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Object cannot be cast from DBNull to other types - using SQL 2000/C#

Posted on 2007-03-27
6
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Last Modified: 2008-01-09
Hi there

Wonder if you can help me, I keep getting the following error:

Object cannot be cast from DBNull to other types.

I have the following code:

dateSend_ADHOC          = myDataReader["DT_SEND"].ToString();

if (dateSend_ADHOC == "")  // I guess I get the error here?
{
  .
  .
  .
  .
}

dateSend_ADHOC could have a date or left blank i.e. in SQL left as null

How do I get this to work?

Many thanks
Mousemat24
0
Comment
Question by:mousemat24
6 Comments
 
LVL 18

Expert Comment

by:DarrenD
ID: 18800181
Hi

not positive but try...

if (dateSend_ADHOC == System.DBNull.Value)  // I guess I get the error here?
{
  .
  .
  .
  .
}
0
 
LVL 18

Expert Comment

by:DarrenD
ID: 18800193
Or...

returnValue = Convert.IsDBNull(value)
0
 

Author Comment

by:mousemat24
ID: 18800216
When I do the following - if (dateSend_ADHOC == System.DBNull.Value)
I get the following error:


Error      1      Operator '==' cannot be applied to operands of type 'string' and 'System.DBNull'      
0
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LVL 13

Expert Comment

by:KarinLoos
ID: 18800253
or jsut query length of resulting string, if its 0, then input field was empty or null
dateSend_ADHOC          = myDataReader["DT_SEND"].ToString();

if (dateSend_ADHOC.Length == 0)  // I guess I get the error here?
{
 ...
}
0
 
LVL 18

Accepted Solution

by:
DarrenD earned 250 total points
ID: 18800273
You should probably use the function

Convert.ISDBNull above to check if it is a Null and then perform whatever logic.

This returns a boolean which you can then test for

if (Convert.ISDBNull(dateSend_ADHOC)) {
}

Darren
0
 
LVL 142

Assisted Solution

by:Guy Hengel [angelIII / a3]
Guy Hengel [angelIII / a3] earned 250 total points
ID: 18801822
you should work this way:

if (myDataReader["DT_SEND"] = DBNull.Value)  // I guess I get the error here?
{
  .
  .
  .
  .
}
else
{
  dateSend_ADHOC          = myDataReader["DT_SEND"].ToString();

}

or, if you don't need to distinguish between a NULL and an empty string, use the COALESCE() function in the query to replace the null directly by empty string.

SELECT COALESCE( yourfield, '' ) as yourfield FROM yourtable ...
0

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