessage = "";
    }

    protected override void OnAttachedTo(Entry bindable)
    {
        bindable.TextChanged += HandleTextChanged;
    }
    protected override void OnDetachingFrom(Entry bindable)
    {
        bindable.TextChanged -= HandleTextChanged;
    }
}

`public bool IsValid
{
    get { return (bool)base.GetValue(IsValidProperty); }
    set { base.SetValue(IsValidPropertyKey, value); }
}
public string ErrorMessage
{
    get { return (string)base.GetValue(ErrorMessageProperty); }
    set { base.SetValue(ErrorMessageProperty, value); }
}`

private set { base.SetValue(ErrorMessageProperty, value); }

private set { base.SetValue(ErrorMessagePropertyKey, value); }