Comparing DefaultValue to DataRow column values

7 May 2006 11:05 PM

James

The code below should it seems to me work but doesnt:

bool rowChanged = false;
foreach (DataColumn col in row.Columns)
if (col != row.Table.PrimaryKey[0])
if (col.DefaultValue != row[col])
rowChanged = true;

When debugging it I found that for a bool type column whose
DefaultValue is true and whose row[col] value is true, the code goes
into the last if statement and sets rowChanged to true. When I get
VS.Net Command window to print out the value of (col.DefaultValue !=
row[col]) it even says false. However somewhat suspiciously the full
dump of the variables col.DefaultValue and row[col] are different
although they imply their values are the same.

I seem to remember something about DataRow column values being held in
types which allow for a DBNull.Value value. So I may be trying to
compare two different types here I suspect.

However I can't find any way of cooercing an object into another type
where that type is held in a variable something like this:

System.Type tp;
object x = (tp)row[col];

I've had to write some hideous code which deals with all the different
possible types on a case by case basis.

Anyone got a better idea?

PS anyone who writes a reply asking why I want to do this without
suggesting any solution will get flamed

8 May 2006 5:15 AM

Cor Ligthert [MVP]

James,

I don't know how you are debugging but the result of this loop will be in my
opinion always be true.

Cor

Show quote

"James" <jim***@hotmail.com> schreef in bericht
news:1147043158.633592.249480@u72g2000cwu.googlegroups.com...
> The code below should it seems to me work but doesnt:
>
> bool rowChanged = false;
> foreach (DataColumn col in row.Columns)
> if (col != row.Table.PrimaryKey[0])
> if (col.DefaultValue != row[col])
> rowChanged = true;
>
> When debugging it I found that for a bool type column whose
> DefaultValue is true and whose row[col] value is true, the code goes
> into the last if statement and sets rowChanged to true. When I get
> VS.Net Command window to print out the value of (col.DefaultValue !=
> row[col]) it even says false. However somewhat suspiciously the full
> dump of the variables col.DefaultValue and row[col] are different
> although they imply their values are the same.
>
> I seem to remember something about DataRow column values being held in
> types which allow for a DBNull.Value value. So I may be trying to
> compare two different types here I suspect.
>
> However I can't find any way of cooercing an object into another type
> where that type is held in a variable something like this:
>
> System.Type tp;
> object x = (tp)row[col];
>
> I've had to write some hideous code which deals with all the different
> possible types on a case by case basis.
>
> Anyone got a better idea?
>
> PS anyone who writes a reply asking why I want to do this without
> suggesting any solution will get flamed
>

8 May 2006 4:12 PM

James

Hi Cor

Thanks for replying. Why should this loop always be true? To simplify
matters I have assumed there are no calculated columns in the table,
however taking this into account would not make the loop always true.
When debugging this means of comparison worked on most of the columns
in the table including null columns and string-valued columns so
demonstrably the loop would be false if the table had only the columns
that I found compared correctly.

James

8 May 2006 4:21 PM

Cor Ligthert [MVP]

James,

Let say it in another way, when it is set to true it will never become false
anymore.

Cor

Show quote

"James" <jim***@hotmail.com> schreef in bericht
news:1147104734.502606.289020@g10g2000cwb.googlegroups.com...
> Hi Cor
>
> Thanks for replying. Why should this loop always be true? To simplify
> matters I have assumed there are no calculated columns in the table,
> however taking this into account would not make the loop always true.
> When debugging this means of comparison worked on most of the columns
> in the table including null columns and string-valued columns so
> demonstrably the loop would be false if the table had only the columns
> that I found compared correctly.
>
> James
>

8 May 2006 5:47 PM

James

Hi Cor

It seems you are unaware of standard C/C# syntax.

if (test)
action();

will only perform action() if test is true. Enclosing {} are
unnecessary for a single statement.
However the intention of the code should have been obvious. If you
have any helpful comments they
would be welcome. Conversely unhelpful comments will not.

11 May 2006 11:04 PM

Mark Ashton

I think what you are running into is reference comparision vs. value
comparision.
This example results in "false, true, true"
        object x = true, y = true;
        Console.WriteLine("{0} {1} {2}", x == y, x != y, x.Equals(y));

If you change your loop to, it should be closer to what you want.

bool rowChanged = false;
DataColumn pkey = row.Table.PrimaryKey[0];
foreach (DataColumn col in row.Columns) {
if (col != pkey && !col.DefaultValue.Equals(row[col])) {
       rowChanged = true;
       break;
    }
}

// side note: DataTable.PrimaryKey returns a new DataColumn[] on every call.

Show quote

"James" wrote:

> The code below should it seems to me work but doesnt:
>
> bool rowChanged = false;
> foreach (DataColumn col in row.Columns)
>   if (col != row.Table.PrimaryKey[0])
>     if (col.DefaultValue != row[col])
>       rowChanged = true;
>
> When debugging it I found that for a bool type column whose
> DefaultValue is true and whose row[col] value is true, the code goes
> into the last if statement and sets rowChanged to true. When I get
> VS.Net Command window to print out the value of (col.DefaultValue !=
> row[col]) it even says false. However somewhat suspiciously the full
> dump of the variables col.DefaultValue and row[col] are different
> although they imply their values are the same.
>
> I seem to remember something about DataRow column values being held in
> types which allow for a DBNull.Value value. So I may be trying to
> compare two different types here I suspect.
>
> However I can't find any way of cooercing an object into another type
> where that type is held in a variable something like this:
>
> System.Type tp;
> object x = (tp)row[col];
>
> I've had to write some hideous code which deals with all the different
> possible types on a case by case basis.
>
> Anyone got a better idea?
>
> PS anyone who writes a reply asking why I want to do this without
> suggesting any solution will get flamed
>
>