I find myself often needing to pass view data into the partial views and I find it quite cumbersome.

I find the below code example to be rather cumbersome, and also it is not obvious what relation the ViewData and the partial have.

@{
    ViewData["flavor"] = "raspberry"
}
<partial name="Pie" />

I too would much prefer:

<partial name="Pie" view-data-flavor="raspberry" />

Also, doing it this way:

@{
    ViewData["flavor"] = "raspberry"
}
<partial name="Pie" />

pollutes the parent ViewData as opposed to only affecting the partial view.

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should this be very easy ?
the partial tag helper can access parent ViewData and add an new IDictionary<string,object> inside partial tag helper ad attributes helper.
OnProcess first populate the parent ViewData to an private IDictionary<string,object> , and then add the attributes IDictionary<string,object>

and after process, clear the private IDictionary<string,object> and the attributes IDictionary<string,object>

We can get around this for the time being by implementing a custom TagHelper.

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 ="_MyPartial" ex-view-data-id="123" ex-view-data-returnUrl="/dashboard" />