how to set database string value to null?

[djmlbp]

there are some value contain in my database branch column for example "B01", i am trying to set my branch column back to null value but i failed to do so, i had tried string.Empty, String.Empty, "", null,DBNull.value, beside manually set my string to "NULL" is successful, the rest method failed, what is the actual problem actually ?*the method that i tried all cant clear my DB value=http://stackoverflow.com/questions/14035696/(\[code\] if (ddlBranches.SelectedIndex > 0 && ddlLocation.SelectedItem.Text.ToUpper() == "BRANCH") { drUpdProb["Branch"] = ddlBranches.SelectedValue.ToString(); drUpdProb["BranchAbbr"] = ddlBranches.SelectedItem.Text; } else { drUpdProb["Branch"] = null; drUpdProb["BranchAbbr"] = null; }\[/code\]