Access Controls in ViewModel using Attached Properties

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Introduction

In MVVM, there are situations when we need to have the access of the particular controls in the ViewModel. For example, if I want to bind IEnumerable to the DataGrid because I am not able to determine the fields till runtime. Now if I bind IEnumerable<dynamic> and set AutogeneratedColumns = true then also DataGrid will not show the records because each row is returned as an "ExpandoObject" with dynamic properties representing the fields. In this case, I have to add the columns dynamically. To do that, I need the DataGrid control. Now to get the dataGrid control in the ViewModel we can use attached property. Let's try.

Background

For better understanding of this article, I would recommend reading of the attached properties.

Using the Code

First of all, define the attached property for the control in the ViewModel.

Public static readonly DependecyProperty DataGridProperty = _  
 DependencyProperty.RegisterAttached("DataGrid", typeof(DataGrid),  
 typeof(MainWindowViewModel),  
 New FrameworkPropertyMetadata(OnDataGridChanged));  
  
 public static void SetDataGrid(DependencyObject element, DataGrid value)  
 {  
 element.SetValue(DataGridProperty, value);  
 }  
  
 public static DataGrid GetDataGrid(DependencyObject element)  
 {  
 return (DataGrid)element.GetValue(DataGridProperty);  
 }

Assign the name property on the control. Now we need to bind the control with this attached property.

<Window x:Class="AccessControlsInViewModel.MainWindow"  
 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"  
 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"  
 xmlns:local="clr-namespace:AccessControlsInViewModel"  
 Title="Access Controls In ViewModel" Height="350" Width="525">  
 <DockPanel>  
 <StackPanel Orientation="Horizontal" DockPanel.Dock="Top" Margin="0,0,0,25">  
 <Button Content="Add Columns" Command="{Binding AddCommand}" />  
 </StackPanel>  
 <DataGrid Name="accessGrid" ItemsSource="{Binding ListOfPerson}"  
 local:MainWindowViewModel.DataGrid="{Binding ElementName=accessGrid}" />  
 </DockPanel>  
 </Window>

Here I have binded the control with the attached property using the ElementName binding.

The reason behind using attached property is here we need to bind the whole control with the property and there is no in-built property available for doing this.

Then define a static variable for the control and assign its value on the attached property changed event.

// DataGrid Instance in the viewModel  
 private static DataGrid dataGrid = null;  
  
 // DataGrid Property changed event  
 public static void OnDataGridChanged  
 (DependencyObject obj, DependencyPropertyChangedEventArgs args)  
 {  
 dataGrid = obj as DataGrid;  
 }

Now we have the control available in the ViewModel. So we can achieve everything that we can do in the code behind file.

For the DataGrid example, on the add button's command we can add the columns like this:

 private static void AddDynamicColumnsToDataGrid()  
 {  
 IEnumerable<IDictionary<string, object>>  
 rows = dataGrid.ItemsSource.OfType<IDictionary<string, object>>();  
 IEnumerable<string> columns = rows.SelectMany  
 (d => d.Keys).Distinct(StringComparer.OrdinalIgnoreCase);  
 foreach (string text in columns)  
 {  
 // now set up a column and binding for each property  
 var column = new DataGridTextColumn  
 {  
 Header = text,  
 Binding = new Binding(text)  
 };  
  
 dataGrid.Columns.Add(column);  
 }  
 }

Points of Interest

To solve this problem, I have added click and command both on the Button and the trick has really worked for me. But as I was using MVVM, I needed to do it in the ViewModel and I found out this solution.