Custom hidden field for Sitecore Forms isn't maintaining value on submit

Question

I have a custom hidden field. The model uses a rendering parameter on the page to set the value of the hidden field, and the same form is used on multiple pages which is why I'm using a rendering parameter on the page to set the value instead of a field on the form field item. When I inspect the loaded page, I can see that value.

[Serializable]
public class CampaignIdFieldModel : InputViewModel<string>
{
    public string CampaignId = "";
    public string PlaceholderText = "";
    public string FieldName = "";

    protected override void InitItemProperties(Item item)
    {
        Assert.ArgumentNotNull(item, "item");
        base.InitItemProperties(item);

        FieldName = item.Name;

        if (Sitecore.Context.PageMode.IsExperienceEditor || Sitecore.Context.Request.QueryString["sc_formmode"] != null)           
        {
            PlaceholderText = "(Campaign Id)";
        }
        else if (!HttpContext.Current.Request.Url.ToString().Contains("form"))
        {
            var renderingRef = Sitecore.Context.Item?.Visualization.GetRenderings(Sitecore.Context.Device, false).FirstOrDefault(x => x.WebEditDisplayName == "Contact Us Form");

            if (renderingRef == null) return;

            var rendering = new Rendering
            {
                RenderingItemPath = renderingRef.RenderingID.ToString(),
                Parameters = new RenderingParameters(renderingRef.Settings.Parameters),
                DataSource = renderingRef.Settings.DataSource
            };

            var campaignId = rendering.Parameters["CampaignId"];

            CampaignId = campaignId;
        }
    }
}

---

@using System.Web.Mvc.Html
@model VelirSite.Feature.Forms.Models.CampaignIdFieldModel

@Model.PlaceholderText
<input type="hidden" id="@Html.IdFor(m => Model.CampaignId)" name="@Html.NameFor(m => Model.CampaignId)" value="@Model.CampaignId" />

However, when the form submits, the campaignid field is there in the data but its value is null.

Notably, when debugging the code while submitting the form, it hits InitItemProperties for CampaignIdFieldModel again and if I inspect this.Value, it's null. So it seems like even though the field on the form has a value, when the form is submitted the field reinitializes and sets the value to null rather than passing in the value that's on the form. What am I doing wrong in my custom hidden field?

EDIT:

I have also tried putting this.Value = CampaignId; in InitItemProperties, it doesn't make a difference. When I submit the form, it reinitializes everything and both CampaignId and Value are reset to null

Answer 1

Value and CampaignId are different variables. 'Value' comes from the InputViewModel class and CampaignId comes from the custom model you created. When you inspect it on the Submit action you're looking into the 'Value' instead of 'CampaignId', but you're not populating the Value variable. You need to decide if you want to use Value or CampaignId, but in case Value is good, you should populate it such as:

protected override void InitItemProperties(Item item)
    {
        <!-- your code -->
        this.Value = campaignId;
    }

Don't forget to change the View (.cshtml) in case you switch to Value.