I often find the need to set values in ViewData when calling partials that are reused across the website. It might not make sense always to make that value part of the Model.

You can do so currently by doing…

<partial name="_ProfileImage" model="image"
	view-data="@(new ViewDataDictionary(ViewData) { { "returnUrl", Context.Request.Path } })"/>

There is already an issue reported on aspnetcore repository to make this experience better.

<partial name="view" view-data-additionalItem="abc" />

A similar experience is already in use when using <a> anchor element and setting route values.

<a asp-controller="User" asp-action="Profile" asp-route-id="123" asp-route-foo="bar">Profile</a>

While we are waiting for the issue to be closed, it is possible to extend the Partial Tag Helper ourselves.

using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.AspNetCore.Mvc.ViewFeatures;
using Microsoft.AspNetCore.Razor.TagHelpers;

namespace Example.TagHelpers;

[HtmlTargetElement("partial", Attributes = viewDataAttributeNamePrefix + "*")]
public class ViewDataTagHelper : TagHelper
{
    private const string viewDataAttributeNamePrefix = "ex-view-data-";

    /// <inheritdoc />
    public override int Order => -1000;

    [HtmlAttributeNotBound, ViewContext]
    public required ViewContext ViewContext { get; set; }

    public override Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
    {
        var attributes = context.AllAttributes.Where(attr =>
            attr.Name.StartsWith(viewDataAttributeNamePrefix)
        );

        foreach (var attribute in attributes)
        {
            var key = attribute.Name.Substring(viewDataAttributeNamePrefix.Length);
            var value = attribute.Value;

            ViewContext.ViewData[key] = value;
        }

        return Task.CompletedTask;
    }
};

Notice the attribute name cannot conflict with view-data, and i had to prefix it with ex-.

Usage:

<partial name ="_Profile" ex-view-data-id="123" ex-view-data-returnUrl="/dashboard" />

We can have multiple values set on ViewData more succinctly.

Don’t forget to add your tag helper to the _ViewImports.cshtml file.

@addTagHelper *, ExampleAssemblyName