OdbcDataReader calling IsDBNull changes the behaviour of Get methods

__John_

Hi, If I do...

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

while (odbcDataReader.Read())
{
    string strName = odbcDataReader.GetString(nColNumber);
    int nNumber = odbcDataReader.GetInt32(nColNumber);
}

GetString() succeeds. GetInt32() throws an exception... [System.InvalidCastException] = {"Specified cast is not valid."} However if I do...

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

while (odbcDataReader.Read())
{
    odbcDataReader.IsDBNull(nColNumber);

    int nNumber = odbcDataReader.GetInt32(nColNumber);

    string strNumber = odbcDataReader.GetString(nColNumber);

}

Now GetInt32() succeeds. And GetString() throws an exception... [System.InvalidCastException] = {"Unable to cast object of type 'System.Int32' to type 'System.String'."} FYI: The column in question 'IQ' is of type 'int'. It seems that calling IsDBNull() first, changes the behaviour of the Get methods. Can anyone explain what is going on? Thanks - John.

kevinnicol

My only guess at this point is in the first test you are calling GetString first, and in the second test you are calling GetInt32 first. It's possible the the reader is casting your value to what you asked for first?

PIEBALDconsult

Well, personally, I never use the Get methods anyway; I just use the indexers.

__John_ wrote:

odbcDataReader.IsDBNull(nColNumber);

Shouldn't you be testing the result?

__John_

Hi kevinnicol, The only reason for putting them in a different order is because I put the call that throws the exception last. Changing the order just means that you never get to second call because the first throws an exception. @PIEBALDconsult: Yes, I should check the return but it makes no differance to my question.

Dan Mos

Shouldn't it be like:

while (odbcDataReader.Read())
{
string strName = odbcDataReader.GetString(nColNumbernColName);
int nNumber = odbcDataReader.GetInt32(nColNumber);
}

__John_ wrote:

@PIEBALDconsult: Yes, I should check the return but it makes no differance to my question.

I beg to differ. If it's true e.g. the value is null it will throw an exception on the int part. Ints can't be null. Either use a nullable int? or check for null first.

All the best, Dan

__John_

Non of the returns are null so checking makes no fifferance.

Dan Mos

Then yes. But you failed to specify that. Honestly I'm out of ideas. Maybe a stupid question like "what is the DB type?" is all I got left. Maybe it should be long or short or... on the C# side. :)

All the best, Dan

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:

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