ViewBag.users =  from c in userModel.userCollection.AsQueryable<User>() where 
             c.UserName ==(TextBox1) select c;    

               foreach(User u in ViewBag.users)
            {
              ViewBag.message = messageModel.findByUser(u.LastName);          
            }

@foreach (var user in ViewBag.users)
            {
                <tr>                 
                    <td>
                        <label><b>Id: </b></label>@user.Id <br>
                        <label><b>Username: </b></label> @user.UserName <br>
                        <label><b> Имя: </b></label>@user.FirstName   @user.LastName<br>
                        <label><b>Сообщения:</b> </label>ViewBag.message

                    </td>

@foreach (var user in ViewBag.users)
            {
                <tr>                 
                    <td>
                        <label><b>Id: </b></label>@user.Id <br>
                        <label><b>Username: </b></label> @user.UserName <br>
                        <label><b> Имя: </b></label>@user.FirstName   @user.LastName<br>
                        <label><b>Сообщения:</b> </label>   @foreach (var mes in ViewBag.message)
                        {
                            @mes.Text
}

                    </td>

  public List<Message> findByUser(string val)
        {
            var bQuery = "{ 'sender_lastname' : '" + val + "'}";

            var filter = MongoDB.Bson.Serialization.BsonSerializer.Deserialize<BsonDocument>(bQuery);
            var result = messageCollection.FindSync(filter).ToList();
          
            return result;
        }

 public List<Message> findByUser(string val)
        {
        
           var filter = new BsonDocument("lastname", val);
           return  messageCollection.Find(filter).ToList();
            
        }

var users = from c in userModel.userCollection.AsQueryable<User>()
                            where c.UserName == (TextBox1)
                            select c;

                foreach (User u in users)
                {                  
                    u.Phone = "999555";
                    u.LastName = "familia";
                    u.FirstName = "name";                    
                }
         
               return View(users);