Unable to map namespaces to xmlns namespaces in xaml

Question

I'm following a tutorial on MVVM and am having some issues.

I created 4 folders which represent 4 different namespaces. In my MainPage.xaml I'm making a reference to these namespaces using the following code:

xmlns:viewModels="using:ViewModels"
xmlns:converters="using:Converters"

These properties are in the Page attributes.

Next, I need to use those using the following code:

<Page.Resources>
   <converters:ObjectExistsToVisible x:Key="ObjectExistsToVisible" />
</Page.Resources>


<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="Auto"/>
        <ColumnDefinition Width="Auto"/>
    </Grid.ColumnDefinitions>

    <StackPanel Grid.Column="0" Orientation="Vertical">
        <ListView  x:Name="MainList" 
          ItemsSource="{x:Bind Organization.People, Mode=OneWay}"  
          SelectedIndex="{x:Bind Organization.SelectedIndex, Mode=TwoWay}" 
          MinWidth="250" Margin="5">
            <ListView.ItemTemplate>
                <!-- The error is with x:DataType="" -->
                <DataTemplate x:DataType="viewModels:PersonViewModel" >
                    <TextBlock Text="{x:Bind Name, Mode=OneWay}" />
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
        <Button Content="Add" Click="{x:Bind  Organization.Add}" Margin="5"/>
    </StackPanel>

    <StackPanel Grid.Column="2" Orientation="Vertical">
        <TextBox 
          Text="{x:Bind Organization.SelectedPerson.Name, Mode=TwoWay, FallbackValue=''}" 
          Margin="5" />
        <TextBox 
          Text="{x:Bind Organization.SelectedPerson.Age, Mode=TwoWay, FallbackValue='0'}" 
          Margin="5" />
        <Button Content="Delete" Click="{x:Bind Organization.Delete}" Margin="5" />
    </StackPanel>
</Grid>

The tutorial can be found at: this link

The problem is, I'm getting the following errors:

The name "ObjectExistsToVisible" does not exist in the namespace "using:Converters"

The name "PersonViewModel" does not exist in the namespace "using:ViewModels"

I'm pretty sure that they do exist.

As requested:

PersonViewModel:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using Data;

namespace ViewModels
{
    public class PersonViewModel : NotificationBase<Person>
    {
        public PersonViewModel(Person person = null) : base(person) { }
        public String Name
        {
            get { return This.Name; }
            set { SetProperty(This.Name, value, () => This.Name = value); }
        }
        public int Age
        {
            get { return This.Age; }
            set { SetProperty(This.Age, value, () => This.Age = value); }
        }
    }
}

Answer 1

After a great comment from Will, the solution is as follows:

First, clean the solution via Build > Clean Solution . Next, rebuild the solution via Build > Rebuild Solution .

This fixed the errors for me.