OdbcDataReader calling IsDBNull changes the behaviour of Get methods
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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
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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.
This is my final reply. First check my post below. Second you're using
nColNumberfor both thenameand theiq. 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
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Thanks dan, But I think my way is more robust and future proof. ie. it will still work even with 'SELECT *'. - John
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In your statement
var Valis the same asint Valcause 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
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.
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Thanks dan, But I think my way is more robust and future proof. ie. it will still work even with 'SELECT *'. - John
__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
nColNumberis appearing too many times in your code? (courtesy MDL) :omg:Luc Pattyn [My Articles] Nil Volentibus Arduum
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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.
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
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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.
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
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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
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. -
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. -
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
There must be at least a small overhead in calling the method.