How do I get datagrid.columns.count using MVVM C#?

To access properties via data binding that are not normally reachable, like the Columns.Count property of the DataGrid control, you can write a wrapper around the control and add a custom property.

public class MyDataGrid : DataGrid, IDisposable
{
    private const int DefaultColumnCountValue = 0;

    private static readonly Type ctrlType = typeof(MyDataGrid);
    private const string ctrlName = nameof(MyDataGrid);

    public static readonly DependencyProperty ColumnCountProperty =
        DependencyProperty.Register(nameof(ColumnCount),
            typeof(int),
            ctrlType,
            new PropertyMetadata(DefaultColumnCountValue,
                                 OnColumnCountChanged));

    [Bindable(true)]
    [Description("Column Count"), Category(ctrlName)]
    public int ColumnCount
    {
        get => (int)GetValue(ColumnCountProperty);
        set => SetValue(ColumnCountProperty, value);
    }

    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();

        // listen for changes
        this.Columns.CollectionChanged += Columns_CollectionChanged;
    }

    private void Columns_CollectionChanged(
        object? sender,
        NotifyCollectionChangedEventArgs e)
        => ColumnCount = this.Columns.Count;

    private static void OnColumnCountChanged(
        DependencyObject d,
        DependencyPropertyChangedEventArgs e)
    {
        // can do something here when the property value is changed
    }

    public void Dispose()
        => this.Columns.CollectionChanged -= Columns_CollectionChanged;
}

Now we can bind to the property:

<local:MyDataGrid x:Name="MyDataGrid"
    ColumnCount="{Binding ColumnCount, Mode=OneWayToSource}"
    ItemsSource="{Binding Items}" />

Here is the complete demo:

1. MainWindow Xaml

<Window x:Class="WpfDataGridCountHelper.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:WpfDataGridCountHelper"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.DataContext>
        <local:MainViewModel />
    </Window.DataContext>

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <local:MyDataGrid x:Name="MyDataGrid"
            ColumnCount="{Binding ColumnCount, Mode=OneWayToSource}"
            ItemsSource="{Binding Items}" />

        <Grid Grid.Row="1"
              Margin="10">
            <Grid.ColumnDefinitions>
                <ColumnDefinition/>
                <ColumnDefinition Width="Auto"/>
            </Grid.ColumnDefinitions>
            <Border BorderBrush="DarkGray"
                    BorderThickness="2" 
                    Padding="20 10">
                <TextBlock Margin="0 10" HorizontalAlignment="Stretch">
                    <Run FontWeight="Bold" Text="Column Count: "/>
                    <Run Text="{Binding ColumnCount}"/>
                </TextBlock>
            </Border>
            <Button Grid.Column="1"
                    Margin="10 0 0 0"
                    Padding="20 10"
                    Content="ADD COLUMN"
                    Click="ButtonBase_OnClick"/>
        </Grid>
    </Grid>
</Window>

2. MainWindow code-behind for the button click event (demo purposes only):

public partial class MainWindow : Window
{
    public MainWindow() => InitializeComponent();

    private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
        => MyDataGrid.Columns.Add(new DataGridTextColumn());
}

3. MainViewModel + ObservableObject (INotifyPropertyChanged Handler class)

public abstract class ObservableObject : INotifyPropertyChanged
{
    public void Set<TValue>(
        ref TValue field, 
        TValue newValue, 
        [CallerMemberName] string propertyName = "")
    {
        if (!EqualityComparer<TValue>.Default.Equals(field, default)
            && field!.Equals(newValue))
            return;

        field = newValue;
        PropertyChanged?.Invoke(this,
            new PropertyChangedEventArgs(propertyName));
    }

    public event PropertyChangedEventHandler? PropertyChanged;
}

public class MainViewModel : ObservableObject
{
    private int columnCount = 0;

    public int ColumnCount
    {
        get => columnCount;
        set => Set(ref columnCount, value);
    }

    public ObservableCollection<PersonModel> Items { get; set; } = new()
    {
        new() { Name = "Freddie", Age = 21 },
        new() { Name = "Milly", Age = 18 },
        new() { Name = "Caddie", Age = 23 },
    };
}


public class PersonModel : ObservableObject
{
    private string? name;
    private int? age;

    public string? Name
    {
        get => name;
        set => Set(ref name, value);
    }

    public int? Age
    {
        get => age;
        set => Set(ref age, value);
    }
}

Clicking on the ADD COLUMN button adds additional blank columns to the MyDataGrid control, then binding triggers and the TextBlock in the footer updates with the ColumnCount, all bound via the DataContext (MainViewModel).

Enjoy!

private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
        => MyDataGrid.Columns.Add(new DataGridTextColumn());

<local:MyDataGrid 
										ColumnCount="{Binding ColumnCount, Mode=OneWayToSource}"
										ItemsSource="{Binding Students}"
										SelectedItem="{Binding SelectedStudent}"
										CanUserAddRows="False"
										CanUserDeleteRows="False"
										SelectionMode="Single"
										SelectedIndex="{Binding SelectedIndex}"
										AutoGenerateColumns="False"
										HeadersVisibility="Column"
                                    <DataGrid.Columns>
                                            <DataGridTextColumn
												Header="Vendor"
												Binding="{Binding Path=Name, Mode=OneWay}"
												Width="350"
												IsReadOnly="True" />
                                            <DataGridTextColumn
												Header="CNDN No."
												Binding="{Binding Path=Age, Mode=OneWay}"
												Width="320"
												IsReadOnly="True" />
											...
                                    </DataGrid.Columns>
                                    <DataGrid.Resources>
                                            <Style
												BasedOn="{StaticResource {x:Type DataGridColumnHeader}}"
												TargetType="{x:Type DataGridColumnHeader}">
                                                <Setter
													Property="Background"
													Value="#AA00FF" />
                                                <Setter
													Property="FontSize"
													Value="15" />
                                                <Setter
													Property="FontWeight"
													Value="DemiBold" />
                                            </Style>
                                            <Style
												TargetType="{x:Type DataGrid}">
                                                <Setter
													Property="ColumnWidth"
													Value="*"></Setter>
                                                <Setter
													Property="FontSize"
													Value="13"></Setter>
                                            </Style>
                                            <!--Selected row color-->
                                            <Style
												TargetType="{x:Type DataGridCell}">
                                                <Style.Triggers>
                                                    <Trigger
														Property="DataGridCell.IsSelected"
														Value="True">
                                                        <Setter
															Property="Background"
															Value="#7CBA52" />
                                                        <Setter
															Property="Foreground"
															Value="#601BD1" />
                                                        <Setter
															Property="FontWeight"
															Value="DemiBold" />
                                                    </Trigger>
                                                    <Trigger
														Property="IsSelected"
														Value="True">
                                                        <Setter
															Property="BorderThickness"
															Value="0" />
                                                    </Trigger>
                                                </Style.Triggers>
                                                <EventSetter
													Event="PreviewMouseLeftButtonDown"
													Handler="DoSelectedRow" />
                                            </Style>
                                    </DataGrid.Resources>
</local:MyDataGrid>