Bind input field to custom object instead of string - asp.net-mvc

Question

I'm using ASP.NET MVC 4 for an internal web application and I have a desire to bind HTML input fields to a custom object rather than string.
In the HTML I have input fields that will look like the following:
<input type="hidden" name="First" value="1;Simple" />
<input type="hidden" name="First" value="2;Sample" />
<input type="hidden" name="Second" value="1;Over" />
<input type="hidden" name="Third" value="22;Complex" />
<input type="hidden" name="Third" value="17;Whosit" />
This will happily bind to ViewModel properties like:
public string[] First { get; set; }
public string[] Second { get; set; }
public string[] Third { get; set; }
Each string is a delimited string of key+value that I'd love to have automatically parsed into a concrete object (I have one already defined.) Ideally I'd want it to bind exactly as above but using my object that would know how to split the delimited string into the proper properties.
I can't figure out how to get MVC to bind to a custom object. I've used constructors and implicit operator definitions but I can't get it to work with anything but string datatype.
I know I could get this to work if I pre-split the values into pairs in the HTML but I'm using a JavaScript library that doesn't give this ability. For instance I know repeating {name}.Label and {name}.Value would work to bind to the string properties on my complex object but this is prohibitive and a non-starter.
I have gotten this to work with a custom object to handle File Uploads but I suspect that worked only because it inherited from the same base object. I can't do this here since string is a sealed type and can't be extended.
My last resort is to find the default model binder code and reflect that to figure out how it's assigning the values to see if it teaches me anything that I can override. I'd prefer not to go the route of a custom binder I'd have to write myself and if it comes down to it I'll just have duplicate ViewModel fields and convert them myself but I'd really love to avoid this if there's already a capability for the model binder to do this for me.

Answer 1

Here is what you can do. Let's say your MyThing class is something like this:
public class MyThing
{
public int Id { get; set; }
public string Name { get; set; }
public override string ToString()
{
return string.Format("{0};{1}", this.Id, this.Name);
}
}
Then, you can create a custom model binder for it like below:
public class MyModelBinder : IModelBinder
{
public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
{
ValueProviderResult valueResult = bindingContext.ValueProvider
.GetValue(bindingContext.ModelName);
ModelState modelState = new ModelState { Value = valueResult };
object actualValue = null;
if (valueResult != null && !string.IsNullOrEmpty(valueResult.AttemptedValue))
{
if(valueResult.AttemptedValue.Contains(';'))
{
try
{
var attemptedValue = valueResult.AttemptedValue.Split(';');
int id = int.Parse(attemptedValue.First());
string name = attemptedValue.Last();
actualValue = new MyThing { Id = id, Name = name };
}
catch(Exception e)
{
modelState.Errors.Add(e);
}
}
else
{
modelState.Errors.Add("Invalid value.");
}
bindingContext.ModelState.Add(bindingContext.ModelName, modelState);
}
return actualValue;
}
}
You'll need to register your ModelBinder in Application_Start event of Global.asax like this:
ModelBinders.Binders.Add(typeof(MyThing), new MyModelBinder());

Answer 2

The question didn't get a single bite so I looked at the default model binder to see what was happening under the covers. There are a number of stages it goes through to see if a value can be converted to the ViewModel type but most of them are inaccessible to me. I did find a segment of code that fell back to using a type converter which I'd never used before.
Using this MSDN Type Converter how-to, I made a simple converter and decorated my class with the appropriate attribute and it just worked. I'm not sure what the performance implications are but it really simplifies my ViewModel code.
This example below is working for me. Keep in mind I'm only converting from the simple string type used by the DefaultModelBinder so it doesn't look like it's doing much but it solves my need and taught me a new feature of the framework.
public class MyThingConverter : TypeConverter
{
public override bool CanConvertFrom(ITypeDescriptorContext context,
Type sourceType)
{
if (sourceType == typeof(string))
return true;
return base.CanConvertFrom(context, sourceType);
}
public override object ConvertFrom(ITypeDescriptorContext context,
CultureInfo culture, object value)
{
if (value is string)
return new MyThing((string)value);
return base.ConvertFrom(context, culture, value);
}
}
[TypeConverter(typeof(MyThingConverter))]
public class MyThing
{
public MyThing(string combinedValue)
{
//Split combinedValue into whatever properties I need
...
}
public override string ToString()
{
return string.Format("{0};{1}", prop1, prop2);
}
...
}
And that's it. So far it's working as expected.