Why am i getting a nullreference here?

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2.00/5 (2 votes)

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I am getting a null reference in this part of the code...

C#

dr["Column" + counter.ToString()] = cell.Value.ToString();

the metrogrid is just like datagridview

What I have tried:

C#

DataTable dt = new DataTable();
            for (int i = 0; i <= metroGrid1.Columns.Count; i++)
            {
                dt.Columns.Add("Column" + i.ToString() );
            }
foreach (DataGridViewRow row in metroGrid1.Rows)
            {
                DataRow dr = dt.NewRow();
                int counter = 0;
                foreach (DataGridViewCell cell in row.Cells)
                {
                        dr["Column" + counter.ToString()] = cell.Value.ToString();
                        counter++;
                        
                }
                dt.Rows.Add(dr);
            }

now i notice that the datagridview add a 1 data from your original data so if your original data is 50, when you transfer it to datagridview is became 51. how can i prevent it from reading the added data cos it throws a null error? thanks!

Posted 23-Feb-16 17:04pm

_bluRe_

Updated 24-Feb-16 1:35am

v4

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_bluRe_ 23-Feb-16 23:05pm

is it because of the counter?

[no name] 23-Feb-16 23:26pm

Just check cell.Value is NULL?

_bluRe_ 23-Feb-16 23:30pm

no sir. it has a rowindex=49

_bluRe_ 23-Feb-16 23:31pm

or maybe because it still count the last value which is nothing?

[no name] 24-Feb-16 0:10am

Just debug and find out at which line the error is creating?

_bluRe_ 24-Feb-16 0:22am

i already use the debuger mode and the error lies in that code. hmmm

PIEBALDconsult 23-Feb-16 23:42pm

Are you _sure_ it's on that statement?

_bluRe_ 23-Feb-16 23:44pm

yes sir. im sure. i have a guess it will still get last thing which is empty so thats why the error occurs.

Sergey Alexandrovich Kryukov 24-Feb-16 0:13am

Wrong way of thinking. Just use the debugger. Such problems are amazingly simple. The debugger.
—SA

_bluRe_ 24-Feb-16 0:23am

already did sir. but i cant figure it out. why it will give a null error

Sergey Alexandrovich Kryukov 24-Feb-16 1:12am

If you did that, you could examine each and every object for null, but, by some reason, you keep complete silence about that. Perhaps you need to understand that running something under the debugger does not mean really using it. Again, this is the simplest possible case.
—SA

_bluRe_ 24-Feb-16 1:37am

okay sir. will look into it again. but i have a question, is it good to still use foreach or just for loop?

Sergey Alexandrovich Kryukov 24-Feb-16 8:20am

It's up to you. The common rule is: try to use what you need. Say, if you do any calculations with loop index, use "for", if you only traverse some set of values, use "foreach".
—SA

_bluRe_ 24-Feb-16 6:05am

i updated the question sir. thanks!

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F. Xaver · Accepted Answer · 2016-02-24T01:35:00

okay.. so that extra row is from the new line, the DataGridView shows for users to add a new line.

just set the DataGridView.AllowUserToAddRows Property[^] to False and that line is gone..
if you need that users can add rows..
you can check if the row you are reading is just an empty new line with the DataGridViewRow.IsNewRow Property (System.Windows.Forms)[^]

as addition .. have a look at this c# - How to make a DataTable from DataGridView without any Datasource? - Stack Overflow[^] accessing the columns by index is sure faster as searching it with that concatenated string ColumnName you use.

and second. Like Sergey already mentioned, you need to check if cell.value is null! I'm pretty sure it can be and you just will get your next Null Exception there!!!

Sergey Alexandrovich Kryukov · Accepted Answer · 2016-02-23T19:17:00

Please see my comments to the question. It's just boring to list what potentially could be null. It could be dr["Column" + counter.ToString()], hardly cell, but cell.Value could again be null. You had to simply check it all, to find the root of the problem. You cannot ask such question every time such things happen, but really have to learn to deal with such very basic situations all by yourself.

Not to worry. This is one of the very easiest cases to detect and fix. It simply means that some member/variable of some reference type is dereferences by using and of its instance (non-static) members, which requires this member/variable to be non-null, but in fact it appears to be null. Simply execute it under debugger, it will stop the execution where the exception is thrown. Put a break point on that line, restart the application and come to this point again. Evaluate all references involved in next line and see which one is null while it needs to be not null. After you figure this out, fix the code: either make sure the member/variable is properly initialized to a non-null reference, or check it for null and, in case of null, do something else.

Please see also: want to display next record on button click. but got an error in if condition of next record function "object reference not set to an instance of an object".

Sometimes, you cannot do it under debugger, by one or another reason. One really nasty case is when the problem is only manifested if software is built when debug information is not available. In this case, you have to use the harder way. First, you need to make sure that you never block propagation of exceptions by handling them silently (this is a crime of developers against themselves, yet very usual). The you need to catch absolutely all exceptions on the very top stack frame of each thread. You can do it if you handle the exceptions of the type System.Exception. In the handler, you need to log all the exception information, especially the System.Exception.StackTrace:
http://msdn.microsoft.com/en-us/library/system.exception.aspx,
http://msdn.microsoft.com/en-us/library/system.exception.stacktrace.aspx.

The stack trace is just a string showing the full path of exception propagation from the throw statement to the handler. By reading it, you can always find ends. For logging, it's the best (in most cases) to use the class System.Diagnostics.EventLog:
http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog.aspx.

Good luck,

—SA

Why am i getting a nullreference here?

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