Yes, my yet another post on Model Binding (previous one is here), but this one uses features presented in MVC 2. How I got to writing this blog? Well, I’m on a project where we’re doing some MVC things for a shopping cart. Let me show you what I was working with. Below are my model classes: 1: public class Product 2: { 3: public int Id { get; set; } 4: public string Name { get; set; } 5: public int Quantity { get; set; } 6: public decimal UnitPrice { get; set; } 7: } 8: 9: public class Totals 10: { 11: public decimal SubTotal { get; set; } 12: public decimal Tax { get; set; } 13: public decimal Total { get; set; } 14: } 15: 16: public class Basket 17: { 18: public List<Product> Products { get; set; } 19: public Totals Totals { get; set;} 20: } The view looks as below:
1: <h2>Shopping Cart</h2> 2: 3: <% using(Html.BeginForm()) { %> 4: 5: <h3>Products</h3> 6: <% for (int i = 0; i < Model.Products.Count; i++) 7: { %> 8: <div style="width: 100px;float:left;">Id</div> 9: <div style="width: 100px;float:left;"> 10: <%= Html.TextBox("ID", Model.Products[i].Id) %> 11: </div> 12: <div style="clear:both;"></div> 13: <div style="width: 100px;float:left;">Name</div> 14: <div style="width: 100px;float:left;"> 15: <%= Html.TextBox("Name", Model.Products[i].Name) %> 16: </div> 17: <div style="clear:both;"></div> 18: <div style="width: 100px;float:left;">Quantity</div> 19: <div style="width: 100px;float:left;"> 20: <%= Html.TextBox("Quantity", Model.Products[i].Quantity)%> 21: </div> 22: <div style="clear:both;"></div> 23: <div style="width: 100px;float:left;">Unit Price</div> 24: <div style="width: 100px;float:left;"> 25: <%= Html.TextBox("UnitPrice", Model.Products[i].UnitPrice)%> 26: </div> 27: <div style="clear:both;"><hr /></div> 28: <% } %> 29: 30: <h3>Totals</h3> 31: <div style="width: 100px;float:left;">Sub Total</div> 32: <div style="width: 100px;float:left;"> 33: <%= Html.TextBox("SubTotal", Model.Totals.SubTotal)%> 34: </div> 35: <div style="clear:both;"></div> 36: <div style="width: 100px;float:left;">Tax</div> 37: <div style="width: 100px;float:left;"> 38: <%= Html.TextBox("Tax", Model.Totals.Tax)%> 39: </div> 40: <div style="clear:both;"></div> 41: <div style="width: 100px;float:left;">Total</div> 42: <div style="width: 100px;float:left;"> 43: <%= Html.TextBox("Total", Model.Totals.Total)%> 44: </div> 45: <div style="clear:both;"></div> 46: <p /> 47: <input type="submit" name="Submit" value="Submit" /> 48: <% } %>
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Nothing fancy, just a bunch of div’s containing textboxes and a submit button. Just make note that
the textboxes have
the same name as
the property they are going to display. Yea, yea, I know. I’m displaying unit price as a textbox instead of a label, but that’s beside
the point (and trust me, this will not be how it’ll look on
the production site!!).
The way my controller works is that initially two dummy products are added to
the basked object and
the Totals are calculated based on what products were added in what quantities and their respective unit price. So when
the page loads in edit mode, where
the user can change
the quantity and hit
the submit button. In
the ‘post’ version of
the action method,
the Totals get recalculated and
the new total will be displayed on
the screen. Here’s
the code:
1: public ActionResult Index() 2: { 3: Product product1 = new Product 4: { 5: Id = 1, 6: Name = "Product 1", 7: Quantity = 2, 8: UnitPrice = 200m 9: }; 10: 11: Product product2 = new Product 12: { 13: Id = 2, 14: Name = "Product 2", 15: Quantity = 1, 16: UnitPrice = 150m 17: }; 18: 19: List<Product> products = new List<Product> { product1, product2 }; 20: 21: Basket basket = new Basket 22: { 23: Products = products, 24: Totals = ComputeTotals(products) 25: }; 26: return View(basket); 27: } 28: 29: [HttpPost] 30: public ActionResult Index(Basket basket) 31: { 32: basket.Totals = ComputeTotals(basket.Products); 33: return View(basket); 34: }
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That’s that. Now I run
the app, I see two products with
the totals section below them. I look at
the view source and I see that
the input
controls have
the right ID,
the right name and
the right value as well.
1: <input id="ID" name="ID" type="text" value="1" /> 2: <input id="Name" name="Name" type="text" value="Product 1" /> 3: ... 4: <input id="ID" name="ID" type="text" value="2" /> 5: <input id="Name" name="Name" type="text" value="Product 2" />
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So just as a regular user would do, I change
the quantity value of one of
the products and hit
the submit button.
The ‘post’ version of
the Index method gets called and I had put a break-point on line 32 in
the above snippet. When I hovered my mouse on
the ‘basked’ object, happily assuming that
the object would be all bound and ready for use, I was surprised to see both basket.Products and basket.Totals were null. Huh?
A little research and I found out that
the reason
the DefaultModelBinder could not do its job is because of a naming mismatch on
the input
controls. What I mean is that when you have to bind to a custom .net type, you need more than just
the property name. You need to pass a qualified name to
the name property of
the input control.
I modified my view and
the emitted code looked as below:
1: <input id="Product_Name" name="Product.Name" type="text" value="Product 1" /> 2: ... 3: <input id="Product_Name" name="Product.Name" type="text" value="Product 2" /> 4: ... 5: <input id="Totals_SubTotal" name="Totals.SubTotal" type="text" value="550" />
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Now, I update
the quantity and hit
the submit button and I see that
the Totals object is populated, but
the Products list is still null. Once again I went: ‘Hmm.. time for more research’. I found out that
the way to do this is to provide
the name as:
1: <%= Html.TextBox(string.Format("Products[{0}].ID", i), Model.Products[i].Id) %> 2: <!-- this will be rendered as --> 3: <input id="Products_0__ID" name="Products[0].ID" type="text" value="1" />
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It was only now that I was able to see both
the products and
the totals being properly bound in
the ‘post’ action method. Somehow, I feel this is kinda ‘clunky’ way of doing things. Seems like people at MS felt in a similar way and offered us a much cleaner way to solve this issue.
The simple solution is that instead of using a Textbox, we can either use a TextboxFor or an EditorFor helper method. This one directly spits out
the name of
the input property as ‘Products[0].ID and so on. Cool right? I totally fell for this and changed my UI to contain EditorFor helper method.
At this point, I ran
the application, changed
the quantity field and pressed
the submit button. Of course my basket object parameter in my action method was correctly bound after these changes. I let
the app complete
the rest of
the lines in
the action method. When
the page finally rendered, I did see that
the quantity was changed to what I entered before
the post. But, wait a minute,
the totals section did not reflect
the changes and showed
the old values.
My status: COMPLETELY PUZZLED! Just to recap, this is what my ‘post’ Index method looked like:
1: [HttpPost] 2: public ActionResult Index(Basket basket) 3: { 4: basket.Totals = ComputeTotals(basket.Products); 5: return View(basket); 6: }
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A careful debug confirmed that
the basked.Products[0].Quantity showed
the updated value and
the ComputeTotals() method also returns
the correct totals. But still when I passed this basket object, it ended up showing
the old totals values only. I began playing a bit with
the code and my first guess was that
the input
controls got their values from
the ModelState object.
For those who don’t know,
the ModelState is a temporary storage area that ASP.NET MVC uses to retain incoming attempted values plus binding and validation errors. Also,
the fact that input
controls populate
the values using data taken from:
Previously attempted values recorded in
the ModelState["name"].Value.AttemptedValue
Explicitly provided value (<%= Html.TextBox("name", "Some value") %>)
ViewData, by calling ViewData.Eval("name") FYI: ViewData dictionary takes precedence over ViewData's Model properties – read more here.
These two indicators led to my guess. It took me quite some time, but finally I hit this post where Brad brilliantly explains why this is
the preferred behavior. My guess was right and I, accordingly modified my code to reflect
the following way:
1: [HttpPost] 2: public ActionResult Index(Basket basket) 3: { 4: // read
the following posts to see why
the ModelState 5: // needs to be cleared before passing it
the view 6: // http://forums.asp.net/t/1535846.aspx 7: // http://forums.asp.net/p/1527149/3687407.aspx 8: if (ModelState.IsValid) 9: { 10: ModelState.Clear(); 11: } 12: 13: basket.Totals = ComputeTotals(basket.Products); 14: return View(basket); 15: }
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What this does is that in
the case where your ModelState IS valid, it clears
the dictionary. This enables
the values to be read from
the model directly and not from
the ModelState.
So
the verdict is this: If you need to pass other parameters (like html attributes and
the like) to your input control, use
1: <%= Html.TextBox(string.Format("Products[{0}].ID", i), Model.Products[i].Id) %>
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Since, in EditorFor, there is no direct and simple way of passing this information to
the input control. If you don’t have to pass any such ‘extra’ piece of information to
the control, then go
the EditorFor way.
The code used in
the post can be found here.
