In this example I will show how to validate the data entered into a datarepeater control. For this example I am added the northwind SQL compact edition database to the project and created a typed dataset for the products table.  So from the toolbox drop a datarepeater on the form.  Inside the datarepeater drag the ProductName, UnitPrice, and Units in stock fields. Your form should look something like

Now in the drawitem event for the datarepeater we can add a handler to validating event.

Private Sub DataRepeater1_DrawItem(ByVal sender As Object, ByVal e As Microsoft.VisualBasic.PowerPacks.DataRepeaterItemEventArgs) Handles DataRepeater1.DrawItem
    Dim currItem As DataRowView = DirectCast(ProductsBindingSource.Item(e.DataRepeaterItem.ItemIndex), DataRowView)
    Dim txt As TextBox = DirectCast(e.DataRepeaterItem.Controls("Unit_PriceTextBox"), TextBox)
    AddHandler txt.Validating, AddressOf TextBox_Validating
End Sub

Private Sub TextBox_Validating(ByVal sender As System.Object, ByVal e As System.ComponentModel.CancelEventArgs)
    Dim dec As Decimal
    If Not Decimal.TryParse(DirectCast(sender, TextBox).Text, dec) Then
        MessageBox.Show("Please enter a valid number")
        e.Cancel = True
    End If
End Sub

The complete code

Public Class Form1

    Private Sub ProductsBindingNavigatorSaveItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ProductsBindingNavigatorSaveItem.Click
        Me.Validate()
        Me.ProductsBindingSource.EndEdit()
        Me.TableAdapterManager.UpdateAll(Me.NorthwindDataSet)

    End Sub

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        'TODO: This line of code loads data into the 'NorthwindDataSet.Products' table. You can move, or remove it, as needed.
        Me.ProductsTableAdapter.Fill(Me.NorthwindDataSet.Products)

    End Sub

    Private Sub DataRepeater1_DrawItem(ByVal sender As Object, ByVal e As Microsoft.VisualBasic.PowerPacks.DataRepeaterItemEventArgs) Handles DataRepeater1.DrawItem
        Dim currItem As DataRowView = DirectCast(ProductsBindingSource.Item(e.DataRepeaterItem.ItemIndex), DataRowView)
        Dim txt As TextBox = DirectCast(e.DataRepeaterItem.Controls("Unit_PriceTextBox"), TextBox)
        AddHandler txt.Validating, AddressOf TextBox_Validating
    End Sub

    Private Sub TextBox_Validating(ByVal sender As System.Object, ByVal e As System.ComponentModel.CancelEventArgs)
        Dim dec As Decimal
        If Not Decimal.TryParse(DirectCast(sender, TextBox).Text, dec) Then
            MessageBox.Show("Please enter a valid number")
            e.Cancel = True
        End If
    End Sub
End Class