Cannot perform runtime binding on a null reference

问题:

using: MVC 4, ASP.NET Razor

I'm getting an error that looks like it shouldn't be possible. It tells me that i'm using a null-reference, States, but clearly it is being set.

Controller:

public ActionResult Index()
{
    Dictionary<int, string> states = new Dictionary<int, string>()
    {
        { -1, "a"},
        { 0, "b"},
        { 1, "c"},
        { 2, "d"},
    };

    //assigning states
    ViewBag.States = states;

    foreach (KeyValuePair<int, string> de in ViewBag.States)
    {
        Debug.WriteLine(de.Key);
    }
    return View();
}

The View:

<div class="search-input">
    <select>
        @foreach (KeyValuePair<int, string> de in ViewBag.States)
        {
            <option value="@de.Key">@de.Value</option>
        }
    </select>
</div>

The error:

Cannot perform runtime binding on a null reference
Line 54: @foreach (KeyValuePair<int, string> de in ViewBag.States)

回答1:

Found solution: I had typo in my view, ViewBag.Typo <-- this caused the error, but the debugger placed the exception at a irrelevant place.

回答2:

This error happen when you have a ViewBag nonexistent in your razor code calling a method.

Controller

public ActionResult Accept(int id)
{
    return View();
}

razor:

<div class="form-group">
      @Html.LabelFor(model => model.ToId, "To", htmlAttributes: new { @class = "control-label col-md-2" })
     <div class="col-md-10">
           @Html.Flag(Model.from)
     </div>
</div>
<div class="form-group">
     <div class="col-md-10">
          <input value="@ViewBag.MaximounAmount.ToString()" />@* HERE is the error *@ 
     </div>
</div>

For some reason the .net aren't able to show the error in the correct line. Normally this cause a lot of wasted time.

回答3:

This exception is also thrown when a non-existent property is being updated dynamically, using reflection.

If one is using reflection to dynamically update property values, it's worth checking to make sure the passed PropertyName is identical to the actual property.

In my case, I was attempting to update Employee.firstName, but the property was actually Employee.FirstName.

Worth keeping in mind. :)

回答4:

My solution to this error was that a copy and paste from another project that had a reference to @Model.Id. This particular page didn't have a model but the error line was so far off from the actual error I about never found it!

回答5:

You must define states not equal to null..

@if (ViewBag.States!= null)
{
    @foreach (KeyValuePair<int, string> de in ViewBag.States)
    {
        value="@de.Key">@de.Value 
    }
}                                

回答6:

Set

 Dictionary<int, string> states = new Dictionary<int, string>()

as a property outside the function and inside the function insert the entries, it should work.