三元运算符意外结果

Question

Can someone comment on the issue I'm having. A ternary operator throws an error and the argument here is that if it evaluates to null then it should ignore the part after the colon. The Watch set up for this indicates an exception:

Int32.Parse(SQLDataReader["TrayId"].ToString())' threw an exception of Type 'System.FormatException

suggesting that it can't convert a null to a string. Is this how it works?

ShipmentId = SQLDataReader["ShipmentId"] == DBNull.Value ? 0 : Int32.Parse(SQLDataReader["ShipmentId"].ToString()),

Answer 1

The column ShipmentId is an integer, but it is also nullable , which means that the c# type is int? .

The error boils down to this code:

Int32.Parse((string)null)

The parsing gives up because you can't turn null into an int .

To fix this, use TryParse .

int ShipmentId = 0;
int.TryParse(SQLDataReader["ShipmentId"].ToString(), out ShipmentId);

This will cover if the value is null , or if for some strange reason the value can't actually be converted to an int (like "asdf121343fds").

Answer 2

建议与DBNull进行比较，使用DBNull.Value.Equals方法，如本页所述： http ： //msdn.microsoft.com/en-us/library/system.dbnull.value.aspx

ShipmentId = DBNull.Value.Equals(SQLDataReader["ShipmentId"]) ? 0 : Int32.Parse(SQLDataReader["ShipmentId"].ToString());