Partial Tag Helper in ASP.NET Core

By Scott Addie

For an overview of Tag Helpers, see Tag Helpers in ASP.NET Core.

View or download sample code (how to download)

Overview

The Partial Tag Helper is used for rendering a partial view in Razor Pages and MVC apps. Consider that it:

  • Requires ASP.NET Core 2.1 or later.
  • Is an alternative to HTML Helper syntax.
  • Renders the partial view asynchronously.

The HTML Helper options for rendering a partial view include:

The Product model is used in samples throughout this document:

namespace TagHelpersBuiltIn.Models
{
    public class Product
    {
        public int Number { get; set; }

        public string Name { get; set; }

        public string Description { get; set; }
    }
}

An inventory of the Partial Tag Helper attributes follows.

name

The name attribute is required. It indicates the name or the path of the partial view to be rendered. When a partial view name is provided, the view discovery process is initiated. That process is bypassed when an explicit path is provided. For all acceptable name values, see Partial view discovery.

The following markup uses an explicit path, indicating that _ProductPartial.cshtml is to be loaded from the Shared folder. Using the for attribute, a model is passed to the partial view for binding.

<partial name="Shared/_ProductPartial.cshtml"
         for="Product" />

for

The for attribute assigns a ModelExpression to be evaluated against the current model. A ModelExpression infers the @Model. syntax. For example, for="Product" can be used instead of for="@Model.Product". This default inference behavior is overridden by using the @ symbol to define an inline expression. The for attribute can't be used with the model attribute.

The following markup loads _ProductPartial.cshtml:

<partial name="_ProductPartial"
         for="Product" />

The partial view is bound to the associated page model's Product property:

using Microsoft.AspNetCore.Mvc.RazorPages;
using TagHelpersBuiltIn.Models;

namespace TagHelpersBuiltIn.Pages
{
    public class ProductModel : PageModel
    {
        public Product Product { get; set; }

        public void OnGet()
        {
            Product = new Product
            {
                Number = 1,
                Name = "Test product",
                Description = "This is a test product"
            };
        }
    }
}

model

The model attribute assigns a model instance to pass to the partial view. The model attribute can't be used with the for attribute.

In the following markup, a new Product object is instantiated and passed to the model attribute for binding:

<partial name="_ProductPartial"
         model='new Product { Number = 1, Name = "Test product", Description = "This is a test" }' />

view-data

The view-data attribute assigns a ViewDataDictionary to pass to the partial view. The following markup makes the entire ViewData collection accessible to the partial view:

@{
    ViewData["IsNumberReadOnly"] = true;
}

<partial name="_ProductViewDataPartial"
         for="Product"
         view-data="ViewData" />

In the preceding code, the IsNumberReadOnly key value is set to true and added to the ViewData collection. Consequently, ViewData["IsNumberReadOnly"] is made accessible within the following partial view:

@model TagHelpersBuiltIn.Models.Product

<div class="form-group">
    <label asp-for="Number"></label>
    @if ((bool)ViewData["IsNumberReadOnly"])
    {
        <input asp-for="Number" type="number" class="form-control" readonly />
    }
    else
    {
        <input asp-for="Number" type="number" class="form-control" />
    }
</div>
<div class="form-group">
    <label asp-for="Name"></label>
    <input asp-for="Name" type="text" class="form-control" />
</div>
<div class="form-group">
    <label asp-for="Description"></label>
    <textarea asp-for="Description" rows="4" cols="50" class="form-control"></textarea>
</div>

In this example, the value of ViewData["IsNumberReadOnly"] determines whether the Number field is displayed as read only.

Migrate from an HTML Helper

Consider the following asynchronous HTML Helper example. A collection of products is iterated and displayed. Per the PartialAsync method's first parameter, the _ProductPartial.cshtml partial view is loaded. An instance of the Product model is passed to the partial view for binding.

@foreach (var product in Model.Products)
{
    @await Html.PartialAsync("_ProductPartial", product)
}

The following Partial Tag Helper achieves the same asynchronous rendering behavior as the PartialAsync HTML Helper. The model attribute is assigned a Product model instance for binding to the partial view.

@foreach (var product in Model.Products)
{
    <partial name="_ProductPartial" model="product" />
}

Additional resources