CODEWITHSUNDEEP
c#asp.net-coreasp.net-core-tag-helpers
Larry Lustig
Larry Lustig

Reputation: 51000

TagHelperAttribute.Value is type string, not type ModelExpression, for novel tag

I have a correctly working tag helper for the <select> element which contains the following working code:

TagHelperAttribute forAttribute;
if (!context.AllAttributes.TryGetAttribute("asp-for", out forAttribute))
{
    throw new Exception("No asp-for attribute found.");
}

var forInfo = (Microsoft.AspNetCore.Mvc.ViewFeatures.ModelExpression)forAttribute.Value;

I have the same code in a different tag helper, for a novel element I'm calling <date-picker>. In this second tag helper, the cast to ModelExpression fails because forAttribute.Value is, in fact, not a ModelExpression but rather a string (it is the property name, "DueDate", to which I'm trying to bind the tag).

It seems my novel date-picker tag is not aware that the asp-for value should be applied to the Razor page model.

How can I ensure that my date-picker receives a correct ModelExpression on which to base my output?

Upvotes: 0

Views: 1246

Answers (2)

Rafi
Rafi

Reputation: 1

create the attribute TAG and property as shown below:

[HtmlAttributeName("asp-for")]
public ModelExpression AspFor { get; set; }

Access the attribute model value,name etc. as in the following way:

AspFor.Name
AspFor.Model

Upvotes: 0

M&#233;toule
M&#233;toule

Reputation: 14482

To ensure your attribute has the proper binding, you need to define a property in your tag helper class, with the HtmlAttributeName attribute. For example:

[HtmlAttributeName("asp-for")]
public ModelExpression For { get; set; }

The reason why the attribute is required is that the HtmlAttributeNameAttribute class does a lot behind the scene to bind the proper value. You can see how the value is bound on github.

Also, this simplifies how you access the value of the attribute, as you don't need to go through the entire list. So instead of this: 

TagHelperAttribute forAttribute;
if (!context.AllAttributes.TryGetAttribute("asp-for", out forAttribute))
{
    throw new Exception("No asp-for attribute found.");
}

var forInfo = (Microsoft.AspNetCore.Mvc.ViewFeatures.ModelExpression)forAttribute.Value;

You can write :

// you can call the For property
var forInfo = For;

Upvotes: 1

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