Custom html helper with LINQ expression in MVC 3

Question

I'm building a custom HTML helper with an expression to draw a tag cloud where the data that goes into the tag cloud comes from the expression. I'll let the code do the talking here:

View Model

public class ViewModel
{
    public IList<MyType> MyTypes { get; set; }
    public IList<MyOtherType> MyOtherTypes { get; set; }
}

View

<div>
    @Html.TagCloudFor(m => m.MyTypes)
</div>

<div>
    @Html.TagCloudFor(m => m.MyOtherTypes)
</div>

Helper

public static MvcHtmlString TagCloudFor<TModel, TProperty>(this HtmlHelper<TModel> helper, Expression<Func<TModel, TProperty>> expression) where TProperty : IList<MyType> where TProperty : IList<MyOtherType>
{
    // So my actual question here is: how do I get my IList<TProperty> collection 
    // so I can iterate through and build my HTML control
}

I've had a quick look around and have done the usual Google searches, but I can't quite seem to find that specific answer. I assume it's somewhere in the region of expression.Compile().Invoke() but I'm not sure what the correct parameters to pass are.

I should also mention that MyType and MyOtherType have a similar property of Id but there's no inheritance here, they are completely separate objects hence I'm constraining my TProperty as IList<MyType> and IList<MyOtherType>. Am I heading down the wrong path here, I feel like this should be obvious, but my brain isn't playing.

If you have to constraints, one where TProperty: IList<MyType> and where TProperty: IList<MyOtherType>, and MyType and MyOtherType are incompatible, than it's not going to work. Both constraints are applied. — Maarten
– Maarten, Commented Aug 23, 2012 at 13:03
@Maarten: yes, I thought this too I've removed the 2nd constraint for now to get the list working, and then will re-look at my inheritance structure. These objects are POCO entities for Entity Framework so it's looking like I'll need a common ancestor for each. — Paul Aldred-Bann
– Paul Aldred-Bann, Commented Aug 23, 2012 at 13:10

Pierluc SS · Accepted Answer · 2012-08-23 13:08:06Z

9

The following should do it:

public static MvcHtmlString TagCloudFor<TModel , TProperty>( this HtmlHelper<TModel> helper , Expression<Func<TModel , TProperty>> expression )
        where TProperty : IList<MyType>, IList<MyOtherType> {

        //grab model from view
        TModel model = (TModel)helper.ViewContext.ViewData.ModelMetadata.Model;
        //invoke model property via expression
        TProperty collection = expression.Compile().Invoke(model);

        //iterate through collection after casting as IEnumerable to remove ambiguousity
        foreach( var item in (System.Collections.IEnumerable)collection ) {
            //do whatever you want
        }

    }

edited Aug 23, 2012 at 13:08

answered Aug 23, 2012 at 12:51

Pierluc SS

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5 Comments

Maarten Over a year ago

If MyType and MyOtherType are incompatible, than the usage of your htmlhelper won't compile since no type will satisfy both your constraints. I think you need two html helpers, on for MyType, one for MyOtherType. The generic argument TProperty is then obsolete.

Paul Aldred-Bann Over a year ago

@Burnzy: that looks really promising, however I'm getting the following errors on the line expression.Compile().Invoke(model) - cannot convert from 'object' to 'TModel' and Delegate 'System.Func<TModel,TProperty>' has some invalid arguments

Pierluc SS Over a year ago

@Maarten I believe TProperty wont be explicitly defined, it'll be implicitly defined by the expression property type.

Pierluc SS Over a year ago

@Terric I believe you had to cast it. Look at newer edit. You posted right before I edited :)

Paul Aldred-Bann Over a year ago

@Burnzy: bingo! Yep that's done it, thanks for that much appreciated!

Bob Vale · Accepted Answer · 2012-08-23 13:23:26Z

1

How about....

public static MvcHtmlString TagCloudFor<TModel , TItemType>( this HtmlHelper<TModel> helper , Expression<Func<TModel , IEnumerable<TItemType>>> expression ) 
// optional --- Where TItemType : MyCommonInterface
 { 

        TModel model = (TModel)helper.ViewContext.ViewData.ModelMetadata.Model; 
        //invoke model property via expression 
        IEnumerable<TItemType> collection = expression.Compile().Invoke(model); 

        foreach( var item in collection ) { 
            //do whatever you want 
        } 

    }

answered Aug 23, 2012 at 13:23

Bob Vale

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