reading null value in a dataset

[monika varshney]

I have to read a database using dataset. My database has some null values.
My code is working fine when there is a value in the database. But when there is a null value in the database, dataset throws an exception.

I have to read null value in a variable. I have tried this....

if (dr["value"] != DBNull.Value)
{
abc = Convert.ToDouble(dr["value"]);
MessageBox.Show("value is:" + abc.ToString());
}
else
{
abc=null;
MessageBox.Show("value is:" + abc.ToString());
}

But it gives an error that null can not be assigned to a double variable(here abc is a double variable). How can I read a null value in a variable.
Can anybody guide me....

[anijos]

Try declaring abc as a nullable variable

double? abc =null;

[Siyodia]

try Convert.isDBNull(database-value-to-check)==true

[kenobewan]

I would have expected your null value to be read as a string by the datareader, hence only needing to test for an empty string.

[rpicilli]

@monika varshney
There some way to do that.

The easiest is to put your code inside a Try Cath constrution

Try
your code
Catch Ex
verify if was throwed a dbnull exception
if this is the case make abc = 0 or other value
End


I hope this help you

Rpicilli

[mldisibio]

Just to clarify:
Your check (dr["value"]!= DbNull.Value) is a correct way of checking for a null value from the database, and is an excellent practice.
[The other way is "dr[0].IsDbNull" if you know the column index.]

The error that null cannot be assigned has nothing to do with your database values. It is only because the double "abc" is a value type. Value types have 0 or false as their default values and cannot be assigned to null. Only reference types (objects such as a DataReader or any other class) can be assigned null directly.

So your option is, as anijos said, declare abc as a Nullable double, which was a new Type added to the Framework in 2.0 for the most part to accomodate null database values. It basically says: I have a double which either has some legitimate value, or is undefined. Beware that using this option brings some extra work. Read best practices for using Nullable types: Nullable Types

The other option is to leave abc as a normal double, but assign it a value that your code knows equates to a null in the database, such as 0, -1 or Double.MinValue. However, if all those are legitimate values from your data field, then you must find a work around (such as nullable) that signifies the source column is null.

[mldisibio]

P.S. kenobewan:
@kenobewan
Using the "dataReader["columnName"] syntax to pull a value returns the value boxed as an object. So if the source query pulls the field as a numeric type, it will be a boxed numeric.

For any datatype, even if varchar or such, if the column value is null, it is pulled as DbNull.Value, not as a string.

:) Mike

[kenobewan]

@mldisibio
They say that you learn something new everyday, good response :).