C#

Hi
I'm trying to create a "FinalDocument" list from another list "OriginDocument" 
The problem is that the "FinalDocument" does not have the correct Client Code.
So I want to get this Client Code from a third List "clients", for that I use the "TaxClienteID" field

It would be something like this:

List<FinalDocument> finalDocument = originDocument.Select(p => new FinalDocument()
{
documentcode = p.documentcode,
date = p.date,
...
codcliente = clients.codcliente where (TaxClienteID = clients.TaxClienteID)
})

I'm not getting it, the best I could get was with this:

List<FinalDocument> finalDocument = originDocument.Select(p => new FinalDocument()
{
documentcode = p.documentcode,
date = p.date,
...
clientcod = clients.Select(j => j.clientcod).Where(u => clients.Select(c => c.TaxClienteID.ToString()).Contains(p.TaxClienteID)).ToString()
}

But I get this in the customer clientcod field:
"System.Linq.Enumerable+WhereEnumerableIterator`1[System.String]"
and I don't know how to extract the value.

I'm sure there is a much simpler way, but I can't find it.
someone can help me?

<br />
Mixed mode assembly is built against version 'v2.0.50727' of runtime and cannot be loaded in the 4.0 runtime without additional configuration.<br />

<?xml version="1.0"?>
<configuration>

  <startup useLegacyV2RuntimeActivationPolicy="true">

    <!-- 4.0 RTM -->
    <supportedRuntime version="v4.0.30319"/>

    <!-- 2.0 RTM -->
    <supportedRuntime version="v2.0.50727"/>

    
  </startup>

</configuration>

GetItemChildAsync

[HttpGet]
public async Task<IActionResult> GetSpecialPostsList()
{
    var posts = await myContext.Posts.Where(x => x.IsSpecialPost == true && x.IsActive == true).ToListAsync();
    if (posts != null)
    {
        var revisedPost = new RevisedPost();
        var revisedPostList = new List<RevisedPost>();
        foreach (var post in posts)
        {
            revisedPost.CreationDate = DateConverter.GregToPersianDate(post.CreationDate.ToShortDateString());
            revisedPost.Description = post.Description;
            revisedPost.ImageFileName = post.ImageFileName;
            revisedPost.IsActive = post.IsActive;
            revisedPost.IsMainPost = post.IsMainPost;
            revisedPost.IsSpecialPost = post.IsSpecialPost;
            revisedPost.Title = post.Title;
            revisedPost.Id = post.Id;

            revisedPostList.Add(revisedPost);
        }
        return Ok(revisedPostList);
    }
    return NotFound("Data not found!");
}