Walking through a DataView |   | One of my favorite objects in ADO.NET is DataView Class. Basically, ADO.NET uses a Database metaphor where the DataSet object corresponds to a "database", the DataTable class corresponds to a table in that database and the dataview corresponds to a "view" object. It's a pretty good metaphor, but it's not perfect. One of the main differences is that a DataView is only based on one table where in a database, it's common practice to create a view on multiple joined tables. With that distinction, the metaphor holds. As such, each tuple in a database table corresponds to a DataRow object. So iterating throught a datatable entails walking through its Rows collection. So a common assumption is that a DataView has a Rows collection as well. After all, there's really not a difference in how you deal with a Row in a database table and a database view. There's a slight distinction in ADO.NET. A DataView is composed of DataRowView objects. In a datatable, you get the number of tuples by using DataTable.Rows.Count property. In a Dataview, you just use the DataView's .Count property.
Anyway, enough background. If you want to iterate through a DataView, I show you three different methods in both VB.NET and C#. I use two different constructors just to show you they are equal (probably doesn't fit in with the rest of the discussion, but I figured I'd mention it). The comments should explain everything, but if you have any questions, my email is provided on my profile and by all means drop me a line:
VB.NET
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim dt As New DataTable
da.Fill(dt)
Debug.WriteLine(dt.Rows.Count.ToString)
'Create 2 new DataViews, both yeild the same result
'Note that there's a third constructor which allows you
'to specify a RowFilter, Sort, and RowStateFilter
Dim dv As DataView = dt.DefaultView
Dim dv2 As DataView = New DataView(dt)
Debug.Assert(dv.Count = dv2.Count)
'Let walk through the first one using IEnumerator
Dim iterator As IEnumerator = dv.GetEnumerator
Dim drv As System.Data.DataRowView
Dim i As Integer = 0
While iterator.MoveNext
drv = CType(iterator.Current, System.Data.DataRowView)
i += 1
End While
Dim x As Integer = 0
For Each drv2 As System.Data.DataRowView In dv2
x += 1
Next
Dim a As Integer = 0
For z As Integer = 0 To dv.Count - 1
a += 1
Next
Debug.Assert(i = dt.Rows.Count)
Debug.Assert(x = dt.Rows.Count)
Debug.Assert(a = dt.Rows.Count)
End Sub |
C#
private void Form1_Load(object sender, System.EventArgs e)
{
DataTable dt = new DataTable();
da.Fill(dt);
Debug.WriteLine(dt.Rows.Count.ToString());
//Create 2 new DataViews, both yeild the same result
//Note that there's a third constructor which allows you
//to specify a RowFilter, Sort, and RowStateFilter
DataView dv = dt.DefaultView;
DataView dv2 = new DataView(dt);
//Assertion does not fail so they are the same.
Debug.Assert(dv.Count == dv2.Count);
IEnumerator iterator = dv.GetEnumerator();
DataRowView drv;
System.Int32 i= 0;
while(iterator.MoveNext())
{
drv = (DataRowView)iterator.Current;
i++;
}
System.Int32 x = 0;
foreach(DataRowView drv2 in dv)
{
x++;
}
System.Int32 a = 0;
//Remember this is C#, so we don't want to subtract the 1 from Count
for(System.Int32 z = 0; z < dv.Count; z++)<BR>
{
a++;
}
Debug.Assert(i == dt.Rows.Count);
Debug.Assert(x == dt.Rows.Count);
Debug.Assert(a == dt.Rows.Count);
} |
|
