Procedura: accedere egli oggetti in un elenco a discesa DataGridViewComboBoxCell Windows Form

Articolo
09/12/2008

Aggiornamento: novembre 2007

Analogamente al controllo ComboBox, i tipi DataGridViewComboBoxColumn e DataGridViewComboBoxCell consentono di aggiungere oggetti arbitrari agli elenchi a discesa. Con questa funzionalità, è possibile rappresentare stati complessi in un elenco a discesa senza dover archiviare gli oggetti corrispondenti in un insieme distinto.

A differenza del controllo ComboBox, i tipi DataGridView non includono una proprietà SelectedItem per il recupero dell'oggetto attualmente selezionato. È invece necessario impostare la proprietà DataGridViewComboBoxColumn.ValueMember o DataGridViewComboBoxCell.ValueMember sul nome di una proprietà nell'oggetto business. Quando l'utente effettua una selezione, la proprietà dell'oggetto business indicata imposta la proprietà Value della cella.

Per recuperare l'oggetto business tramite il valore della cella, è necessario che la proprietà ValueMember indichi una proprietà che restituisce un riferimento all'oggetto business stesso. Pertanto, se il tipo dell'oggetto business non è sotto controllo, è necessario aggiungere tale proprietà estendendo il tipo tramite l'ereditarietà.

Nelle procedure seguenti viene illustrato come compilare un elenco a discesa con oggetti business e recuperare gli oggetti tramite la proprietà Value della cella.

Per aggiungere oggetti business all'elenco a discesa

Creare un nuovo controllo DataGridViewComboBoxColumn e compilare il relativo insieme Items. In alternativa, è possibile impostare la proprietà DataSource della colonna sull'insieme di oggetti business. In tal caso, tuttavia, non è possibile aggiungere "unassigned" all'elenco a discesa senza creare un oggetto business corrispondente nell'insieme.

Dim assignedToColumn As New DataGridViewComboBoxColumn()

' Populate the combo box drop-down list with Employee objects. 
For Each e As Employee In employees
    assignedToColumn.Items.Add(e)
Next

' Add "unassigned" to the drop-down list and display it for 
' empty AssignedTo values or when the user presses CTRL+0. 
assignedToColumn.Items.Add("unassigned")
assignedToColumn.DefaultCellStyle.NullValue = "unassigned"

DataGridViewComboBoxColumn assignedToColumn = 
    new DataGridViewComboBoxColumn();

// Populate the combo box drop-down list with Employee objects. 
foreach (Employee e in employees) assignedToColumn.Items.Add(e);

// Add "unassigned" to the drop-down list and display it for 
// empty AssignedTo values or when the user presses CTRL+0. 
assignedToColumn.Items.Add("unassigned");
assignedToColumn.DefaultCellStyle.NullValue = "unassigned";

Impostare le proprietà DisplayMember e ValueMember. DisplayMember indica la proprietà dell'oggetto business da visualizzare nell'elenco a discesa. ValueMember indica la proprietà che restituisce un riferimento all'oggetto business.
```
assignedToColumn.DisplayMember = "Name"
assignedToColumn.ValueMember = "Self"
```
```
assignedToColumn.DisplayMember = "Name";
assignedToColumn.ValueMember = "Self";
```
Assicurarsi che il tipo di oggetto business contenga una proprietà che restituisce un riferimento all'istanza corrente. Questa proprietà deve essere denominata con il valore assegnato a ValueMember nel passaggio precedente.
```
Public ReadOnly Property Self() As Employee
    Get
        Return Me
    End Get
End Property
```
```
public Employee Self 
{ 
    get { return this; } 
}
```

Per recuperare l'oggetto business attualmente selezionato

Ottenere la proprietà Value della cella ed eseguirne il casting sul tipo di oggetto business.

' Retrieve the Employee object from the "Assigned To" cell.
Dim assignedTo As Employee = TryCast(dataGridView1.Rows(e.RowIndex) _
    .Cells("Assigned To").Value, Employee)

// Retrieve the Employee object from the "Assigned To" cell.
Employee assignedTo = dataGridView1.Rows[e.RowIndex]
    .Cells["Assigned To"].Value as Employee;

Esempio

Nell'esempio completo viene illustrato l'utilizzo degli oggetti business nell'elenco a discesa. Un controllo DataGridView viene associato a un insieme di oggetti Task. Ogni oggetto Task include una proprietà AssignedTo che indica l'oggetto Employee attualmente assegnato a tale attività. Nella colonna Assigned To viene visualizzato il valore della proprietà Name per ogni dipendente assegnato, o "unassigned" se il valore della proprietà Task.AssignedTo è null.

Per visualizzare il comportamento di questo esempio, attenersi alla procedura seguente:

Modificare le assegnazioni nella colonna Assigned To selezionando valori diversi dagli elenchi a discesa o premendo CTRL+0 in una cella della casella combinata.
Fare clic su Generate Report per visualizzare le assegnazioni correnti. Questo dimostra che eseguendo una modifica nella colonna Assigned To viene automaticamente aggiornato l'insieme tasks.
Fare clic su un pulsante Request Status per eseguire una chiamata al metodo RequestStatus dell'oggetto Employee corrente per tale riga. Questo dimostra che l'oggetto selezionato è stato recuperato correttamente.

Imports System
Imports System.Text
Imports System.Collections.Generic
Imports System.Windows.Forms

Public Class Form1
    Inherits Form

    Private employees As New List(Of Employee)
    Private tasks As New List(Of Task)
    Private WithEvents reportButton As New Button
    Private WithEvents dataGridView1 As New DataGridView

    <STAThread()> _
    Public Sub Main()
        Application.Run(New Form1)
    End Sub

    Sub New()
        dataGridView1.Dock = DockStyle.Fill
        dataGridView1.AutoSizeColumnsMode = _
            DataGridViewAutoSizeColumnsMode.AllCells
        reportButton.Text = "Generate Report"
        reportButton.Dock = DockStyle.Top

        Controls.Add(dataGridView1)
        Controls.Add(reportButton)
        Text = "DataGridViewComboBoxColumn Demo"
    End Sub

    ' Initializes the data source and populates the DataGridView control.
    Private Sub Form1_Load(ByVal sender As Object, _
        ByVal e As EventArgs) Handles Me.Load

        PopulateLists()
        dataGridView1.AutoGenerateColumns = False
        dataGridView1.DataSource = tasks
        AddColumns()

    End Sub

    ' Populates the employees and tasks lists. 
    Private Sub PopulateLists()
        employees.Add(New Employee("Harry"))
        employees.Add(New Employee("Sally"))
        employees.Add(New Employee("Roy"))
        employees.Add(New Employee("Pris"))
        tasks.Add(New Task(1, employees(1)))
        tasks.Add(New Task(2))
        tasks.Add(New Task(3, employees(2)))
        tasks.Add(New Task(4))
    End Sub

    ' Configures columns for the DataGridView control.
    Private Sub AddColumns()

        Dim idColumn As New DataGridViewTextBoxColumn()
        idColumn.Name = "Task"
        idColumn.DataPropertyName = "Id"
        idColumn.ReadOnly = True

        Dim assignedToColumn As New DataGridViewComboBoxColumn()

        ' Populate the combo box drop-down list with Employee objects. 
        For Each e As Employee In employees
            assignedToColumn.Items.Add(e)
        Next

        ' Add "unassigned" to the drop-down list and display it for 
        ' empty AssignedTo values or when the user presses CTRL+0. 
        assignedToColumn.Items.Add("unassigned")
        assignedToColumn.DefaultCellStyle.NullValue = "unassigned"

        assignedToColumn.Name = "Assigned To"
        assignedToColumn.DataPropertyName = "AssignedTo"
        assignedToColumn.AutoComplete = True
        assignedToColumn.DisplayMember = "Name"
        assignedToColumn.ValueMember = "Self"

        ' Add a button column. 
        Dim buttonColumn As New DataGridViewButtonColumn()
        buttonColumn.HeaderText = ""
        buttonColumn.Name = "Status Request"
        buttonColumn.Text = "Request Status"
        buttonColumn.UseColumnTextForButtonValue = True

        dataGridView1.Columns.Add(idColumn)
        dataGridView1.Columns.Add(assignedToColumn)
        dataGridView1.Columns.Add(buttonColumn)

    End Sub

    ' Reports on task assignments. 
    Private Sub reportButton_Click(ByVal sender As Object, _
        ByVal e As EventArgs) Handles reportButton.Click

        Dim report As New StringBuilder()
        For Each t As Task In tasks
            Dim assignment As String
            If t.AssignedTo Is Nothing Then
                assignment = "unassigned"
            Else
                assignment = "assigned to " + t.AssignedTo.Name
            End If
            report.AppendFormat("Task {0} is {1}.", t.Id, assignment)
            report.Append(Environment.NewLine)
        Next
        MessageBox.Show(report.ToString(), "Task Assignments")

    End Sub

    ' Calls the Employee.RequestStatus method.
    Private Sub dataGridView1_CellClick(ByVal sender As Object, _
        ByVal e As DataGridViewCellEventArgs) _
        Handles dataGridView1.CellClick

        ' Ignore clicks that are not on button cells. 
        If e.RowIndex < 0 OrElse Not e.ColumnIndex = _
            dataGridView1.Columns("Status Request").Index Then Return

        ' Retrieve the task ID.
        Dim taskID As Int32 = CInt(dataGridView1(0, e.RowIndex).Value)

        ' Retrieve the Employee object from the "Assigned To" cell.
        Dim assignedTo As Employee = TryCast(dataGridView1.Rows(e.RowIndex) _
            .Cells("Assigned To").Value, Employee)

        ' Request status through the Employee object if present. 
        If assignedTo IsNot Nothing Then
            assignedTo.RequestStatus(taskID)
        Else
            MessageBox.Show(String.Format( _
                "Task {0} is unassigned.", taskID), "Status Request")
        End If

    End Sub

End Class

Public Class Task

    Sub New(ByVal id As Int32)
        idValue = id
    End Sub

    Sub New(ByVal id As Int32, ByVal assignedTo As Employee)
        idValue = id
        assignedToValue = assignedTo
    End Sub

    Private idValue As Int32
    Public Property Id() As Int32
        Get
            Return idValue
        End Get
        Set(ByVal value As Int32)
            idValue = value
        End Set
    End Property

    Private assignedToValue As Employee
    Public Property AssignedTo() As Employee
        Get
            Return assignedToValue
        End Get
        Set(ByVal value As Employee)
            assignedToValue = value
        End Set
    End Property

End Class

Public Class Employee

    Sub New(ByVal name As String)
        nameValue = name
    End Sub

    Private nameValue As String
    Public Property Name() As String
        Get
            Return nameValue
        End Get
        Set(ByVal value As String)
            nameValue = value
        End Set
    End Property

    Public ReadOnly Property Self() As Employee
        Get
            Return Me
        End Get
    End Property

    Public Sub RequestStatus(ByVal taskID As Int32)
        MessageBox.Show(String.Format( _
            "Status for task {0} has been requested from {1}.", _
            taskID, nameValue), "Status Request")
    End Sub

End Class

using System;
using System.Text;
using System.Collections.Generic;
using System.Windows.Forms;

public class Form1 : Form
{
    private List<Employee> employees = new List<Employee>();
    private List<Task> tasks = new List<Task>();
    private Button reportButton = new Button();
    private DataGridView dataGridView1 = new DataGridView();

    [STAThread]
    public static void Main()
    {
        Application.Run(new Form1());
    }

    public Form1()
    {
        dataGridView1.Dock = DockStyle.Fill;
        dataGridView1.AutoSizeColumnsMode = 
            DataGridViewAutoSizeColumnsMode.AllCells;
        reportButton.Text = "Generate Report";
        reportButton.Dock = DockStyle.Top;
        reportButton.Click += new EventHandler(reportButton_Click);

        Controls.Add(dataGridView1);
        Controls.Add(reportButton);
        Load += new EventHandler(Form1_Load);
        Text = "DataGridViewComboBoxColumn Demo";
    }

    // Initializes the data source and populates the DataGridView control.
    private void Form1_Load(object sender, EventArgs e)
    {
        PopulateLists();
        dataGridView1.AutoGenerateColumns = false;
        dataGridView1.DataSource = tasks;
        AddColumns();
    }

    // Populates the employees and tasks lists. 
    private void PopulateLists()
    {
        employees.Add(new Employee("Harry"));
        employees.Add(new Employee("Sally"));
        employees.Add(new Employee("Roy"));
        employees.Add(new Employee("Pris"));
        tasks.Add(new Task(1, employees[1]));
        tasks.Add(new Task(2));
        tasks.Add(new Task(3, employees[2]));
        tasks.Add(new Task(4));
    }

    // Configures columns for the DataGridView control.
    private void AddColumns()
    {
        DataGridViewTextBoxColumn idColumn = 
            new DataGridViewTextBoxColumn();
        idColumn.Name = "Task";
        idColumn.DataPropertyName = "Id";
        idColumn.ReadOnly = true;

        DataGridViewComboBoxColumn assignedToColumn = 
            new DataGridViewComboBoxColumn();

        // Populate the combo box drop-down list with Employee objects. 
        foreach (Employee e in employees) assignedToColumn.Items.Add(e);

        // Add "unassigned" to the drop-down list and display it for 
        // empty AssignedTo values or when the user presses CTRL+0. 
        assignedToColumn.Items.Add("unassigned");
        assignedToColumn.DefaultCellStyle.NullValue = "unassigned";

        assignedToColumn.Name = "Assigned To";
        assignedToColumn.DataPropertyName = "AssignedTo";
        assignedToColumn.AutoComplete = true;
        assignedToColumn.DisplayMember = "Name";
        assignedToColumn.ValueMember = "Self";

        // Add a button column. 
        DataGridViewButtonColumn buttonColumn = 
            new DataGridViewButtonColumn();
        buttonColumn.HeaderText = "";
        buttonColumn.Name = "Status Request";
        buttonColumn.Text = "Request Status";
        buttonColumn.UseColumnTextForButtonValue = true;

        dataGridView1.Columns.Add(idColumn);
        dataGridView1.Columns.Add(assignedToColumn);
        dataGridView1.Columns.Add(buttonColumn);

        // Add a CellClick handler to handle clicks in the button column.
        dataGridView1.CellClick +=
            new DataGridViewCellEventHandler(dataGridView1_CellClick);
    }

    // Reports on task assignments. 
    private void reportButton_Click(object sender, EventArgs e)
    {
        StringBuilder report = new StringBuilder();
        foreach (Task t in tasks)
        {
            String assignment = 
                t.AssignedTo == null ? 
                "unassigned" : "assigned to " + t.AssignedTo.Name;
            report.AppendFormat("Task {0} is {1}.", t.Id, assignment);
            report.Append(Environment.NewLine);
        }
        MessageBox.Show(report.ToString(), "Task Assignments");
    }

    // Calls the Employee.RequestStatus method.
    void dataGridView1_CellClick(object sender, DataGridViewCellEventArgs e)
    {
        // Ignore clicks that are not on button cells. 
        if (e.RowIndex < 0 || e.ColumnIndex !=
            dataGridView1.Columns["Status Request"].Index) return;

        // Retrieve the task ID.
        Int32 taskID = (Int32)dataGridView1[0, e.RowIndex].Value;

        // Retrieve the Employee object from the "Assigned To" cell.
        Employee assignedTo = dataGridView1.Rows[e.RowIndex]
            .Cells["Assigned To"].Value as Employee;

        // Request status through the Employee object if present. 
        if (assignedTo != null)
        {
            assignedTo.RequestStatus(taskID);
        }
        else
        {
            MessageBox.Show(String.Format(
                "Task {0} is unassigned.", taskID), "Status Request");
        }
    }

}

public class Task
{
    public Task(Int32 id)
    {
        idValue = id;
    }

    public Task(Int32 id, Employee assignedTo)
    {
        idValue = id;
        assignedToValue = assignedTo;
    }

    private Int32 idValue;
    public Int32 Id
    {
        get { return idValue; }
        set { idValue = value; }
    }

    private Employee assignedToValue;
    public Employee AssignedTo
    {
        get { return assignedToValue; }
        set { assignedToValue = value; }
    }
}

public class Employee
{
    public Employee(String name)
    {
        nameValue = name;
    }

    private String nameValue;
    public String Name
    {
        get { return nameValue; }
        set { nameValue = value; }
    }

    public Employee Self 
    { 
        get { return this; } 
    }

    public void RequestStatus(Int32 taskID)
    {
        MessageBox.Show(String.Format(
            "Status for task {0} has been requested from {1}.", 
            taskID, nameValue), "Status Request");
    }
}