The type arguments cannot be inferred from the usage. Try specifying the type arguments explicitly

Question

Could someone please clarify something for me. In my ASP.NET MVC 2 app, I've got a BaseViewModel class which includes the following method:

public virtual IDictionary<string, object> GetHtmlAttributes<TModel, TProperty>
                        (Expression<Func<TModel, TProperty>> propertyExpression)
{
    return new Dictionary<string, object>();
}

The idea being that each child viewmodel can override this method and provide a suitable set of html attributes, based on some logic, to be rendered in the view:

<%: Html.TextBoxFor(model => model.MyProperty, Model.GetHtmlAttributes
                                                 (model => model.MyProperty)) %>

However when used as in the line above, I get a compilation error when I hit the view:

The type arguments for method '...BaseViewModel.GetHtmlAttributes<TModel,TProperty> Expression<System.Func<TModel,TProperty>)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

I have to do the following:

<%: Html.TextBoxFor(model => model.MyProperty, Model.GetHtmlAttributes
                             <ChildModel, string>(model => model.MyProperty)) %>

I'm just looking for some clarity as to how it tries to infer the type, it has no problem doing so in the HtmlHelper/TextBoxFor extension method?

Is it because HtmlHelper in the view will automatically be for the same type as is specified in the ViewUserControl at the top of the page, whereas my code can be for any type inheriting from BaseViewModel? Is is possible to write this in such a way that it can infer my model/property types?

Answer 1

I know this question already has an accepted answer, but for me, a .NET beginner, there was a simple solution to what I was doing wrong and I thought I'd share.

I had been doing this:

@Html.HiddenFor(Model.Foo.Bar.ID)

What worked for me was changing to this:

@Html.HiddenFor(m => m.Foo.Bar.ID)

(where "m" is an arbitrary string to represent the model object)

Answer 2

In your example, the compiler has no way of knowing what type should TModel be. You could do something close to what you are probably trying to do with an extension method.

static class ModelExtensions
{
   public static IDictionary<string, object> GetHtmlAttributes<TModel, TProperty>
      (this TModel model, Expression<Func<TModel, TProperty>> propertyExpression)
   {
       return new Dictionary<string, object>();
   }
}

But you wouldn't be able to have anything similar to virtual, I think.

EDIT:

Actually, you can do virtual, using self-referential generics:

class ModelBase<TModel>
{
    public virtual IDictionary<string, object> GetHtmlAttributes<TProperty>
        (Expression<Func<TModel, TProperty>> propertyExpression)
    {
        return new Dictionary<string, object>();
    }
}

class FooModel : ModelBase<FooModel>
{
    public override IDictionary<string, object> GetHtmlAttributes<TProperty>
        (Expression<Func<FooModel, TProperty>> propertyExpression)
    {
        return new Dictionary<string, object> { { "foo", "bar" } };
    }
}

Answer 3

I had this same problem, my solution:
In the web.config file :

<compilation debug="true>
had to be changed to
<compilation debug="true" targetFramework="4.0">

Answer 4

This error is also related with a cache issue.

I had the same problem and it was solved just cleaning and building the solution again.

Answer 5

C# compiler have only lambda

arg => arg.MyProperty

for infer type of arg(TModel) an type of arg.MyProperty(TProperty). It's impossible.

Answer 6

I was actually searching for a similar error and Google sent me here to this question. The error was:

The type arguments for method 'IModelExpressionProvider.CreateModelExpression(ViewDataDictionary, Expression>)' cannot be inferred from the usage

I spent maybe 15 minutes trying to figure it out. It was happening inside a Razor .cshtml view file. I had to comment portions of the view code to get to where it was barking since the compiler didn't help much.

<div class="form-group col-2">
    <label asp-for="Organization.Zip"></label>
    <input asp-for="Organization.Zip" class="form-control">
    <span asp-validation-for="Zip" class="color-type-alert"></span>
</div>

Can you spot it? Yeah... I re-checked it maybe twice and didn't get it at first!

See that the ViewModel's property is just Zip when it should be Organization.Zip. That was it.

So re-check your view source code... :-)

Answer 7

In case it helps, I've ran into this problem when passing null into a parameter for a generic TValue, to get around this you have to cast your null values:

(string)null

(int)null

etc.

Answer 8

You are referring to the type rather than the instance. Make 'Model' lowercase in the example in your second and fourth code samples.

Model.GetHtmlAttributes

should be

model.GetHtmlAttributes