Store complex object in TempData

I've been trying to pass data to an action after a redirect by using TempData like so:

if (!ModelState.IsValid)
{
	TempData["ErrorMessages"] = ModelState;
	return RedirectToAction("Product", "ProductDetails", new { code = model.ProductCode });
}

but unfortunately it's failing with the following message:

> 'System.InvalidOperationException The > Microsoft.AspNet.Mvc.SessionStateTempDataProvider' cannot serialize an > object of type 'ModelStateDictionary' to session state.'

I've found an issue in the MVC project in Github, but while it explains why I'm getting this error, I can't see what would be a viable alternative.

One option would be to serialize the object to a json string and then deserialize it back and reconstruct the ModelState. Is this the best approach? Are there any potential performance issues I need to take into account?

And finally, are there any alternatives for either serializing complex object or using some other pattern that doesn't involve using TempData?

You can create the extension methods like this:

public static class TempDataExtensions
{
	public static void Put<T>(this ITempDataDictionary tempData, string key, T value) where T : class
	{
		tempData[key] = JsonConvert.SerializeObject(value);
	}

	public static T Get<T>(this ITempDataDictionary tempData, string key) where T : class
	{
		object o;
		tempData.TryGetValue(key, out o);
		return o == null ? null : JsonConvert.DeserializeObject<T>((string)o);
	}
}

And, you can use them as follows:

Say objectA is of type ClassA. You can add this to the temp data dictionary using the above mentioned extension method like this:

TempData.Put("key", objectA);

And to retrieve it you can do this:

var value = TempData.Get<ClassA>("key") where value retrieved will be of type ClassA

I can not comment but I added a PEEK as well which is nice to check if there or read and not remove for the next GET.

public static T Peek<T>(this ITempDataDictionary tempData, string key) where T : class
{
    object o = tempData.Peek(key);
    return o == null ? null : JsonConvert.DeserializeObject<T>((string)o);
}

Example

var value = TempData.Peek<ClassA>("key") where value retrieved will be of type ClassA

Using System.Text.Json in .Net core 3.1 & above

using System.Text.Json;

    public static class TempDataHelper
    {
        public static void Put<T>(this ITempDataDictionary tempData, string key, T value) where T : class
        {
            tempData[key] = JsonSerializer.Serialize(value);
        }

        public static T Get<T>(this ITempDataDictionary tempData, string key) where T : class
        {
            tempData.TryGetValue(key, out object o);
            return o == null ? null : JsonSerializer.Deserialize<T>((string)o);
        }

        public static T Peek<T>(this ITempDataDictionary tempData, string key) where T : class
        {
            object o = tempData.Peek(key);
            return o == null ? null : JsonSerializer.Deserialize<T>((string)o);
        }
    }