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?

Answer 1

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.

Answer 2

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.