How can I access a vm object declared in main window in Wpf Page

Question

Hai

I have declared an object of my view model in my main window. I want to know how to access this in my xaml

my code is

Main window

      private void Button_Click(object sender, RoutedEventArgs e)
            {
                ViewModel VM = new ViewModel();
                SubPage Sp = new SubPage();
                Sp.DataContext = VM;
                MainFrame.Content = Sp;
            }

in page

some thing like this

please help me how to access the main window object.

Thanks

Accepted Answer

1.Delete the part which related to validation in Model.cs, then you update the ViewModel.cs like the below:

public ObservableCollection viewModels { get; set; }

        public ViewModel()
        {
            viewModels = new ObservableCollection()
            {
            new Model {Name="Name1",Height=170,Age=10}
             };
        }

2.Update the SubPage.xaml like below,the DataGrid access data from VM which declared in MainWindow.xaml.cs

Answer

Hi, if you want to use ViewModel or properties of ViewModel in ValidationRules you must add DependencyPropety to ValidationRule. Since ValidationRule does not inherit from DependencyObject you cannot create a DependecyProperty in your custom validation class. You can have a normal property in your validation class which is of a type that inherits from DependecyObject and create a DependencyProperty in that class. The following ValidationRule check the length of "M_Name" property in data item of row.

  [ContentProperty("MaxLength")]
  public class ValidateRow : ValidationRule
  {
    public MappingValue MaxLength { get; set; }

    public override ValidationResult Validate(object value, CultureInfo cultureInfo)
    {
      BindingGroup bg = value as BindingGroup;
      if (bg == null && bg.Items.Count != 1) return new ValidationResult(false, "invalid use ValidateRow");
      Student s = bg.Items[0] as Student;
      if (s == null) return new ValidationResult(false, "invalid use ValidateRow");
      if (s.M_Name.Length> (int)MaxLength.Value) return new ValidationResult(false, "invalid length");
      return ValidationResult.ValidResult;
    }
  }

MappingValue is a simple class that inherits from DependencyObject and has a DependencyProperty:

  public class MappingValue : DependencyObject
  {
    public object Value
    {
      get { return this.GetValue(ValueProperty); }
      set { this.SetValue(ValueProperty, value); }
    }
    public static readonly DependencyProperty ValueProperty = DependencyProperty.Register(
        nameof(Value),
        typeof(object),
        typeof(MappingValue),
        new PropertyMetadata(default(int), OnValueChanged));

    private static void OnValueChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
      MappingValue comparisonValue = (MappingValue)d;
      BindingExpressionBase bindingExpressionBase = 
        BindingOperations.GetBindingExpressionBase(comparisonValue, BindingToTriggerProperty);
      bindingExpressionBase?.UpdateSource();
    }

    public object BindingToTrigger
    {
      get { return this.GetValue(BindingToTriggerProperty); }
      set { this.SetValue(BindingToTriggerProperty, value); }
    }
    public static readonly DependencyProperty BindingToTriggerProperty = DependencyProperty.Register(
        nameof(BindingToTrigger),
        typeof(object),
        typeof(MappingValue),
        new FrameworkPropertyMetadata(default(object), FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));
  }

The binding does not work correctly since the ValidationRules is not part of visual tree and therefore cannot get the bound property correctly. You can use a proxy class:

  public class BindingProxy : Freezable
  {
    protected override Freezable CreateInstanceCore() => new BindingProxy();

    public object Data
    {
      get { return this.GetValue(DataProperty); }
      set { this.SetValue(DataProperty, value); }
    }
    public static readonly DependencyProperty DataProperty = 
      DependencyProperty.Register(nameof(Data), typeof(object), typeof(BindingProxy), new UIPropertyMetadata(null));
  }

In XAML you can use this BindingProxy in StaticResource lik in following demo. I insert a TextBox for MaxLength (property in ViewModel).

Answer

Welcome to our Microsoft Q&A platform!

Your description make me little confused, I will show you my sample for Validation which use MVVM.

NotifyObject.cs

public class NotifyObject : INotifyPropertyChanged { public event PropertyChangedEventHandler PropertyChanged;

              public void RaisePropertyChanged(string propertyName)
                 {
                     if (PropertyChanged != null)
                     {
                         PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
                     }
                 }

             }

Model.cs

public class Model : NotifyObject, IDataErrorInfo
    {
        private string name;
        public string Name
        {
            get
            {
                return name;
            }
            set
            {
                name = value;
                RaisePropertyChanged("Name");
            }
        }

        private decimal height;
        public decimal Height
        {
            get
            {
                return height;
            }
            set
            {
                height = value;
                RaisePropertyChanged("Height");
            }
        }

        private int age;
        public int Age
        {
            get
            {
                return age;
            }
            set
            {
                age = value;
                RaisePropertyChanged("Age");
            }
        }

        public string Error
        {
            get { throw new NotImplementedException(); }
        }

        public string this[string columnName]
        {
            get
            {
                string result = string.Empty;
                switch (columnName)
                {
                    case "Name": if (string.IsNullOrEmpty(Name)) result = "Name is required!"; break;
                    case "Height": if ((Height < 52) || (Height > 250)) result = "Height must be between 52 and 250"; break;
                    case "Age": if ((Age < 1) || (Age > 100)) result = "Age must be between 1 and 100"; break;
                };
                return result;
            }
        }
    }

ViewModel.cs

public class ViewModel : NotifyObject { public Model validModel { get; set; }

         public ViewModel()
         {
             validModel = new Model();
         }
     }

SubPage.xaml

MainWindow.xaml

MainWindow.xaml.cs

public MainWindow()
         {
             InitializeComponent();
             ViewModel VM = new ViewModel();
             SubPage Sp = new SubPage();
             Sp.DataContext = VM;
             MainFrame.Content = Sp;
         }

Answer

This is not I want (validation)
I want to know how to access the same object declared in main window (VM) button click in my page XAML.

Thanks.

Answer

Hi, the logic of your code is unclear to me. I have revised your code, deleted all code-behind (except InitializeComponent) and concentrated all logic in a ViewModel. A better approach would be to use IDataError in StudentModel.

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

namespace Experiment
{
  public class ViewModel : RowValidation, INotifyPropertyChanged
  {
    public ViewModel()
    {
      StudentCollection.Add(new StudentModel { M_STId = 1, M_Name = "Santhosh", M_Maths = 70, M_Science = 56, M_English = 45 });
      StudentCollection.Add(new StudentModel { M_STId = 2, M_Name = "Mathew", M_Maths = 60, M_Science = 65, M_English = 77 });
      StudentCollection.Add(new StudentModel { M_STId = 3, M_Name = "Poly", M_Maths = 40, M_Science = 66, M_English = 45 });
      StudentCollection.Add(new StudentModel { M_STId = 4, M_Name = "Tony", M_Maths = 20, M_Science = 88, M_English = 49 });
      //
      cvsStudentCollection.Source = StudentCollection;
      cvsStudentCollection.View.CurrentChanged += StudentCollection_CurrentChanged;
      //
      cvsStudentCollectionSub.Source = StudentCollectionSub;
      //
      Frame1 = new Page1() { DataContext = this };
      Frame2 = new Page2() { DataContext = this };
    }

    public Page Frame1 { get; set; }
    public Page Frame2 { get; set; }

    private ObservableCollection StudentCollection { get; set; } = new ObservableCollection();
    private ObservableCollection StudentCollectionSub { get; set; } = new ObservableCollection();

    private CollectionViewSource cvsStudentCollection = new CollectionViewSource();
    private CollectionViewSource cvsStudentCollectionSub = new CollectionViewSource();

    public ICollectionView Vm_StudentCollection { get => cvsStudentCollection.View; }
    public ICollectionView Vm_StudentCollectionSub { get => cvsStudentCollectionSub.View; }

    private void StudentCollection_CurrentChanged(object sender, System.EventArgs e)
    {
      if (Vm_StudentCollection.CurrentItem != null)
      {
        StudentCollectionSub.Clear();
        StudentCollectionSub.Add(((StudentModel)Vm_StudentCollection.CurrentItem).Copy());
      }
    }

    public ICommand Cmd { get => new RelayCommand(CmdExec); }

    private void CmdExec(object obj)
    {
      for (int i = StudentCollectionSub.Count-1; i >= 0; i--)
      {
        var s = StudentCollectionSub[i]; ;
        StudentCollection.Add(s);
        StudentCollectionSub.Remove(s);
      }
    }

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

using System.ComponentModel;
using System.Runtime.CompilerServices;

namespace Experiment
{
  public class StudentModel : INotifyPropertyChanged
  {
    public StudentModel() { }
    public int M_STId { get; set; }
    public string M_Name { get; set; }
    public int M_Maths { get; set; }
    public int M_Science { get; set; }
    public int M_English { get; set; }
    public int M_Total { get; set; }

    public StudentModel Copy() => new StudentModel()
    {
      M_English = this.M_English,
      M_Maths = this.M_Maths,
      M_Name = this.M_Name,
      M_Science = this.M_Science,
      M_STId = this.M_STId,
      M_Total = this.M_Total
    };      

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

using System;
using System.Windows.Input;

namespace Experiment
{
  public class RelayCommand : ICommand
  {
    private readonly Predicate

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How can I access a vm object declared in main window in Wpf Page

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