Process of comparing sting and int (get from query) value in c#

Question

I want to do auto generate the value of a field which type is int. Below is my code and it works successfully when I entry one column data manually in database.

But when the DB is totally empty then the output of this query is 'null'. I want when the output of this query is 'null' then the value of createCode will be 0 (zero).

ConnectionObj.Open();
string query = "SELECT MAX(t_code) FROM teacher_table";
CommandObj.CommandText = query;
int createCode = Convert.ToInt32(CommandObj.ExecuteScalar());
if (createCode == 'null')  //can't compare string and int  
{
    createCode = 0;
    return createCode;
}
else
{
    return createCode;
}

How I can do it?

Error is telling you what is wrong. `createCode` is of type `int` and you are trying to compare it with `null` — Habib, May 12 '14 at 16:37
possible duplicate of [How to get nullable out of the database](http://stackoverflow.com/questions/9503698/). — Dour High Arch, May 12 '14 at 16:39
Please read my question again. I think you don;t understand my problem @Habib — Ikr, May 12 '14 at 16:41
@Ikr, your `createCode` is `int`, it can never be null, you will get an exception with `Convert.ToInt32`. — Habib, May 12 '14 at 16:45
@Habib bro I know that. Please see my whole question with code again. — Ikr, May 12 '14 at 16:47

score 1 · Accepted Answer · answered May 12 '14 at 16:40

You need to check for DBNull.Value before casting it to an int:

ConnectionObj.Open();
string query = "SELECT MAX(t_code) FROM teacher_table";
CommandObj.CommandText = query;
object code = CommandObj.ExecuteScalar();
if (code == DBNull.Value)
{
    return 0;
}

return Convert.ToInt32(code);

score 1 · Answer 2 · answered May 12 '14 at 16:43

1

In Mysql, you can use IFNULL() like this:

string query = "SELECT IFNULL(MAX(t_code),0) FROM teacher_table";

In Sql Server you can use ISNULL() like this:

  string query = "SELECT ISNULL(MAX(t_code),0) FROM teacher_table";

answered May 12 '14 at 16:43

Ehsan Sajjad

61,834
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score 0 · Answer 3 · answered May 12 '14 at 16:38

0

Change if (createCode == 'null') to if (createCode == DBNull.Value).

answered May 12 '14 at 16:38

rory.ap

34,009
10
83
174

score 0 · Answer 4 · answered May 12 '14 at 16:40

0

Use:

object obj = CommandObj.ExecuteScalar();
if (obj == null || obj == DBNull.Value)
    return 0;
else
    return Convert.ToInt32(obj);

Since your createCode is of type int, it can never be null, instead your current code will not even compile.

answered May 12 '14 at 16:40

user2711965

1,795
2
14
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Process of comparing sting and int (get from query) value in c#

4 Answers4