<Window x:Class="WpfApp1.MainWindow"
        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:local="clr-namespace:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" Height="300" Width="500">
    <Grid>
        <Label Content="{Binding myLabel}"/>
    </Grid>
</Window>

using System.Windows;

namespace WpfApp1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        private string label = "我的标签";
        public string myLabel
        {
            get { return label; }
            set { label = value; }
        }
        public MainWindow()
        {
            InitializeComponent();
            DataContext = this;
        }
    }
}

namespace WpfApp1
{
    public class ClassA
    {
        private string label = "新建类A的标签";
        public string myLabel
        {
            get { return label; }
            set { label = value; }
        }
    }
}

    public partial class MainWindow : Window
    {
        private string label = "我的标签";
        public string myLabel
        {
            get { return label; }
            set { label = value; }
        }
        public MainWindow()
        {
            InitializeComponent();
            //DataContext = this;
            DataContext = new ClassA();
        }
    }