Populating a Windows Forms ComboBox

[ Team LiB ]


Recipe 7.11 Populating a Windows Forms ComboBox
Problem
You need to populate a ComboBox from a database, bind a field in a database to the
ComboBox so that the correct value is selected from the list, and use the value selected in
the ComboBox to update the database.
Solution
You must:
•
Fill a ComboBox from a database (pay attention to the difference between the
SelectedIndex and SelectedValue).
•
Bind a ComboBox to a field in a result set so that the value is selected in the
ComboBox corresponding to the value in a field for the record displayed.
•
Use the selection events returned by the ComboBox.
•
Update the database with the value selected in the ComboBox.
The schema of table TBL0711 used in this solution is shown in Table 7-11
.
Table 7-11. TBL0711 schema
Column name Data type Length Allow nulls?
Id int 4 No
ComboBoxItemId int 4 No
Field1 nvarchar 50 Yes
The schema of table TBL0711_ComboBoxSource used in this solution is shown in Table
7-12.
Table 7-12. TBL0711_ComboBoxSource schema
Column name Data type Length Allow nulls?

ComboBoxItemId int 4 No
Description nvarchar 50 No
The sample code contains seven event handlers:
Form.Load
Sets up the sample by creating a DataAdapter with the logic to select all records
from table TBL0709 in the sample database and to update changes made back to
the database. The DataAdapter is used to fill a DataTable in a new DataSet with
the schema and data from TBL0709. A DataTable is filled from table
TBL0709_ComboBoxSource and added to the DataSet. A DataRelation is created
between the two tables in the DataSet.
The ComboBox control is filled by linking its properties to table
TBL0709_ComboBoxSource in the DataSet. The text boxes displaying the data
for the ID and Field1 columns and the ComboBox displaying data for the
ComboBoxItemId column are bound to the DataTable TBL0709. Finally, the
BindingManagerBase is obtained for the TBL0709 table in the DataSet.
Update Button.Click
Ends the current edit and uses the DataAdapter created in the Form.Load event
handler to update the database.
ComboBox.SelectionChangeCommitted
Displays the values for the SelectedIndex, SelectedValue, and SelectedText
properties of the ComboBox after its selected item has changed.
Move First Button.Click
Sets the current record of the bound controls to the first record by setting the
Position property of the BindingManagerBase object to 0.
Move Previous Button.Click
Sets the current record of the bound controls to the previous record by
decrementing the Position property of the BindingManagerBase object by 1.
Move Next Button.Click
Sets the current record of the bound controls to the next record by incrementing
the Position property of the BindingManagerBase object by 1.

Move Last Button.Click
Sets the current record of the bound controls to the last record by setting the
Position property of the BindingManagerBase object to the total number of
records, as returned by the Count property, minus 1.
The C# code is shown in Example 7-19
.
Example 7-19. File: ComboBoxForm.cs
// Namespaces, variables, and constants
using System;
using System.Configuration;
using System.Windows.Forms;
using System.Data;
using System.Data.SqlClient;

private const String TABLENAME = "TBL0711";
private const String TABLENAME_COMBOBOXSOURCE =
"TBL0709_ComboBoxSource";
private const String RELATIONNAME = "REL0711";

private DataSet ds;
private SqlDataAdapter da;

private BindingManagerBase bm;

// . . .

private void ComboBoxForm_Load(object sender, System.EventArgs e)
{
// Create the DataSet.
ds = new DataSet( );

// Create the select and update commands for the DataAdapter.
String selectCommand = "SELECT Id, ComboBoxItemId, Field1 FROM " +
TABLENAME;
String updateCommand = "UPDATE " + TABLENAME + " " +
"SET ComboBoxItemId = @ComboBoxItemId, Field1 = @Field1 " +
"WHERE Id = @Id";

// Create the DataAdapter.
da = new SqlDataAdapter(selectCommand,
ConfigurationSettings.AppSettings["Sql_ConnectString"]);
da.UpdateCommand = new SqlCommand(updateCommand,
da.SelectCommand.Connection);
da.UpdateCommand.CommandType = CommandType.Text;
da.UpdateCommand.Parameters.Add("@Id", SqlDbType.Int, 0, "Id");
da.UpdateCommand.Parameters.Add("@ComboBoxItemId", SqlDbType.Int, 0,
"ComboBoxItemId");
da.UpdateCommand.Parameters.Add("@Field1", SqlDbType.NVarChar, 50,
"Field1");

// Retrieve the data and schema for the table.
da.FillSchema(ds, SchemaType.Source, TABLENAME);
da.Fill(ds, TABLENAME);

// Create and fill the schema for the ComboBox table.
String sqlText = "SELECT ComboBoxItemId, Description FROM " +
TABLENAME_COMBOBOXSOURCE;
SqlDataAdapter daCB = new SqlDataAdapter(sqlText,
da.SelectCommand.Connection);
DataTable comboBoxSourceTable =

new DataTable(TABLENAME_COMBOBOXSOURCE);
daCB.FillSchema(comboBoxSourceTable, SchemaType.Source);
// Sdd the instructions for the user as the first element.
comboBoxSourceTable.Rows.Add(new object[] {-1, "<Select>"});
// Fill the rest of the data for the ComboBox.
daCB.Fill(comboBoxSourceTable);

// Add the ComboBox source table to the DataSet.
ds.Tables.Add(comboBoxSourceTable);

// Relate the parent and ComboBox tables.
ds.Relations.Add(new DataRelation(RELATIONNAME,
ds.Tables[TABLENAME_COMBOBOXSOURCE].Columns["ComboBoxItemId"],
ds.Tables[TABLENAME].Columns["ComboBoxItemId"],
true));

// Set up the ComboBox with the DataSet.
comboBox.DataSource = ds.Tables[TABLENAME_COMBOBOXSOURCE];
comboBox.ValueMember = "ComboBoxItemId";
comboBox.DisplayMember = "Description";

// Bind all of the controls to the DataSet.
idTextBox.DataBindings.Add("Text", ds, TABLENAME + ".Id");
comboBox.DataBindings.Add("SelectedValue", ds,
TABLENAME + ".ComboBoxItemId");
field1TextBox.DataBindings.Add("Text", ds, TABLENAME + ".Field1");

// Get the binding manager base for the parent table.
bm = BindingContext[ds, TABLENAME];
}

private void updateButton_Click(object sender, System.EventArgs e)
{
// End the current update and update the record using the DataAdapter.
bm.EndCurrentEdit( );
da.Update(ds.Tables[TABLENAME]);
}

private void comboBox_SelectionChangeCommitted(object sender, System.EventArgs e)
{
resultTextBox.Text="SelectedIndex = " + comboBox.SelectedIndex +
Environment.NewLine +
"SelectedValue = " + comboBox.SelectedValue +
Environment.NewLine +
"SelectedText = " +
((DataRowView)comboBox.Items[comboBox.SelectedIndex])
["Description"];
}

private void moveFirstButton_Click(object sender, System.EventArgs e)
{
bm.Position = 0;
}

private void movePreviousButton_Click(object sender, System.EventArgs e)
{
bm.Position -= 1;
}

private void moveNextButton_Click(object sender, System.EventArgs e)

{
bm.Position += 1;
}

private void moveLastButton_Click(object sender, System.EventArgs e)
{
bm.Position = bm.Count - 1;
}