How can the DataView object reference not be set?

Question

I have the following sample where the SourceData class would represent a DataView resulting from an Sql query:

class MainClass
{
    private static SourceData Source;
    private static DataView View;
    private static DataView Destination;
        
    public static void Main (string[] args)
    {
        Source = new SourceData();
        View = new DataView(Source.Table);
        Destination = new DataView();
            
        Source.AddRowData("Table1", 100);
        Source.AddRowData("Table2", 1500);
        Source.AddRowData("Table3", 1300324);
        Source.AddRowData("Table4", 1122494);
        Source.AddRowData("Table5", 132545);

        Console.WriteLine(String.Format("Data View Records: {0}", View.Count));         

        foreach(DataRowView drvRow in View)
        {
            Console.WriteLine(String.Format("Source {0} has {1} records.", drvRow["table"], drvRow["records"]));
            DataRowView newRow = Destination.AddNew();
            newRow["table"] = drvRow["table"];
            newRow["records"] = drvRow["records"];
        }
            
        Console.WriteLine();
        Console.WriteLine(String.Format("Destination View Records: {0}", Destination.Count));
            
        foreach(DataRowView drvRow in Destination)
        {
            Console.WriteLine(String.Format("Destination {0} has {1} records.", drvRow["table"], drvRow["records"]));
        }

    }
}

class SourceData
{
    public DataTable Table
    {
        get{return dataTable;}
    }
        
    private DataTable dataTable;
    
    public SourceData()
    {
        dataTable = new DataTable("TestTable");
        dataTable.Columns.Add("table", typeof(string));
        dataTable.Columns.Add("records", typeof(int));
    }
        
    public void AddRowData(string tableName, int tableRows)
    {
        dataTable.Rows.Add(tableName, tableRows);
    }
}

My output is:

Data View Records: 5
Source Table1 has 100 records.

Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object at System.Data.DataView.AddNew () [0x0003e] in /usr/src/packages/BUILD/mono-2.4.2.3 /mcs/class/System.Data/System.Data/DataView.cs:344 at DataViewTest.MainClass.Main (System.String[] args) [0x000e8] in /home/david/Projects/DataViewTest/SourceData.cs:29

I did some reading here: DataView:AddNew Method...
...and it would appear that I am doing this the right way. How come I am getting the Object reference not set?

What is your actual goal? Are you wanting to display the data on a DataGridView? The DataView is generally used as a DataSource container. It isn't a GUI object displays Data. — galford13x, May 27 '10 at 03:16
Keep in mind this is a test to work out an issue with why the dataview wasn't working. In the end, yes, the destination dataview will be displayed in a datagrid. — IAbstract, May 27 '10 at 03:21

score 0 · Accepted Answer · answered May 27 '10 at 02:59

0

The DataView has to be a view of something. The way you've got it, it's not a view of anything (using the default constructor) - you need to at least set the DataView.Table property to something.

I think you actually want to create a new DataTable for "destination", not a DataView.

answered May 27 '10 at 02:59

Dean Harding

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+1 - I may indeed create a DataTable instead of a DataView. Not really sure at this point. This was to solve a problem I was facing earlier and the solution was not clearly obvious by anything found on MSDN - especially being basically unfamiliar with DataViews in the first place. – IAbstract May 27 '10 at 03:04

score -1 · Answer 2 · answered May 27 '10 at 03:00

The problem resides in the declaration of the Destination dataview. In order for the AddNew() to function, there must be an object (in this case, a DataTable) in which to reference and add the new record.

By correcting the following:

Destination = new DataView(new SourceData().Table);

...the dataview will have it's requirements met. This may not make sense at first - but realize that the DataView class isn't really complete until it has a DataTable (or some other form of record set) on which to build.

The output is now:

Data View Records: 5
Source Table1 has 100 records.
Source Table2 has 1500 records.
Source Table3 has 1300324 records.
Source Table4 has 1122494 records.
Source Table5 has 132545 records.

Destination View Records: 5
Destination Table1 has 100 records.
Destination Table2 has 1500 records.
Destination Table3 has 1300324 records.
Destination Table4 has 1122494 records.
Destination Table5 has 132545 records.

How can the DataView object reference not be set?

2 Answers2