OdbcDataReader calling IsDBNull changes the behaviour of Get methods

[Dan Mos]

What is the query that gets executed on the DB while doing a cmd.ExecuteReader()? Maybe there is something missing/wrong there.

All the best, Dan

[__John_]

Hi MDL=>Moshu, thanks for the reply. The cast that fails is internal to the Get methods. So for example...

var Val = odbcDataReader.GetInt32(nColNumber);

Still thows the same exception. I am using the data provider for MS SQL Sever that came with vs2010. SQL Server version is 2008. Thanks - John.

[__John_]

Hi Moshu, The command that I execute is as follows..

odbcSelectCommand.CommandText = @"SELECT name, iq FROM Table_People WHERE iq < 3 ORDER BY iq";

I get the 'name' and 'iq' but I am only concurned with 'iq' for now.

[Dan Mos]

In your statement var Val is the same as int Val cause GetInt32() returns a int on success. var Val = odbcDataReader[nColNumber]; is what you should use. Val will be an object of type Object and you can check for null and do the casting later. This is obviously for debugging purposes. Cause the cast and checking are not necessarily once you figure out the problem.

All the best, Dan

[Dan Mos]

OK. Heres what I would do. Run that query in SQLManagementStudio and check the results. Are there any? Or some with a field null or...

All the best, Dan

[Dan Mos]

Also since you are not using a Select * statement, I would drop the lines of code that get the ordinals. You know them name is 0 and iq is 1 so in a nutshell:

 while(dr.Read()){
 string name = dr\[0\].ToString();
 int iq = (int)dr\[1\];

}

All the best, Dan

[__John_]

Hi MDL=Moshu, thanks for looking at this. The code now looks like this...

int nColName = odbcDataReader.GetOrdinal(@"name");
int nColNumber = odbcDataReader.GetOrdinal(@"iq");

while (odbcDataReader.Read())
{
    if(odbcDataReader.IsDBNull(nColNumber))
    {
        continue;
    }

    int nNumber = odbcDataReader.GetInt32(nColNumber);

    string strNumber = odbcDataReader.GetString(nColNumber);

}

No nulls are returned and the behaviour is the same ie. with the call to IsDBNull() GetString() fails, and without it, GetInt32() fails.

[__John_]

Thanks dan, But I think my way is more robust and future proof. ie. it will still work even with 'SELECT *'. - John

[Dan Mos]

This is my final reply. First check my post below. Second you're using nColNumber for both the name and the iq. That's not right and I pointed it out on my first post to this thread. If for some reason you want a string representation of the number/iq then just use strNumber = nNumber.ToString(); Hope it helps. I'm out of CP for today. Going to program some beers now. :)

All the best, Dan

[Dan Mos]

True. Then use dr["name"] and dr["iq"] and it will be future proof.

All the best, Dan

[__John_]

Hi Dan, Using the indexer, casting to an int always works and casting to a string always fails (invalid cast). So the behavour is predictable and my problem 'kind of' goes away. So for now I will use the indexer and not use the Get???? methods. @PIEBALDconsult: You mentioned indexers earlier but I was not sure what you were getting at. Turns out you were on to something. Thanks.

[__John_]

Hi MDL=>Moshu, Using an indexer as you sagest (with column name) is fine if you only have a small number of fields, but if the number of fields is higher then I think you will take a performance hit.

[Luc Pattyn]

__John_ wrote:

my way is more robust

That doesn't sound very convincing as your code does not produce the expected results to begin with. Have you already figured nColNumber is appearing too many times in your code? (courtesy MDL) :omg:

Luc Pattyn [My Articles] Nil Volentibus Arduum

[Dan Mos]

Again you used nColNumber twice. You should have used nColNumber and nColName. That was your problem. And yeah I don't use GetXXX either. But that wasn't what caused the exception. You tried to get a string from an int DB value or a int form a varchar DB value courtesy of using twice nColNumber.

All the best, Dan

[Ennis Ray Lynch Jr]

Your code changed. When you test things you should change 1 item at a time, however, you have also changed order and type. In your last example, you are calling GetString and GetInt32 on nColNumber which refers to an integer, your code should read:

int nameColumn = odbcDataReader.GetOrdinal(@"name");
int idColumn = odbcDataReader.GetOrdinal(@"iq");
while (odbcDataReader.Read()){
odbcDataReader.IsDBNull(idColumn);
int iqValue= odbcDataReader.GetInt32(idColumn);
string nameValue = odbcDataReader.GetString(nameColumn );
}

I just typed it in the post window but you can see how different variable names make it easier to see.

Need custom software developed? I do custom programming based primarily on MS tools with an emphasis on C# development and consulting. I also do Android Programming as I find it a refreshing break from the MS. "And they, since they Were not the one dead, turned to their affairs" -- Robert Frost

[PIEBALDconsult]

MDL=>Moshu wrote:

string name = dr[0].ToString();

It's already a string, just cast it -- string name = (string) dr[0]; , no need to call a method.

[Dan Mos]

True. Is there a performance reason too, or just style? Cause if there is one, I might need to change some lines of code. I've always happily used ToString() on datareader where the element is a text/string. :)

All the best, Dan

[PIEBALDconsult]

There must be at least a small overhead in calling the method.