C# WPF) DataGrid.Items.Count returns 0, How to get the real number?

// this timer starts in another thread(dependency) with about 5 seconds tick interval 
// after the dataGrid_Test is populated to check it's populated abnormally or not.

private void timer_Tick(object sender, EventArgs e)
{
    if (dataGrid_Test.ItemsSource == null)
    {
        needed_to_load_again = true;
    }

    dataGrid_Test.UpdateLayout();

    if (dataGrid_Test.Items.Count == 0)
    {
        needed_to_load_again = true;
    }
}

A DataTable is bound to the dataGrid_Test. So, the first 'if' returns false.

However, eventhough a DataTable is bound to the dataGrid and I can see the rows with real data of dataGrid_Test, the dataGrid_Test.Items.Count returns 0.

If I check the dataGrid_Test.Items.Count with another test button, the dataGrid_Test.Items.Count returns 1 (real number).

I tried UpdateLayout() but it still return 0.

Can anybody advise here ?

Thank you !