how to get maxid using method fro re use in same from

Below I give you some advice on your code. However I have grave concerns about this function. It looks as if you are attempting to find the maximum current id in order to assign the next value. This is a very bad design! What if another user adds another record while this code is running? (Very likely in multi-user environments). Far better is to use the right tool for the job - an Identity column[^] on the database


Code Review:

First look at the line

FunGetMaxID() = (reader["id"]);

That is very VB6 in style and will result in the compile error

Quote:

The left-hand side of an assignment must be a variable, property or indexer

Next - learn about the using statement[^] - it is especially useful for working with sql connections. You do not need to explicitly close the connection nor Dispose of it.
While we're on that subject, look at

con = new SqlConnection(cs.Dbcon);
con.Close();

There is no point in that .Close() - you have only just created the object!

Now consider your sql statement

SELECT ISNULL(MAX(ClassId),0)+1  AS Id FROM Class

It is better to handle the potential to be null at the source rather than escalating a null up calculations/functions/etc i.e. this would be better

SELECT MAX(ISNULL(ClassId,0)) AS Id FROM Class

The next problem is that you have

private string FunGetMaxID ()

but the maximum Id is likely to be an integer, so you should be returning an integer from your function. Note as well, the way you have named the function implies that you want the existing maximum value for id - but you are returning the maximum plus 1. Either rename your function or return the correct value

As Manas_Kumar has suggested, it is more appropriate to use ExecuteScalar[^] in this instance (you don't have to, it's just more appropriate)

Regarding the Try-Catch block, you may also find these articles useful: Exception Handling Best Practices in .NET[^] and Best Practices for Exceptions - MSDN[^]

I would do something like

private int FunGetMaxID()
{
    var maxId = 0;

    try
    {
        using (var con = new SqlConnection(cs.Dbcon))
        {
            con.Open();
            const string sql = "Select max(IsNull(ClassId,0)) as Id from Class ";

            using (var cmd = new SqlCommand(sql, con))
            {
                cmd.CommandType = CommandType.Text;
                maxId = (int)cmd.ExecuteScalar();
            }
        }
    }
    catch (SqlException ex)
    {
        MessageBox.Show("Error", "Message" + ex.Message);
    }
    return maxId;
}