You can use the IsNull operator.

// the outer quotes are double-quotes
// the inner quotes are two single-quotes
// Fax is the column mapping name
dataView1.RowFilter = "IsNull(Fax, '') = ''";

// for a numeric null in custNumber
dataView1.RowFilter = "IsNull(custNumber, 0) = 0";

George Shepherd, Syncfusion, and Bob Gibson