C# 4 and .NET 4

Managing Data and relationships: The Dataset Class ❘ 837
ProPerTy
AllowDBNull
AutoIncrement
AutoIncrementSeed
AutoIncrementStep
Caption
ColumnMapping
ColumnName
DataType
DefaultValue
Expression
desCriPTion
If true, permits the column to be set to DBNull.
Defines that this column value is automatically generated as an incrementing number.
Defines the initial seed value for an AutoIncrement column.
Defines the step between automatically generated column values, with a default of one.
Can be used for displaying the name of the column onscreen.
Defines how a column is mapped into XML when a DataSet class is saved by calling
DataSet.WriteXml.
The name of the column; this is auto-generated by the runtime if not set in the
constructor.
Defines the System.Type value of the column.
Can define a default value for a column.
Defines the expression to be used in a computed column.
Data rows
This class makes up the other part of the DataTable class. The columns within a data table are defined in
terms of the DataColumn class. The actual data within the table is accessed by using the DataRow object.
The following example shows how to access rows within a data table. First, the connection details:
string source = "server=(local);" +
" integrated security=SSPI;" +
"database=northwind";
string select = "SELECT ContactName,CompanyName FROM Customers";
SqlConnection conn = new SqlConnection(source);
The following code introduces the SqlDataAdapter class, which is used to place data into a DataSet class.
SqlDataAdapter issues the SQL clause and fills a table in the DataSet class called Customers with the
output of the following query. (For more details on the SqlDataAdapter class, see the section “Populating a
DataSet” later in this chapter.)
SqlDataAdapter da = new SqlDataAdapter(select, conn);
DataSet ds = new DataSet();
da.Fill(ds, "Customers");
In the following code, you might notice the use of the DataRow indexer to access values from within that
row. The value for a given column can be retrieved by using one of the several overloaded indexers. These
permit you to retrieve a value knowing the column number, name, or DataColumn:
foreach(DataRow row in ds.Tables["Customers"].Rows)
Console.WriteLine("'{0}' from {1}", row[0],row[1]);
One of the most appealing aspects of DataRow is that it is versioned. This permits you to receive various
values for a given column in a particular row. The versions are described in the following table.
daTaroW Version Value
Current
Default
Original
Proposed
desCriPTion
The value existing at present within the column. If no edit has occurred, this will be
the same as the original value. If an edit (or edits) has occurred, the value will
be the last valid value entered.
The default value (in other words, any default set up for the column).
The value of the column when originally selected from the database. If the DataRow’s
AcceptChanges() method is called, this value will update to the Current value.
When changes are in progress for a row, it is possible to retrieve this modified
value. If you call BeginEdit() on the row and make changes, each column will
have a proposed value until either EndEdit() or CancelEdit() is called.
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