Object reference not set to an instance of an object when I try to join 2 DataTables

Question

I'm trying to join 2 DataTables but I' m getting this error:

Object reference not set to an instance of an object

this is what I'm doing:

DataTable NodeDataTable = new DataTable();
DataTable sdosDataTable = new DataTable();
private DataTable NodedataTable()
    {
        XmlTextReader xmlreader = new XmlTextReader(Server.MapPath("stuff.xml"));
        DataSet ds = new DataSet();
        ds.ReadXml(xmlreader);
        xmlreader.Close();
        if (ds.Tables.Count != 0)
        {
            NodeDataTable = ds.Tables[22];
        }
        return NodeDataTable;
    }
    private DataTable SdosDataTable()
    {
        XmlTextReader xmlreader = new XmlTextReader(Server.MapPath("stuff.xml"));
        DataSet ds = new DataSet();
        ds.ReadXml(xmlreader);
        xmlreader.Close();
        if (ds.Tables.Count != 0)
        {
            sdosDataTable = ds.Tables[10];
        }
        return sdosDataTable;
    }

and to join both DataTables:

private void JoinNodeSdosDT()
    {
        DataColumn obj_NodeID, obj_SdosID;
        DataSet ds1 = new DataSet();    
        NodeDataTable = NodeDataTable.Copy();
        sdosDataTable = sdosDataTable.Copy();
        ds1.Tables.Add(NodeDataTable);
        ds1.Tables.Add(sdosDataTable);
        obj_NodeID = ds1.Tables["node"].Columns["node_Id"];
        obj_SdosID = ds1.Tables["sdos"].Columns["node_Id"];    
        sdosDataTable.Columns.Add("typeCodee");           
        DataRelation obj_NodeandSdosRelation = new DataRelation("dept_reln", obj_NodeID, obj_SdosID);
        ds1.Relations.Add(obj_NodeandSdosRelation); 
        foreach (DataRow dr_NodeSods in ds1.Tables["sdos"].Rows)
        {
            DataRow dr_NondeeeR = dr_NodeSods.GetParentRow("dept_reln");
            dr_NodeSods["typeCodee"] = dr_NondeeeR["typeCode"];
        }
        DataTable dtResult = ds1.Tables["sdos"].DefaultView.ToTable(false, "node_Id", "typeCode", "sdos_Id");
        GridView1.DataSource = dtResult;           
    }

there is some any matching ID what can I do here to resolve my problem.

I removed The datatable Images there is no use of them.

Post the full stack trace please.. Edit: Check that dr_NondeeeR["typeCode"] has value. — Oscar, Jun 13 '13 at 11:59
That isn't a misspelling is it? Is it really `typeCodee` vs `typeCode`? — DonBoitnott, Jun 13 '13 at 12:00
Almost all cases of `NullReferenceException` are the same. Please see "[What is a NullReferenceException in .NET?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-in-net)" for some hints. — John Saunders, Jun 13 '13 at 19:57

score 1 · Accepted Answer · answered Jun 13 '13 at 12:01

1

Looks like dr_NondeeeR is null:

DataRow dr_NondeeeR = dr_NodeSods.GetParentRow("dept_reln");
dr_NodeSods["typeCodee"] = dr_NondeeeR["typeCode"];

because for whatever reason GetParentRow is returning null.

answered Jun 13 '13 at 12:01

Mike Perrenoud

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Are you sure? The documentation of the `DataRow.Item` property states that this should throw an `ArgumentException` when accessing a non existing column, not a `NullReferenceException` as he gets. – Jan Dörrenhaus Jun 13 '13 at 12:05
Just posted that answer myself. Looks like we came to the same conclusion, then :) – Jan Dörrenhaus Jun 13 '13 at 12:07
@Clement, that would kind of be the issue though. The `GetParentRow` method is returning `null` because there is no related row. Just check for `null` and move on if no related row exists. There's nothing to merge. – Mike Perrenoud Jun 13 '13 at 12:32
Thanks man I think I' m getting confused a lot on this things. it real worked after checking for null. – Clement Jun 13 '13 at 12:44
@Clement, I'm glad I could be of assistance! – Mike Perrenoud Jun 13 '13 at 12:47

score 1 · Answer 2 · answered Jun 13 '13 at 12:07

As the documentation of DataRow.Item (String) states, accessing a non-existing column should give an ArgumentException. What you are getting is a NullReferenceException. If that happens actually in the line you gave, then I can only assume that

DataRow dr_NondeeeR = dr_NodeSods.GetParentRow("dept_reln");

gives a null reference.

Object reference not set to an instance of an object when I try to join 2 DataTables

2 Answers2