设置列值时出现StrongTypingException

Question

Can anyone please tell me why I get a StrongTypingException when ASSIGNING a value to a column in a strongly typed DataTable? (I understand why I get it if I were to READ the column with DBNull value)

In the example below I'm trying to assign a value from one DataTable to another (all columns in the example are of type Int32 ). I can assign a value to the 'newOrderRow.items' column but when I do the same with the 'newOrderRow.debcode' column an Exception is thrown! Why?!

Some of the things I've tried so far (without any luck):
- Assign hard coded value instead of 'calclineRow.debcode'
- Call newOrderRow.SetdebcodeNull() before assigning another value
- Changed DefaultValue property on 'debcode' column in 'orderrows' table from DBNull to -1 and it STILL throws the Exception and says it's DBNull !!!

myDataSet.orderrowsRow newOrderRow;

foreach (MyDataSet.calclinesRow calclineRow in myDataSet.calclines)
{
    newOrderRow = myDataSet.orderrows.NeworderrowsRow();  //Create new 'orderrows' row

    //Assign values from one DataTable to another
    if (!calclineRow.IsitemsNull())
        newOrderRow.items = calclineRow.items;  //calclineRow.items == 1. Assignment successful
    if (!calclineRow.IsdebcodeNull()) 
        newOrderRow.debcode = calclineRow.debcode; //calclineRow.debcode == 556. Assignment raises System.Data.StrongTypingException ! (See message below)


    myDataSet.orderrows.AddorderrowsRow(newOrderRow);
}

/*Exception Message:
=====================

System.Data.StrongTypingException: The value for column 'debcode' in table 'orderrows' is DBNull. 
---> System.InvalidCastException: Specified cast is not valid. 
at MyProject.MyDataSet.orderrowsRow.get_debcode() in Q:\MyProjFolder\DataSets\MyDataSet.Designer.cs:line 21680
*/

Answer 1

You have to use the auto-generated SetNull methods if the nullable property is null:

if (!calclineRow.IsitemsNull())
    newOrderRow.items = calclineRow.items;  
else
    newOrderRow.SetitemsNull();

if (!calclineRow.IsdebcodeNull()) 
    newOrderRow.debcode = calclineRow.debcode; 
else
    newOrderRow.SetdebcodeNull();

You also have to ad the new DataRow to the table in the loop since NeworderrowsRow does this not automatically.

myDataSet.orderrows.AddNeworderrowsRow(newOrderRow);

The line where the exception occurs(MyDataSet.Designer.cs:line 21680) suggests that it's raised from an auto-generated method of the DataSet which reads this property. Since you haven't used SetdebcodeNull it does not know that it's null and throws the StrongTypingException when it tries to read it.

Answer 2

You could try this:

if (!calclineRow.IsdebcodeNull()) 
    newOrderRow["debcode"] = calclineRow.debcode;

While I admit it doesn't make a lot of sense, it appears as if calling the Set for newOrderRow.debcode has the effect of calling the Get , which as noted throws an exception if the underlying property is DbNull.

Answer 3

SOLVED. Sorry, my bad.

I forgot that I was doing things with the 'debcode' column in the OnColumnChanging event handler on my DataTable. When I disabled that it all worked as it should.

Thanks anyway!