It's much easier to read large numbers when thousands are separated by commas. But I can never remember how the numeric format with thousands comma-separated is defined for .NET String.Format() method and for the databinding. So more as a note to self, here it is:
string
output = string.Format("{0:#,#}", 123456789); // Will produce 123,456,789
The same goes for data binding data sources to data controls like DataGridView. Specify format as "{0:#,#}".