如果参数为null,则阻止执行MVC操作方法

下面是我提出的实现(在等待更好的想法时:)

这是一种通用的方法,而且我认为它具有很强的可扩展性——在提供我正在寻找的错误自动响应功能(当模型状态包含一个或多个错误时)的同时,希望允许类似于模型验证的参数深度验证。

我希望这不是太多的代码,所以答案(!);我有一大堆的文档注释,我把它们取出来以缩短它。

• 对将从中构造参数值的XML进行架构验证失败
• 缺少(null)参数值

模式验证目前是在模型绑定期间执行的,并且会自动将模型错误添加到ModelState中—所以这很好。所以我需要一种方法来执行自动空值检查。

最后,我创建了两个类来结束验证:

[AttributeUsage(AttributeTargets.Parameter, 
 AllowMultiple = false, Inherited = false)]
public abstract class ValidateParameterAttribute : Attribute
{
  private bool _continueValidation = false;

  public bool ContinueValidation 
  { get { return _continueValidation; } set { _continueValidation = value; } }

  private int _order = -1;
  public int Order { get { return _order; } set { _order = value; } }

  public abstract bool Validate
    (ControllerContext context, ParameterDescriptor parameter, object value);

  public abstract ModelError CreateModelError
    (ControllerContext context, ParameterDescriptor parameter, object value);

  public virtual ModelError GetModelError
    (ControllerContext context, ParameterDescriptor parameter, object value)
  {
    if (!Validate(context, parameter, value))
      return CreateModelError(context, parameter, value);
    return null;
  }
}

[AttributeUsage(AttributeTargets.Parameter, 
 AllowMultiple = false, Inherited = false)]
public class RequiredParameterAttribute : ValidateParameterAttribute
{
  private object _missing = null;

  public object MissingValue 
    { get { return _missing; } set { _missing = value; } }

  public virtual object GetMissingValue
    (ControllerContext context, ParameterDescriptor parameter)
  {
    //using a virtual method so that a missing value could be selected based
    //on the current controller's state.
    return MissingValue;
  }

  public override bool Validate
    (ControllerContext context, ParameterDescriptor parameter, object value)
  {
    return !object.Equals(value, GetMissingValue(context, parameter));
  }

  public override ModelError CreateModelError
    (ControllerContext context, ParameterDescriptor parameter, object value)
  {
    return new ModelError(
      string.Format("Parameter {0} is required", parameter.ParameterName));
  }
}

有了这个我就可以做到:

public void ActionMethod([RequiredParameter]MyModel p1){ /* code here */ }

输入 ParameterValidationAttribute :

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method,
                Inherited = false)]
public class ParameterValidationAttribute : ActionFilterAttribute
{
  public override void OnActionExecuting(ActionExecutingContext filterContext)
  {
    var paramDescriptors = filterContext.ActionDescriptor.GetParameters();
    if (paramDescriptors == null || paramDescriptors.Length == 0)
      return;

    var parameters = filterContext.ActionParameters;
    object paramvalue = null;
    ModelStateDictionary modelState 
      = filterContext.Controller.ViewData.ModelState;
    ModelState paramState = null;
    ModelError modelError = null;

    foreach (var paramDescriptor in paramDescriptors)
    {
      paramState = modelState[paramDescriptor.ParameterName];
      //fetch the parameter value, if this fails we simply end up with null
      parameters.TryGetValue(paramDescriptor.ParameterName, out paramvalue);

      foreach (var validator in paramDescriptor.GetCustomAttributes
                (typeof(ValidateParameterAttribute), false)
                .Cast<ValidateParameterAttribute>().OrderBy(a => a.Order)
              )
      {
        modelError = 
          validator.GetModelError(filterContext, paramDescriptor, paramvalue);

        if(modelError!=null)
        {
          //create model state for this parameter if not already present
          if (paramState == null)
            modelState[paramDescriptor.ParameterName] = 
              paramState = new ModelState();

          paramState.Errors.Add(modelError);
          //break if no more validation should be performed
          if (validator.ContinueValidation == false)
            break;
        }
      }
    }

    base.OnActionExecuting(filterContext);
  }
}

呼!快到了。。。

所以,现在我们可以这样做:

[ParameterValidation]
public ActionResult([RequiredParameter]MyModel p1)
{
  //ViewData.ModelState["p1"] will now contain an error if null when called
}

为了完成这个难题,我们需要一些能够调查模型错误并在有错误时自动响应的东西。这是最不整洁的类(我讨厌使用的名称和参数类型),我可能会在我的项目中更改它,但它可以工作,所以我还是会发布它:

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, 
                Inherited = false)]
public abstract class RespondWithModelErrorsAttribute : ActionFilterAttribute
{
  public override void OnActionExecuting(ActionExecutingContext filterContext)
  {
    ModelStateDictionary modelState = 
      filterContext.Controller.ViewData.ModelState;

    if (modelState.Any(kvp => kvp.Value.Errors.Count > 0))
      filterContext.Result = CreateResult(filterContext, 
                     modelState.Where(kvp => kvp.Value.Errors.Count > 0));

    base.OnActionExecuting(filterContext);
  }

  public abstract ActionResult CreateResult(
    ActionExecutingContext filterContext, 
    IEnumerable<KeyValuePair<string, ModelState>> modelStateWithErrors);
}

在我的应用程序中,我有一个XmlResult,它接受一个模型实例,并使用DataContractSerializer或XmlSerializer序列化响应—因此我创建了 RespondWithXmlModelErrorsAttribute Errors

[ParameterValidation]
[RespondWithXmlModelErrors(Order = int.MaxValue)]
public ActionResult([RequiredParameter]MyModel p1)
{
  //now if p1 is null, the method won't even be called.
}

但是对于web服务(XML或JSON)来说,能够将错误报告转移到其他东西上,使得编写实际操作方法变得更加容易,而且更具表现力,我觉得。

routes.MapRoute ("SomeWebService", "service/{userId}",
                 new { controller = "Service", action = "UserService" },
                 new { userId = @"\d+" });

或者,可以创建自定义约束,将路由值作为一个包一起验证。这对你来说可能是个更好的策略。请看这里: Creating a Custom Route Constraint