casting nullable decimal from DataTable

Question

I have a DataTable table. from that table I read value from a column, which is typeof(decimal). I try to cast this value to decimal? dec. When value is null, I get an exception. I tried it this way:

decimal? dec= row["myRow"] == null ? null : (decimal?)row["myRow"]; 
//this thorws "Specified cast is not valid." Exception

decimal? dec= myRdr["myValue"] == DBNull.Value ? null : (decimal?)myRdr["myValue"]; 
//this does NOT throw an Exception??

I tried the second solution when reading from SQL data table using SQLDataReader and it worked with no problems.

The second way is the correct way. DbNull and null are not equivalent. — dbugger, Feb 06 '14 at 22:28
@dbugger: first one is reading from DataTable, second one is reading from SQL database table. Is a null value in DataTable also DBNull? — user1080533, Feb 06 '14 at 22:30
Yes, it will be. Another syntax would be `if (myRdr["myValue"] is DBNull) ...`. — DonBoitnott, Feb 06 '14 at 23:21
Yes. If you feed the table from a database, it will have DbNull values. — dbugger, Feb 07 '14 at 00:35

score 2 · Accepted Answer · answered Feb 06 '14 at 22:37

As said in comments, DBNull.Value and null are not same thing. So when you are writing:

decimal? dec= row["myRow"] == null ? null : (decimal?)row["myRow"];

you are trying doing this:

if(DBNull.Value == null)
{
    return null;
}
else
{
    return (decimal?)row["myRow"];
}

which is false so the second part (in else)

(decimal?)row["myRow"];

is executed. This throws an exception.

But in second case:

if(DBNull.Value == DBNull.Value)
{
    return null;
}
else
{
    return (decimal?)row["myRow"];
}

is true so the cast statement is not executed and null is returned.

casting nullable decimal from DataTable

1 Answers1