c# - 如何让 ModelBinder 为参数返回 null？

Question

我有一个 POCO，我将其用作 MVC3 中操作的参数。像这样:

我的类型

public class SearchData
{
    public string Property1 { get; set; }
    public string Property2 { get; set; }
    public string Property3 { get; set; }
}

我的行动

public ActionResult Index(SearchData query)
{
    // I'd like to be able to do this
    if (query == null)
    {
        // do something
    }
}

目前，query作为 SearchData 的实例传递所有属性为 null .我宁愿得到一个 null对于 query所以我可以做上面代码中的空检查。

我总能看到 ModelBinder.Any()或者只是 ModelBinder 中的各种键查看它是否具有 query 的任何属性，但我不想使用反射来遍历 query 的属性.另外，我只能使用 ModelBinder.Any()检查查询是否是我唯一的参数。一旦我添加了额外的参数，该功能就会中断。

使用 MVC3 中当前的模型绑定(bind)功能，是否有可能获得为操作的 POCO 参数返回 null 的行为？

Answer 1

您需要实现自定义模型绑定(bind)器才能执行此操作。您可以只扩展 DefaultModelBinder。

public override object BindModel(
    ControllerContext controllerContext, 
    ModelBindingContext bindingContext)
{
    object model = base.BindModel(controllerContext, bindingCOntext);
    if (/* test for empty properties, or some other state */)
    {
        return null;
    }

    return model;
}

具体实现

这是绑定(bind)器的实际实现，如果所有属性都为 null，它将为模型返回 null。

/// <summary>
/// Model binder that will return null if all of the properties on a bound model come back as null
/// It inherits from DefaultModelBinder because it uses the default model binding functionality.
/// This implementation also needs to specifically have IModelBinder on it too, otherwise it wont get picked up as a Binder
/// </summary>
public class SearchDataModelBinder : DefaultModelBinder, IModelBinder
{
    public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        // use the default model binding functionality to build a model, we'll look at each property below
        object model = base.BindModel(controllerContext, bindingContext);

        // loop through every property for the model in the metadata
        foreach (ModelMetadata property in bindingContext.PropertyMetadata.Values)
        {
            // get the value of this property on the model
            var value = bindingContext.ModelType.GetProperty(property.PropertyName).GetValue(model, null);

            // if any property is not null, then we will want the model that the default model binder created
            if (value != null)
                return model;
        }

        // if we're here then there were either no properties or the properties were all null
        return null;
    }
}

将其添加为 global.asax 中的 Binder

protected void Application_Start()
{
    AreaRegistration.RegisterAllAreas();
    ModelBinders.Binders.Add(typeof(SearchData), new SearchDataModelBinder());
    RegisterGlobalFilters(GlobalFilters.Filters);
    RegisterRoutes(RouteTable.Routes);

    MvcHandler.DisableMvcResponseHeader = true;
}