How to return ViewDataDictionary with Partial

I have a razor pages handler

 public IActionResult OnGetAddObjective(string newObjective)
 {
     ViewData["MyTestData"] = "hi";
     return Partial("_ObjectiveRow", new ObjectiveViewModel { ObjectiveText = newObjective });
 }

However, in the _ObjectiveRow partial, the ViewData is not being passed. I can't seem to find an overload that allows this. Which calling the partial on the main page, the ViewData from the parent is accessible.

How do I pass the ViewDataDictionary to the partial?

In ASP.NET Core Razor Pages, the Partial method itself does not support directly passing ViewData. While you cannot directly override the built-in Partial method, you can achieve similar functionality through alternative approaches to ensure that your partial views receive the necessary data.

1 Use a custom method PartialView with PartialViewResult to return a partial view:

namespace PartialDataReturnTest.Pages
{
    public class TestModel : PageModel
    {
        public void OnGet() { }

        public ObjectiveViewModel Model { get; set; }

        public IActionResult OnGetPartial(string newObjective)
        {
            var model = new TestModel()
            {
                Model = new ObjectiveViewModel
                {
                    ObjectiveText = newObjective
                }
            };

            return PartialView("_ObjectiveRow", model);
        }

        [NonAction]
        public virtual PartialViewResult PartialView(string viewName, object model)
        {
            ViewData.Model = model;
            ViewData["MyTestData"] = "hi";

            return new PartialViewResult()
            {
                ViewName = viewName,
                ViewData = ViewData,
            };
        }
    }

    public class ObjectiveViewModel
    {
        public string ObjectiveText { get; set; }
    }
}

In the Test.cshtml:

@page
@model PartialDataReturnTest.Pages.TestModel

<h1>AJAX Partial View Example</h1>
<button id="loadPartialButton">Load Partial View</button>
<div id="partialContainer"></div>

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.6.0/jquery.min.js"></script>
<script>
    $(document).ready(function() {
        $('#loadPartialButton').click(function() {
            var data = "Some text";
            $.ajax({
                url: '@Url.Page("Test", "Partial")', // Adjust URL if necessary
                type: 'GET',
                data: {
                    newObjective: data
                },
                success: function(result) {
                    $('#partialContainer').html(result);
                },
                error: function() {
                    alert('Failed to load partial view.');
                }
            });
        });
    });
</script>

Then in the _ObjectiveRow Partial:

@model PartialDataReturnTest.Pages.TestModel
@{
    var myTestData = ViewData["MyTestData"];
}

<div>
    <p>Test Data: @myTestData</p>
    <p>Objective Text: @Model.Model.ObjectiveText</p>
</div>

My Test Result:

2 Or you can use the Html.PartialAsync in the page, refer to the tutorial Access data from partial views and the code example in this link.

Sample test:

public class Test2Model : PageModel
{
    public ObjectiveViewModel Model { get; set; }
    public void OnGet()
    {
        Model = new ObjectiveViewModel()
        {
            ObjectiveText = "Test text.",
        };
    }
}

In the Test2.cshtml:

@page
@model PartialDataReturnTest.Pages.Test2Model

@{
    var viewData = new ViewDataDictionary(ViewData)
            {
                ["MyTestData"] = "Hi."
            };
}

<h1>Partial View Example2</h1>

@await Html.PartialAsync("_ObjectiveRow1", Model, viewData)

In the _ObjectiveRow1 Partial:

@model PartialDataReturnTest.Pages.Test2Model
@{
    var myTestData = ViewData["MyTestData"];
}

<div>
    <p>Test Data: @myTestData</p>
    <p>Objective Text:@Model.Model.ObjectiveText </p>
</div>

The Test2 Result: