用户名密码验证器捕获异常

我有一个UserNamePasswordValidator：

 public class EmployeeValidator : UserNamePasswordValidator
 {
    public override void Validate(String userName, String password)
    {
        //...
        throw new MessageSecurityException("Username and password cannot be empty.");
        //...         
    }
 }

在ViewModel：

private void Login()
{
     try
     {
          _cl.ClientCredentials.ServiceCertificate.Authentication.CertificateValidationMode = X509CertificateValidationMode.None;
          _cl.ClientCredentials.UserName.UserName = UserName;
          _cl.ClientCredentials.UserName.Password = Password;
          _cl.Login(UserName)
     }
     catch
     {  /*...*/  }
}

问题是，我无法捕捉到异常。我做错了什么？