当我的视图模型具有构造函数时发布数据不起作用 [英] Posting data when my view model has a constructor does not work

问题描述

我有以下代码:

[HttpGet]
public ActionResult Edit(int req)
{
    var viewModel = new  EditViewModel();
    viewModel.RequestId = int;
    return View(viewModel);
}

[HttpPost]
Public ActionResult Edit(EditViewModel viewModel)
{
// some code here...
}

它工作正常:当编辑表单被发布时，我有被调用的动作控制器.

It works fine: when the edit form is posted, I have the action controller who is called.

现在我像这样修改一些代码:

Now I modify some little bit my code like this:

[HttpGet]
public ActionResult Edit(int req)
{
    var viewModel = new  EditViewModel(req);
    return View(viewModel);
}

[HttpPost]
Public ActionResult Edit(EditViewModel viewModel)
{
// some code here...
}

public class EditViewModel()
{
    public EditViewModel(int req)
    {
        requestId = req; 
    }
    ...
}

在这个新版本中，我有一个带有构造函数的视图模型.

In this new version, I have a view model with a contructor.

这一次，当我的表单被回发时，动作控制器永远不会被触发.

This time, when my form is posted back, the action controller is never triggered.

有什么想法吗?

谢谢.

推荐答案

这很正常.默认模型绑定器无法再实例化您的视图模型，因为它没有无参数构造函数.如果要使用没有默认构造函数的视图模型，则必须编写自定义模型绑定器.

That's normal. The default model binder can no longer instantiate your view model as it doesn't have a parameterless constructor. You will have to write a custom model binder if you want to use view models that don't have a default constructor.

通常您不需要这样的自定义构造函数.您可以简单地拥有这样的视图模型:

Normally you don't need such custom constructor. You could simply have your view model like that:

public class EditViewModel()
{
    public int RequestId { get; set; }
}

和这样的 POST 操作:

and the POST action like that:

[HttpPost]
public ActionResult Edit(EditViewModel viewModel)
{
    // some code here...
}

现在您所要做的就是发布 requestId 参数而不是 req 并且默认模型绑定器将完成这项工作.

and now all you have to do is POST the requestId parameter instead of req and the default model binder will do the job.

如果出于某种原因您想使用带有自定义构造函数的视图模型，以下是自定义模型绑定器的外观示例:

And if for some reason you wanted to use a view model with custom constructor, here's an example of how the custom model binder might look like:

public class EditViewModelBinder : DefaultModelBinder
{
    protected override object CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType)
    {
        var req = bindingContext.ValueProvider.GetValue("req");
        if (req == null)
        {
            throw new Exception("missing req parameter");
        }
        int reqValue;
        if (!int.TryParse(req.AttemptedValue, out reqValue))
        {
            throw new Exception(string.Format("The req parameter contains an invalid value: {0}", req.AttemptedValue));
        }

        return new EditViewModel(reqValue);
    }
}

将在您的 Application_Start 中注册:

ModelBinders.Binders.Add(typeof(EditViewModel), new EditViewModelBinder());

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