How can I correctly bind a ComboBox in my custom Control to its dependency property in a WPF C# application?

Question

I have a ComboBox in my custom Control, and I want to bind its "Text" and "Converter" properties to their dependency properties correctly so that I can reuse the custom Control. In some instances, I want to use ComboBoxItems as the source. For example, I have two instances of my CustomControl, one for "StudentName" and the other for "Student's BloodGroup." For StudentName, I want to bind it to StudentName, and for the BloodGroup, I want to use <ComboBoxItem>. How can I achieve that? You can find a sample of my code on Github here:

https://github.com/MeshkaM/FloatingLabelComboBox

sample

https://github.com/MeshkaM/FloatingLabelComboBox

sample

Accepted Answer

HI,
if you want use converters in your approach you can try following demo. In this demo ComboBoxItems are marked (BackGround) if BloodGroup is not set.

XAML:

Code:

using System;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Globalization;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Media;

namespace WpfApp128
{
	public class ViewModel : INotifyPropertyChanged
	{
		public event PropertyChangedEventHandler PropertyChanged;
		private void OnPropertyChanged([CallerMemberName] String propertyName = "")
			=> PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));

		// contructor: load StudentList 
		public ViewModel()
		{
			for (int i = 1; i <= 10; i++)
				_studentList.Add(new Student() { StudentId = i, StudentName = $"Student {i}" });
			_cvsStudentList.Source= _studentList;
		}

		internal ObservableCollection _studentList { get; set; } = new ObservableCollection();
		private CollectionViewSource _cvsStudentList = new CollectionViewSource();
		public ICollectionView StudentsView { get => _cvsStudentList.View; }
		public ObservableCollection BloodGroups { get; set; } = new ObservableCollection() { "A", "B", "AB", "0" };

		private Student _student;
		public Student Student
		{
			get => _student;
			set
			{
				_student = value;
				OnPropertyChanged();
				StudentsView.Refresh();	
			}
		}

	}
	public class Student : INotifyPropertyChanged
	{
		public event PropertyChangedEventHandler PropertyChanged;

		private void OnPropertyChanged([CallerMemberName] String propertyName = "") =>
			PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));

		public int StudentId { get; set; }

		private string _studentName;
		public string StudentName
		{
			get => _studentName;
			set
			{
				_studentName = value;
				OnPropertyChanged();
			}
		}

		private string _bloodgroup;
		public string BloodGroup
		{
			get => _bloodgroup;
			set
			{
				_bloodgroup = value;
				OnPropertyChanged();
			}
		}
	}
	public class FloatingLabelComboBox : Control, INotifyPropertyChanged
	{
		static FloatingLabelComboBox()
		{
			DefaultStyleKeyProperty.OverrideMetadata(typeof(FloatingLabelComboBox), new FrameworkPropertyMetadata(typeof(FloatingLabelComboBox)));
		}
		public FloatingLabelComboBox() => this.Loaded += FloatingLabelComboBox_Loaded;

		private void FloatingLabelComboBox_Loaded(object sender, RoutedEventArgs e)
		{
			Style st = new Style(typeof(ComboBoxItem));
			Binding b = new Binding() { Converter = CBConverter };
			st.Setters.Add(new Setter(ComboBoxItem.BackgroundProperty, b));
			if (CBConverter != null) this.Resources.Add(typeof(ComboBoxItem), st);
		}

		public static readonly DependencyProperty LabelTextProperty =
			 DependencyProperty.Register("LabelText", typeof(string),
				 typeof(FloatingLabelComboBox), new PropertyMetadata(string.Empty));
		public string LabelText
		{
			get { return GetValue(LabelTextProperty).ToString(); }
			set { SetValue(LabelTextProperty, value); }
		}

		public static readonly DependencyProperty LabelForegroundProperty =
				DependencyProperty.Register("LabelForeground", typeof(Brush),
					typeof(FloatingLabelComboBox), new PropertyMetadata(Brushes.AliceBlue));
		public Brush LabelForeground
		{
			get { return (Brush)GetValue(LabelForegroundProperty); }
			set { SetValue(LabelForegroundProperty, value); }
		}

		public static readonly DependencyProperty LabelFontSizeProperty =
				DependencyProperty.Register("LabelFontSize", typeof(double),
					typeof(FloatingLabelComboBox), new PropertyMetadata(10.0));
		public double LabelFontSize
		{
			get { return (double)GetValue(LabelFontSizeProperty); }
			set { SetValue(LabelFontSizeProperty, value); }
		}

		public static readonly DependencyProperty ItemsSourceProperty =
				DependencyProperty.Register("ItemsSource", typeof(object),
					typeof(FloatingLabelComboBox), new PropertyMetadata(null));
		public object ItemsSource
		{
			get { return GetValue(ItemsSourceProperty); }
			set { SetValue(ItemsSourceProperty, value); }
		}

		public static readonly DependencyProperty SelectedItemProperty =
				DependencyProperty.Register("SelectedItem", typeof(object),
					typeof(FloatingLabelComboBox),
						new FrameworkPropertyMetadata(null,
						FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
						OnSelectedItemChanged));
		public object SelectedItem
		{
			get { return GetValue(SelectedItemProperty); }
			set { SetValue(SelectedItemProperty, value); }
		}
		private static void OnSelectedItemChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
		{
			FloatingLabelComboBox cc = d as FloatingLabelComboBox;
			cc.SelectedItem = e.NewValue;
			if (cc != null) cc.OnPropertyChanged(nameof(SelectedItem));
		}

		public static readonly DependencyProperty DisplayMemberPathProperty =
				DependencyProperty.Register("DisplayMemberPath", typeof(string),
					typeof(FloatingLabelComboBox), new PropertyMetadata(null));
		public string DisplayMemberPath
		{
			get { return GetValue(DisplayMemberPathProperty).ToString(); }
			set { SetValue(DisplayMemberPathProperty, value); }
		}

		public static readonly DependencyProperty SelectedValueProperty =
				DependencyProperty.RegisterAttached("SelectedValue", typeof(object),
					typeof(FloatingLabelComboBox),
						new FrameworkPropertyMetadata(null,
						FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
						new PropertyChangedCallback(OnSelectedValueChanged)));
		public object SelectedValue
		{
			get { return GetValue(SelectedValueProperty); }
			set { SetValue(SelectedValueProperty, value); }
		}
		private static void OnSelectedValueChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
		{
			FloatingLabelComboBox cc = d as FloatingLabelComboBox;
			cc.SelectedValue = e.NewValue;
			if (cc != null) cc.OnPropertyChanged(nameof(SelectedValue));
		}

		public static readonly DependencyProperty CBConverterProperty =
		DependencyProperty.Register("CBConverter", typeof(IValueConverter),
			typeof(FloatingLabelComboBox), new PropertyMetadata(null));
		public IValueConverter CBConverter
		{
			get { return (IValueConverter)GetValue(CBConverterProperty); }
			set { SetValue(CBConverterProperty, value); }
		}

		public event PropertyChangedEventHandler PropertyChanged;
		private void OnPropertyChanged([CallerMemberName] String propertyName = "")
			=> PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
	}

	public class BoolToColorConverter : IValueConverter
	{
		public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
		{
			Student st = value as Student;
			return (st != null && !string.IsNullOrEmpty(st.BloodGroup)) ? Brushes.LightGreen : Brushes.LightPink;
		}
		public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
		{
			throw new NotImplementedException();
		}
	}
}

Result:

Answer

Hi,
you can try following demo.

XAML:

Code:

using System;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Windows;

namespace WpfApp126
{
	/// 
	/// Interaction logic for Window126.xaml
	/// 
	public partial class Window126 : Window
	{
		public Window126()
		{
			InitializeComponent();
		}
	}

	public class ViewModel : INotifyPropertyChanged
	{
		public event PropertyChangedEventHandler PropertyChanged;
		private void OnPropertyChanged([CallerMemberName] String propertyName = "")
			=> PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));

		// contructor: load StudentList 
		public ViewModel() { for (int i = 1; i <= 10; i++) StudentList.Add(new Student() { Name = $"Student {i}" }); }

		public ObservableCollection StudentList { get; set; } = new ObservableCollection();
		public ObservableCollection BloodGroups { get; set; } = new ObservableCollection() { "A", "B", "AB", "0"};

		private Student _student;
		public Student Student
		{
			get => _student;
			set
			{
				_student = value;
				OnPropertyChanged();
			}
		}

	}
	public class Student : INotifyPropertyChanged
	{
		public event PropertyChangedEventHandler PropertyChanged;

		private void OnPropertyChanged([CallerMemberName] String propertyName = "")
		{
			PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
		}

		private string _name;
		public string Name
		{
			get => _name;
			set
			{
				_name = value;
				OnPropertyChanged();
			}
		}

		private string _bloodgroup;
		public string BloodGroup
		{
			get => _bloodgroup;
			set
			{
				_bloodgroup = value;
				OnPropertyChanged();
			}
		}
	}

}

Result:

Answer

Hi,
try following demo with CustomControl:

XAML:

Code (ViewModel, Data, CustomControl):

using System;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;

namespace WpfApp127
{
	public class ViewModel : INotifyPropertyChanged
	{
		public event PropertyChangedEventHandler PropertyChanged;
		private void OnPropertyChanged([CallerMemberName] String propertyName = "")
			=> PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));

		// contructor: load StudentList 
		public ViewModel() { for (int i = 1; i <= 10; i++) StudentList.Add(new Student() { Name = $"Student {i}" }); }

		public ObservableCollection StudentList { get; set; } = new ObservableCollection();
		public ObservableCollection BloodGroups { get; set; } = new ObservableCollection() { "A", "B", "AB", "0" };

		private Student _student;
		public Student Student
		{
			get => _student;
			set
			{
				_student = value;
				OnPropertyChanged();
			}
		}

	}
	public class Student : INotifyPropertyChanged
	{
		public event PropertyChangedEventHandler PropertyChanged;

		private void OnPropertyChanged([CallerMemberName] String propertyName = "")
		{
			PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
		}

		private string _name;
		public string Name
		{
			get => _name;
			set
			{
				_name = value;
				OnPropertyChanged();
			}
		}

		private string _bloodgroup;
		public string BloodGroup
		{
			get => _bloodgroup;
			set
			{
				_bloodgroup = value;
				OnPropertyChanged();
			}
		}
	}

	public class CustomControl : UserControl, INotifyPropertyChanged
	{
		public CustomControl() => CreateContent();
		private Label header = new Label();
		private ComboBox cb = new ComboBox();

		public static readonly DependencyProperty HeaderProperty =
				DependencyProperty.Register("Header", typeof(string), typeof(CustomControl), new PropertyMetadata(string.Empty));
		public string Header
		{
			get { return GetValue(HeaderProperty).ToString(); }
			set { SetValue(HeaderProperty, value); }
		}

		public static readonly DependencyProperty ItemsSourceProperty =
				DependencyProperty.Register("ItemsSource", typeof(object), typeof(CustomControl), new PropertyMetadata(null));
		public object ItemsSource
		{
			get { return GetValue(ItemsSourceProperty); }
			set { SetValue(ItemsSourceProperty, value); }
		}

		public static readonly DependencyProperty SelectedItemProperty =
				DependencyProperty.Register("SelectedItem", typeof(object),
					typeof(CustomControl),
					new FrameworkPropertyMetadata(null,
						FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
						new PropertyChangedCallback(OnSelectedItemChanged)));
		public object SelectedItem
		{
			get { return GetValue(SelectedItemProperty); }
			set { SetValue(SelectedItemProperty, value); }
		}
		private static void OnSelectedItemChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
		{
			CustomControl cc = d as CustomControl;
			if (cc != null) cc.OnPropertyChanged(nameof(SelectedItem));
		}

		public static readonly DependencyProperty DisplayMemberPathProperty =
					DependencyProperty.Register("DisplayMemberPath", typeof(string), typeof(CustomControl), new PropertyMetadata(null));
		public string DisplayMemberPath
		{
			get { return GetValue(DisplayMemberPathProperty).ToString(); }
			set { SetValue(DisplayMemberPathProperty, value); }
		}

		public static readonly DependencyProperty SelectedValueProperty =
		DependencyProperty.RegisterAttached("SelectedValue", typeof(object),
			typeof(CustomControl),
			new FrameworkPropertyMetadata(null,
				FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
				new PropertyChangedCallback(OnSelectedValueChanged)));
		public object SelectedValue
		{
			get { return GetValue(SelectedValueProperty); }
			set { SetValue(SelectedValueProperty, value); }
		}
		private static void OnSelectedValueChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
		{
			CustomControl cc = d as CustomControl;
			if (cc != null) cc.OnPropertyChanged(nameof(SelectedValue));
		}

		public event PropertyChangedEventHandler PropertyChanged;
		private void OnPropertyChanged([CallerMemberName] String propertyName = "")
			=> PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));

		void CreateContent()
		{
			Grid _grid = new Grid();
			_grid.RowDefinitions.Add(new RowDefinition());
			_grid.RowDefinitions.Add(new RowDefinition());
			_grid.Children.Add(header);
			header.Foreground = System.Windows.Media.Brushes.Green;
			header.SetBinding(Label.ContentProperty, new Binding("Header") { Source = this });
			_grid.Children.Add(cb);
			cb.SetValue(Grid.RowProperty, 1);
			cb.SetBinding(ComboBox.ItemsSourceProperty, new Binding("ItemsSource") { Source = this });
			cb.SetBinding(ComboBox.SelectedItemProperty, new Binding("SelectedItem") { Source = this });
			cb.SetBinding(ComboBox.DisplayMemberPathProperty, new Binding("DisplayMemberPath") { Source = this });
			cb.SetBinding(ComboBox.SelectedValueProperty, new Binding("SelectedValue") { Source = this });
			this.Content = _grid;
		}
	}
}

Result:

Answer

Hi,
if you want to use XAML-Template you can try following demo:

XAML:

Code:

using System;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;

namespace WpfApp128
{
	public class ViewModel : INotifyPropertyChanged
	{
		public event PropertyChangedEventHandler PropertyChanged;
		private void OnPropertyChanged([CallerMemberName] String propertyName = "")
			=> PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));

		// contructor: load StudentList 
		public ViewModel() { for (int i = 1; i <= 10; i++) 
				StudentList.Add(new Student() { StudentId = i, StudentName = $"Student {i}" }); }

		public ObservableCollection StudentList { get; set; } = new ObservableCollection();
		public ObservableCollection BloodGroups { get; set; } = new ObservableCollection() { "A", "B", "AB", "0" };

		private Student _student;
		public Student Student
		{
			get => _student;
			set
			{
				_student = value;
				OnPropertyChanged();
			}
		}

	}
	public class Student : INotifyPropertyChanged
	{
		public event PropertyChangedEventHandler PropertyChanged;

		private void OnPropertyChanged([CallerMemberName] String propertyName = "") =>
			PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));

		public int StudentId { get; set; }

		private string _studentName;
		public string StudentName
		{
			get => _studentName;
			set
			{
				_studentName = value;
				OnPropertyChanged();
			}
		}

		private string _bloodgroup;
		public string BloodGroup
		{
			get => _bloodgroup;
			set
			{
				_bloodgroup = value;
				OnPropertyChanged();
			}
		}
	}
	public class FloatingLabelComboBox : Control, INotifyPropertyChanged
	{
		static FloatingLabelComboBox()
		{
			DefaultStyleKeyProperty.OverrideMetadata(typeof(FloatingLabelComboBox), new FrameworkPropertyMetadata(typeof(FloatingLabelComboBox)));
		}

		public static readonly DependencyProperty LabelTextProperty =
			 DependencyProperty.Register("LabelText", typeof(string),
				 typeof(FloatingLabelComboBox), new PropertyMetadata(string.Empty));
		public string LabelText
		{
			get { return GetValue(LabelTextProperty).ToString(); }
			set { SetValue(LabelTextProperty, value); }
		}

		public static readonly DependencyProperty LabelForegroundProperty =
				DependencyProperty.Register("LabelForeground", typeof(Brush),
					typeof(FloatingLabelComboBox), new PropertyMetadata(Brushes.AliceBlue));
		public Brush LabelForeground
		{
			get { return (Brush)GetValue(LabelForegroundProperty); }
			set { SetValue(LabelForegroundProperty, value); }
		}

		public static readonly DependencyProperty LabelFontSizeProperty =
				DependencyProperty.Register("LabelFontSize", typeof(double),
					typeof(FloatingLabelComboBox), new PropertyMetadata(10.0));
		public double LabelFontSize
		{
			get { return (double)GetValue(LabelFontSizeProperty); }
			set { SetValue(LabelFontSizeProperty, value); }
		}

		public static readonly DependencyProperty ItemsSourceProperty =
				DependencyProperty.Register("ItemsSource", typeof(object),
					typeof(FloatingLabelComboBox), new PropertyMetadata(null));
		public object ItemsSource
		{
			get { return GetValue(ItemsSourceProperty); }
			set { SetValue(ItemsSourceProperty, value); }
		}

		public static readonly DependencyProperty SelectedItemProperty =
				DependencyProperty.Register("SelectedItem", typeof(object),
					typeof(FloatingLabelComboBox),
						new FrameworkPropertyMetadata(null,
						FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
						OnSelectedItemChanged));
		public object SelectedItem
		{
			get { return GetValue(SelectedItemProperty); }
			set { SetValue(SelectedItemProperty, value); }
		}
		private static void OnSelectedItemChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
		{
			FloatingLabelComboBox cc = d as FloatingLabelComboBox;
			cc.SelectedItem = e.NewValue;
			if (cc != null) cc.OnPropertyChanged(nameof(SelectedItem));
		}

		public static readonly DependencyProperty DisplayMemberPathProperty =
				DependencyProperty.Register("DisplayMemberPath", typeof(string),
					typeof(FloatingLabelComboBox), new PropertyMetadata(null));
		public string DisplayMemberPath
		{
			get { return GetValue(DisplayMemberPathProperty).ToString(); }
			set { SetValue(DisplayMemberPathProperty, value); }
		}

		public static readonly DependencyProperty SelectedValueProperty =
				DependencyProperty.RegisterAttached("SelectedValue", typeof(object),
					typeof(FloatingLabelComboBox),
						new FrameworkPropertyMetadata(null,
						FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
						new PropertyChangedCallback(OnSelectedValueChanged)));
		public object SelectedValue
		{
			get { return GetValue(SelectedValueProperty); }
			set { SetValue(SelectedValueProperty, value); }
		}
		private static void OnSelectedValueChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
		{
			FloatingLabelComboBox cc = d as FloatingLabelComboBox;
			cc.SelectedItem = e.NewValue;
			if (cc != null) cc.OnPropertyChanged(nameof(SelectedValue));
		}

		public event PropertyChangedEventHandler PropertyChanged;
		private void OnPropertyChanged([CallerMemberName] String propertyName = "")
			=> PropertyChanged?.BeginInvoke(this, new PropertyChangedEventArgs(propertyName), null, null);
	}
}

Result:

Answer

Hi,@Mesh Ka. Welcome to Microsoft Q&A Forum.

I'm trying to implement functionality, but FloatingLabelComboBox can't to set binding data to codebehind.
I'm still looking for the cause of the problem and will come back with updates.

Currently I have the code below. Maybe it will inspire you a little bit.

FloatingLabelComboBox :


 public class FloatingLabelComboBox : ComboBox
  {
        static FloatingLabelComboBox()
        {
            DefaultStyleKeyProperty.OverrideMetadata(typeof(FloatingLabelComboBox), new FrameworkPropertyMetadata(typeof(FloatingLabelComboBox)));
        }
        public static readonly DependencyProperty LabelTextProperty =
           DependencyProperty.Register("LabelText", typeof(string), typeof(FloatingLabelComboBox), new PropertyMetadata("LabelText"));

        public string LabelText
        {
            get { return (string)GetValue(LabelTextProperty); }
            set { SetValue(LabelTextProperty, value); }
        }

        public static readonly DependencyProperty LabelForegroundProperty =
            DependencyProperty.Register("LabelForeground", typeof(Brush), typeof(FloatingLabelComboBox), new PropertyMetadata(Brushes.AliceBlue));

        public Brush LabelForeground
        {
            get { return (Brush)GetValue(LabelForegroundProperty); }
            set { SetValue(LabelForegroundProperty, value); }
        }


        public static readonly DependencyProperty LabelFontSizeProperty =
            DependencyProperty.Register("LabelFontSize", typeof(double), typeof(FloatingLabelComboBox), new PropertyMetadata(10.0));

        public double LabelFontSize
        {
            get { return (double)GetValue(LabelFontSizeProperty); }
            set { SetValue(LabelFontSizeProperty, value); }
        }
}

StudentModel:


 public class StudentModel : INotifyPropertyChanged
    {
        private int _studentId;
        public int StudentId
        {
            get => _studentId;
            set
            {
                if (value == _studentId) return;
                _studentId = value;
                OnPropertyChanged(nameof(_studentId));
            }
        }

   
    private string _studentName;
    public string StudentName
    {
      get => _studentName;
      set
      {
        _studentName = value;
        OnPropertyChanged();
      }
    }

    private ICollection _bloodgroup;
    public ICollection BloodGroup
    {
      get => _bloodgroup;
      set
      {
        _bloodgroup = value;
        OnPropertyChanged();
      }
    }


    public event PropertyChangedEventHandler PropertyChanged;

        private void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

      }

ViewModel:


  public class ViewModel : INotifyPropertyChanged
  {
    
    public ViewModel()
    {
      this.Students = new ObservableCollection()
            { new StudentModel(){ StudentName = "Jane", BloodGroup = new ObservableCollection() { "A", "B", "AB", "0"  }},
              new StudentModel(){ StudentName = "Jone",  BloodGroup = new ObservableCollection()  { "A", "B", "AB", "0" }},
              new StudentModel(){ StudentName = "Tina", BloodGroup = new ObservableCollection() {"A", "B", "AB", "0"}}
            };
    

      this.SelectedStudent = Students.FirstOrDefault();
    }
    public ObservableCollection Students { get; private set; }
    public ObservableCollection BloodGroups { get; private set; }


    public StudentModel _selectedStudent;
    public StudentModel SelectedStudent
    {
      get { return _selectedStudent; }
      set
      {
        _selectedStudent = value;
        if (_selectedStudent != null)
        {
          BloodGroups = _selectedStudent.BloodGroup;
          OnPropertyChanged(nameof(BloodGroups));
        }

        OnPropertyChanged();
      }
    }

    public string _selectedBloodGroup ;
    public string SelectedBloodGroup
    {
      get { return _selectedBloodGroup; }
      set
      {
        _selectedBloodGroup = value;
        OnPropertyChanged();
      }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private void OnPropertyChanged([CallerMemberName] String propertyName = "")
    {
      PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
  }

MainWindow:

...

The result:

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How can I correctly bind a ComboBox in my custom Control to its dependency property in a WPF C# application?

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