<UserControl x:Class="TestApp.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:my="clr-namespace:TestApp"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400">

    <UserControl.Resources>
        <my:User x:Name="CurrentUser" x:Key="CurrentUser" />
    </UserControl.Resources>
    
    <StackPanel x:Name="LayoutRoot">
        <TextBox x:Name="_loginTextBox" 
                 Text="{Binding Login, Mode=TwoWay, Source={StaticResource CurrentUser}}" 
                 Width="100" Height="30" />
    </StackPanel>

</UserControl>

using System.Windows.Controls;

namespace TestApp
{
    public partial class MainPage : UserControl
    {
        public MainPage()
        {
            InitializeComponent();
            this._loginTextBox.Text = "asdfasf"; // _loginTextBox существует
            MessageBox.Show(this.CurrentUser.ToString()); // CurrentUser НЕ существует !!!
        }

    }

    public class User : INotifyPropertyChanged
    {
        private string _login;
        private string Login
        {
            get { return this._login; }
            set
            {
                this._login = value;
                this.OnPropertyChanged("Login");
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
        private void OnPropertyChanged(string propName)
        {
            if (this.PropertyChanged != null)
                this.PropertyChanged(this, new PropertyChangedEventArgs(propName));
        }

        public override string ToString()
        {
            return this._login;
        }
    }
}