How to create Tag Helper whose value is a Model Property (without using @Model)?

Question

Tag helper attributes have the ability to be created in such as way that the attribute value is a model property.

An example of this is the following:

//model
public class MyModel{
    public int MyField {get;set;} = 10;
}

//in the view
@model MyModel
...
<input asp-for="MyField" />

In the above example the asp-for tag helper for the input tag directed references a property from the model. The documentation says that

The asp-for attribute value is a ModelExpression and the right hand side of a lambda expression. Therefore, asp-for="Property1" becomes m => m.Property1 in the generated code which is why you don't need to prefix with Model.

That same documentation appears to call this an "Expression name".

How do I create such a property in my own custom tag helper?

Daniel J.G. · Accepted Answer · 2017-04-08 10:56:37Z

18

Just declare the parameter in your TagHelper as of type ModelExpression and then use it to generate the contents.

For example:

public class FooTagHelper : TagHelper
{
    public ModelExpression For { get; set; }

    public override void Process(TagHelperContext context, TagHelperOutput output)
    {
        output.TagName = "div";
        output.Content.SetHtmlContent(
$@"You want the value of property <strong>{For.Name}</strong>
which is <strong>{For.Model}</strong>");
    }
}

If you use it in a view like this:

@model TestModel

<foo for="Id"></foo>
<foo for="Val"></foo>

And pass a model like new TestModel { Id = "123", Val = "some value" } then you will get the following output in your view (formatted for clarity):

<div>
    You want the value of property <strong>Id</strong>
    which is <strong>123</strong>
</div>
<div>
    You want the value of property <strong>Val</strong>
    which is <strong>some value</strong>
</div>

answered Apr 8, 2017 at 10:56

Daniel J.G.

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1 Comment

Jeroen Over a year ago

Is this possible for entire objects?

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