mvc模型绑定和null检查

Question

I have a simple model as below

public class TestModel
{
    public string property1 {get;set;}
    public string property2 {get;set;}
}

On my razor view i have two textboxes thats mapping to these two properties. Only one of the text box is a required field and other one can be left empty.(that is either property1 or property2 can be entered or both can be entered..however both cannot be left blank)

My controller action method is quite simple

     public ActionResult MyMethod(TestModel tmodel)
     {
          //code fails on this null check..object reference not set to instance of object 
          if (!string.IsNullOrEmpty(tmodel.property1))
          {
          }
      }

If i do this on my attribute of my model, the above statement works..why is null alone failing ?

 [DisplayFormat(ConvertEmptyStringToNull = false)]

What i dont understand is why is the string.IsNullOrEmpty fails?

Here is the view..

     <div class="form-group">
        @Html.LabelFor(m => m.property1, new { @class = "col-sm-3 control-label" })
        <div class="col-sm-4">
            @Html.TextBoxFor(m => m.property1, new { @class = "form-control"})
        </div>          
    </div>

    <div class="form-group">
         @Html.LabelFor(m => m.property2, new { @class = "col-sm-3 control-label" })
         <div class="col-sm-4">
        @Html.TextBoxFor(m => m.property2, new { @class = "form-control" })
    </div>

Thanks for your thoughts...

Answer 1

With this code, I guess that you'll never reach the string.IsNullOrEmpty code. But you have to show us the view to solve the real problem.

    public ActionResult MyMethod(TestModel tmodel)
    {
       if(tmodel != null)
       {
          if (!string.IsNullOrEmpty(tmodel.property1))
          {
          }
       }              
   }

Answer 2

-- EDIT -- To perform complex validation logic, implement IValidatableObject on your ViewModel:

public class TestModel : IValidatableObject {

    public string property1 {get;set;}
    public string property2 {get;set;}

     public IEnumerable<ValidationResult> Validate(ValidationContext validationContext) {
         if (string.IsNullOrWhiteSpace(property1) && string.IsNullOrWhiteSpace(property2)) {
              yield return new ValidationResult(
                  "some error message", 
                  new[] { "property1", "property2"}
              );
         }
     }
}

Check the ModelState in your controller action:

public ActionResult MyMethod(TestModel tmodel) {
   if(!ModelState.IsValid) {
      // error handling here
   }              
}

Answer 3

You can check like this:

public ActionResult MyMethod(TestModel tmodel)
        {
            if (string.IsNullOrEmpty(tmodel.property1) && string.IsNullOrEmpty(tmodel.property2))
            {
                ModelState.AddModelError("property1", "The field property1 can only be empty if property2 is not");
                ModelState.AddModelError("property2", "The field property2 can only be empty if property1 is not");
            }

            if (ModelState.IsValid)
            {
                return RedirectToAction("otherAction");   //or do whatever you want            
            }
            else
            {
                //back to form with the errors highlighted
                return View(tmodel);
            }
        }