Asp.net Core Mvc6 TempData List<string> is null

Question

I am doing an Asp.Net Core MVC6 App.

I am using TempData to use it from the View

I am using like this.

private async void CreateClaimsByUserRole(string role, string usertType)
        {
            List<string> permission = await _iUIConfig.CreateClaimsByUserRole(role, usertType);
            TempData["MyList"] = permission;
            TempData.Keep();
        }

I am saving a List<string>

Here is some other functions

  public async Task<List<string>> CreateClaimsByUserRole(string role, string usertType)
        {
            List<RolesAccessModel>? oResponse = await GetRolesPerApplication();
            List<string> permissions = TransformRowsIntoPermissions(oResponse,role, usertType);
            return permissions;
        }

And

private List<string> TransformRowsIntoPermissions(List<RolesAccessModel>? rows, string role, string usertType)
        {
             List<string> permissionList = new();
            if(rows!=null)
            { 
                foreach (RolesAccessModel row in rows)
                {
                    if (row.Roles!=string.Empty && row.Roles != null && !row.Roles.Contains(role))
                        continue;
                    if (row.UserType != string.Empty && row.UserType != null && !row.UserType.Contains(usertType))
                        continue;

                       // if we hget here we have a match
                       if (!permissionList.Contains(row.EventName))
                        permissionList.Add(row.EventName);
                }
            }
            return permissionList;
        }

As it says here

I can do this in the same Method and works fine..

List<string> SomeList = TempData["MyList"] as List<string>;

But if I want to retrieve the data in another Method, It is null..

The only way to retrieve data is using

var SomeList = TempData["MyList"] ;

I need to retrieve the data from the View, I have the same problem

 @if (TempData["Claims"] != null)
    {
        var claims = TempData["MyList"] as List<string>;
       
            @foreach (string permission in claims)
            {
                <p>@permission</p>
            }
    }

Where var claims = TempData["MyList"] as List<string>; is null

Reading this page, I also add in Program.cs

builder.Services.Configure<CookieTempDataProviderOptions>(options => {
    options.Cookie.IsEssential = true;
});

But still does not work.

What I am missing?

Thanks

It works now if I use string[] SomeList = TempData["MyList"] as string[]; I dont know why... But for your question.. I do not use it anywhere else.. Thanks — Diego
– Diego, Commented Dec 13, 2022 at 21:21

Ruikai Feng · Accepted Answer · 2022-12-15 01:16:43Z

It was related a serialize/deserialize issue,If you are not trying with simple types,I recomand you to serialize it before adding it to the dictionary

If you try as below :

List<decimal> strlist = new List<decimal> { 1, 2, 3 };
TempData["SomeList"] = strlist;

You would got an error:

It indicates your TempDataDictionary would be serialized before appended to cookie

For Complex types, you could try as below:

 List<string> strlist = new List<string> { "q", "w", "e" };
 var jsonstr= JsonSerializer.Serialize(strlist);                
 TempData["SomeList2"] = jsonstr;

deserialize:

var tempobj2 = TempData["SomeList2"];
var list = JsonSerializer.Deserialize<List<string>>(tempobj2 as string);

and You could cast the value to string[] indicates it was serialized/deserialized like below:

List<string> strlist = new List<string> { "q", "w", "e" };
var jsonStr= JsonSerializer.Serialize(strlist);
var obj = JsonSerializer.Deserialize<string[]>(jsonStr) as object;

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