ASP.NET MVC中的模型ID属性null c＃

Question

I have a model expense

public class Expense
{
     public int expenseID {get;set;}
     public string expenseName {get;set;}
     public int reportID {get;set;}
}

I have a controller method which should pass an expense object to the view

public ActionResult Create(int reportID)
{
    Expense ex = new Expense(){ 
        reportID = reportID
    };
    return View(ex);
}

Then in the view the user can set the expenseName property and I have included

@Html.HiddenFor(model => model.expenseID)
@Html.HiddenFor(model => model.reportID)

Then another controller method accepts the Expense model and should save it to the database however the expenseID is null and a database error occurs This is the controller method

public ActionResult Create(Expense ex)
{
    if(ModelState.IsValid)
    {
        db.Expenses.Add(ex);
        db.SaveChanges();
        return RedirectToAction("Index");
   }
   return View(ex);
}

Can anyone suggest why the expenseID property is null when we try and add expense to our database thanks!

Answer 1

expenseID property is null. Because You setting property expenseID not null and you insert expenseID = null in database => error. And model expense you need update again same that:

public class Expense
{
     [Key]
     public int expenseID {get;set;}
     public string expenseName {get;set;}
     public int reportID {get;set;}
}

And Property expenseID in database if you don't setting "Identity Specification" => expenseID don't auto have new key ID.

Answer 2

Make sure the HiddenFor properties are within your BeginForm using statement:

@using (Html.BeginForm("Create", "YourController", FormMethod.Post))
{
    @Html.HiddenFor(model => model.expenseID)
    @Html.HiddenFor(model => model.reportID)

    <input class="btn btn-default" type="submit" value="Post Data">
}

Have you set a breakpoint on the Create Action Method to make sure that the Expense model is definitely not null itself when it arrives, if its not then my above suggestion should fix it.

edit This is assuming your ID is populated when you pass it out to your view in the first place, and is not yet to be actually created in the db as Linh Tuan suggested.

If you are using EF, you should make sure your config is something like this:

public ExpenseConfiguration()
{
    HasKey(x => x.expenseId);
}

You should check the Design of that table, and see whether Identity Specification is set to true, if it is, it doesn't matter whether the expenseId is null all the way to INSERT, the db will assign it an incrementing ID once it get to the point of insert.

Your GET method seems to be expecting a reportId as a parameter, could you tell us what is feeding this ActionMethod in the first place, as if you already have an ID that you wish to re-use for your Expense object table then it might just be a case of not passing the HiddenFor property correctly in the first place.

Answer 3

Unless the expenseID is set as an Identity Column in your database and mapping, you will have to set expenseID explicitly.

Something like this :

public ActionResult Create(int reportID)
{
    Expense ex = new Expense(){ 
        reportID = reportID,
        expenseID = **Some value** //Not advisable though

    };
    return View(ex);
}

However the preferred method would be to make your expenseId Column as Identity column and make it automatically incremented, like this :

public class Expense
{
     [Key]
     public int expenseID {get;set;}
     public string expenseName {get;set;}
     public int reportID {get;set;}
}

Answer 4

将[HttpPost]属性添加到第二个ActionMethod.And参数必须是“Expense model”

Answer 5

My hidden field is inside form.

Model:

public class Operations
{        
    public decimal firstValue { get; set; }        
}

First Action Method:

[HttpGet]
public ActionResult Index()
{
    Operations model = new Operations();
    model.firstValue = 100M;
    return View(model);
}

View:

@Html.HiddenFor(m => m.firstValue)
@Html.ActionLink("MyLink", "SampleGet", Model)//(for redirection)

Target Action Method:

[HttpGet]
public ActionResult SampleGet(Operations model)
{
    decimal myValue = model.firstValue;
    return View(model);
}

Answer 6

You can solve this in 2 ways.

1. From create method you not sending any expenseID to the view through model. You have to initilize ex.expenseID in action method Create.

   public ActionResult Create(int reportID, int expenseID )
    {
        Expense ex = new Expense(){ 
            reportID = reportID,
            expenseID = expenseID /** Or expenseID = max(expenseID) +1 from expense table in database*/  
        };
        return View(ex);
    }

2. Or in post back create method

public ActionResult Create(Expense ex)
{
    if(ModelState.IsValid)
    {
        ex.expenseID = db.Expenses.Max( x => x.expenseID) + 1 ; 
        db.Expenses.Add(ex);
        db.SaveChanges();
        return RedirectToAction("Index");
   }
   return View(ex);

Answer 7

You will have to either pass the expenseID property value while creating new record. Else u can make expenseid as identity column by including this in your Context Class.

protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {           

            modelBuilder.Entity<Models.Expense>().HasKey(a => a.expenseID);
            modelBuilder.Entity<Models.Expense>().Property(a => a.expenseID).HasDatabaseGeneratedOption(System.ComponentModel.DataAnnotations.Schema.DatabaseGeneratedOption.Identity);

            base.OnModelCreating(modelBuilder);
        }

Answer 8

You must explicitly initialised your expenseID .

Yes, I know, we can expect that int will be initialised with 0 - but in my case it's not working when you passing object controller -> view (with hidden) -> controller . I'm not sure why.

Try something like that:

public ActionResult Create(int reportID)
{
    Expense ex = new Expense(){ 
        expenseID = 0,
        reportID = reportID,
    };
    return View(ex);
}

I suppose that expenseID is your Primary Key in Db, so EF will compute proper Id when you set expenseID = 0 .

Answer 9

If expenseID is your key, you should modify your code with these two line before

[Key]
[DatabaseGeneratedAttribute(DatabaseGeneratedOption.None)]
public int expenseID {get;set;}