Custom Model Binders in Visual Basic .NET

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Custom Model Binders
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The default binder does excellent work, but it is a general-purpose tool aimed at working with most possible types in a way that is not specific to any of them. The Bind attribute gives you some more control over the binding process, but some reasonable boundaries still exist. In these cases, all you do is create a custom binder for a specific type. There are two main reasons you should be willing to create a custom binder. The most important reason is that the default binder is limited to taking into account a one-to-one correspondence between posted values and properties on the model. Sometimes the target model has a different granularity than the one expressed by form fields. The canonical example is when you employ multiple input fields to let users enter content for a single property for example, distinct input fields for day, month, and year that then map to a single DateTime value. The second reason to go beyond the standard model binder is to avoid the use of prefixed IDs in the view. To create a custom binder, you can implement the IModelBinder interface:
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public interface IModelBinder { object BindModel( ControllerContext controllerContext, ModelBindingContext bindingContext); }
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6 Inside Models
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Implementing the interface is recommended if you need total control over the binding process. If, say, all you need to do is keep the default behavior and simply force the binder to use a non-default constructor for a given type, inheriting from DefaultModelBinder is the best approach. Here s the schema to follow:
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public CustomerViewModelBinder : DefaultModelBinder { protected override object CreateModel( ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType) { ... return new CustomerViewModel( ... ); } }
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Another common scenario for simply overriding the default binder is when all you want is the ability to validate against a specific type. In this case, you override OnModelUpdated and insert your own validation logic, as shown here:
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protected override void OnModelUpdated(ControllerContext controllerContext, ModelBindingContext bindingContext) { var obj = bindingContext.Model as CustomerViewModel; if (obj == null) return; // Apply validation logic here for the whole model if (String.IsNullOrEmpty(obj.CompanyName)) { bindingContext.ModelState.AddModelError("CompanyName", ...); } ... }
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You override OnModelUpdated if you prefer to keep in a single place all validations for any properties. You resort to OnPropertyValidating if you prefer to validate properties individually.
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Implementing a Model Binder
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Here s an example of a custom binder that implements the IModelBinder interface:
public class CustomerViewModelBinder : IModelBinder { public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) { if (bindingContext == null) throw new ArgumentNullException("bindingContext"); // Get the model instance or create one if needed var obj = (CustomerViewModel) (bindingContext.Model new CustomerViewModel());
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// obj.CompanyName = FromPostedData<string>(bindingContext, "CompanyName"); ... return obj; } // Helper routine private T FromPostedData<T>(ModelBindingContext context, string key) { // Get the value from any of the input collections ValueProviderResult result; context.ValueProvider.TryGetValue(key, out result); // Set the state of the model property resulting from value context.ModelState.SetModelValue(key, result); // Return the value converted (if possible) to the target type return (T) result.ConvertTo(typeof(T)); }
Note that when writing a model binder you are in no way restricted to getting information for the model uniquely from the posted data which represents only the most common scenario. You can grab information from anywhere for example, from the ASP.NET cache and session state parse it, and store it in the model.
Registering a Custom Binder
You can associate a model binder with its target type globally or locally. In the former case, any occurrence of model binding for the type will be resolved through the registered custom binder. In the latter case, you apply the binding to just one occurrence of one parameter in a controller method. Global association takes place in the global.asax file as follows:
void Application_Start() { ... ModelBinders.Binders[typeof(CustomerViewModelBinder)] = new CustomerViewModelBinder(); }
Local association requires the following syntax:
public ActionResult Edit( [ModelBinder(typeof(CustomerViewModelBinder))] CustomerViewModel customerInfo) { ... }
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