Tài liệu Adding Records with a GUID Primary Key pdf

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Recipe 4.6 Adding Records with a GUID Primary Key
Problem
You want to add records to a DataTable that uses a GUID as its primary key and has
related child records.
Solution
Use the DataTable.RowChanging event handler.
The sample code contains three event handlers:
Form.Load
Sets up the sample by creating a DataSet containing two tables, a parent and child,
both having a GUID primary key column with the DefaultValue set to a new
GUID. A relation based on the parent table GUID and a foreign key GUID in the
child table is added to the DataSet. The default view of the
p
arent table is bound to
the data grid on the form. Event handlers are added for the RowChanging event in
both the parent and child DataTable objects.
Parent DataTable.RowChanging
Sets the default value of the primary key column in the parent table to a new
GUID value when a new row has been added to the parent table.
Child DataTable.RowChanging
Sets the default value of the primary key column in the child table to a new GUID
value when a new row has been added to the child table.
The C# code is shown in Example 4-9
.
Example 4-9. File: AddGuidPKRecordForm.cs
// Namespaces, variables, and constants
using System;
using System.Data;
using System.Data.SqlClient;

// Table name constants
private const String PARENTTABLENAME = "ParentTable";
private const String CHILDTABLENAME = "ChildTable";

// Table column name constants for Parent table
private const String PARENTID_FIELD = "ParentId";
private const String FIELD1_FIELD = "Field1";
private const String FIELD2_FIELD = "Field2";

// Table column parameter name constants for Parent table
private const String CHILDID_FIELD = "ChildId";
private const String FIELD3_FIELD = "Field3";
private const String FIELD4_FIELD = "Field4";

private DataSet ds;

// . . .

private void AddGuidPKRecordForm_Load(object sender, System.EventArgs e)
{
DataColumnCollection cols;
DataColumn col;

// Build the parent table.
DataTable parentTable = new DataTable(PARENTTABLENAME);
cols = parentTable.Columns;
col = cols.Add(PARENTID_FIELD, typeof(Guid));
col.DefaultValue = Guid.NewGuid( );
parentTable.PrimaryKey = new DataColumn[] {col};

cols.Add(FIELD1_FIELD, typeof(String)).MaxLength = 50;
cols.Add(FIELD2_FIELD, typeof(String)).MaxLength = 50;

// Build the child table.
DataTable childTable = new DataTable(CHILDTABLENAME);
cols = childTable.Columns;
col = cols.Add(CHILDID_FIELD, typeof(Guid));
col.DefaultValue = Guid.NewGuid( );
childTable.PrimaryKey = new DataColumn[] {col};
cols.Add(PARENTID_FIELD, typeof(Guid)).AllowDBNull = false;
cols.Add(FIELD3_FIELD, typeof(String)).MaxLength = 50;
cols.Add(FIELD4_FIELD, typeof(String)).MaxLength = 50;

// Add the tables to the DataSet and create the relationship.
ds = new DataSet( );
ds.Tables.Add(parentTable);
ds.Tables.Add(childTable);
ds.Relations.Add(new DataRelation("Parent_Child_Relation",
parentTable.Columns[PARENTID_FIELD],
childTable.Columns[PARENTID_FIELD], true));

// Bind the parent table default view to the grid.
dataGrid.DataSource = parentTable.DefaultView;

// Event handlers to generate new GUIDs for primary keys
parentTable.RowChanging +=
new DataRowChangeEventHandler(parentTable_RowChanging);
childTable.RowChanging +=
new DataRowChangeEventHandler(childTable_RowChanging);
}

private void parentTable_RowChanging(object sender,
DataRowChangeEventArgs e)
{
if(e.Action == DataRowAction.Add)
ds.Tables[PARENTTABLENAME].Columns[
PARENTID_FIELD].DefaultValue = Guid.NewGuid( );
}

private void childTable_RowChanging(object sender,
DataRowChangeEventArgs e)
{
if(e.Action == DataRowAction.Add)
ds.Tables[CHILDTABLENAME].Columns[
CHILDID_FIELD].DefaultValue = Guid.NewGuid( );
}
Discussion
A Globally Unique Identifier (GUID) is a 128-bit integer that is statistically unique; you
can use it wherever a unique identifier is needed. The System.Guid type is a .NET
structure that contains members to facilitate working with GUIDs.
The RowChanging event of the DataTable is raised when a DataRow is changing. The
action that occurred on the row can be determined by the Action property of the
DataRowChangingEventArgs argument of the event handler. The Action property is set
to one of the DataRowAction values detailed in Table 4-2
.
Table 4-2. DataRowAction enumeration
Value Description
Add The row has been added to the table.
Change The row has been changed.
Commit The changes made to the row have been committed.

Delete The row has been deleted from the table.
Nothing The row has not been changed.
Rollback The changes made to the row have been rolled back.
The DefaultValue of the GUID primary key column in both parent and child tables is set
to a new GUID using the NewGuid( ) method of the Guid structure when the tables are
defined. This causes a new GUID value to be assigned to the primary key when the first
row is added. Because the DefaultValue for the column is calculated once when the
property is set rather than as each new row is added, it must be changed after each row is
added to the table so that each row has a different GUID primary key value. This is done
by handling the RowChanging event for each table. When a row has been added, that is,
the Action property of the DataRowChangingEventArgs argument is Add, the
DefaultValue for the primary key column is set to a new GUID.

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