Datagrid Textbox validation for numeric values in MVVM

<UserControl x:Class="TestGrid.GridControl"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             mc:Ignorable="d"
           >

    <StackPanel >
        <DataGrid GridLinesVisibility="Vertical" ScrollViewer.HorizontalScrollBarVisibility="Auto" ScrollViewer.VerticalScrollBarVisibility="Auto"
                                      MinHeight="100" MaxHeight="300" Width="Auto" Name="McDataGrid" Grid.Row="1" Grid.Column="2" AutoGenerateColumns="False" CanUserAddRows="False"
                              CanUserDeleteRows="False" ItemsSource="{Binding Path=TestData}" DataContext="{Binding}"
                  >
            <DataGrid.Columns>
                <DataGridCheckBoxColumn Binding="{Binding Path=IsSelected, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" IsReadOnly="False">
                    <DataGridCheckBoxColumn.Header>
                        <CheckBox Name="selectallchkbox3" IsChecked="{Binding DataContext.SelectAll1, Mode=TwoWay, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type DataGrid}},
                                            UpdateSourceTrigger=PropertyChanged }"/>

                    </DataGridCheckBoxColumn.Header>
                </DataGridCheckBoxColumn>
                <DataGridTextColumn Header="Name" Binding="{Binding Path=Name,Mode=OneWay}" IsReadOnly="True" Width="250"/>
                <DataGridTemplateColumn>
                    <DataGridTemplateColumn.CellTemplate>
                        <DataTemplate>
                            <TextBox Text="{Binding Path=InstruQuantity, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Width="100" />
                        </DataTemplate>
                    </DataGridTemplateColumn.CellTemplate>
                </DataGridTemplateColumn>
            </DataGrid.Columns>
        </DataGrid>
    </StackPanel>
</UserControl>

namespace ViewModel
{
    public class Authors: ViewModel.ViewProperty
    {
        public Authors()
        {
            LoadCollectionData();
        }

        private DataTable authors = new DataTable();

        public DataTable TestData
        {
            get
            {
                return authors;
            }
        }

        public void LoadCollectionData()
        {
            DataColumn col = new DataColumn();
            col.ColumnName = "Name";
            col.DataType = typeof(string);
            authors.Columns.Add(col);

            DataColumn col2 = new DataColumn();
            col2.ColumnName = "InstruQuantity";
            col2.DataType = typeof(string);
            authors.Columns.Add(col2);

            DataColumn colcheck = new DataColumn("IsSelected", typeof(bool));
            authors.Columns.Add(colcheck);

            DataRow row1 = authors.NewRow();
            row1[0] = "Ganga";
            row1[1] = "10";
            authors.Rows.Add(row1);

            DataRow row2 = authors.NewRow();
            row2[0] = "Bhagya";
            row2[1] = "20";
            authors.Rows.Add(row2);
        }

        private bool _SelectAll;
        public bool SelectAll1
        {
            get { return _SelectAll; }
            set {
                    _SelectAll = value;
                    OnPropertyChanged("SelectAll1");
                    SelectAll();
                }
        }

        public void SelectAll()
        {
            if (SelectAll1 == true)
            {
                foreach (DataRow row in authors.Rows)
                {
                    row["IsSelected"] = true;
                }
            }
            else
            {
                foreach (DataRow row in authors.Rows)
                {
                    row["IsSelected"] = false;
                }
            }
        }
    }
}