ComboBox.DropDown Evento

Definizione

Spazio dei nomi:

System.Windows.Forms

Assembly:

System.Windows.Forms.dll

Si verifica quando viene visualizzata la parte a discesa di un oggetto ComboBox.

public:
 event EventHandler ^ DropDown;

public event EventHandler? DropDown;

member this.DropDown : EventHandler 

Public Custom Event DropDown As EventHandler 

Tipo evento

EventHandler

Esempio

Nell'esempio di codice seguente viene illustrato come inizializzare un ComboBox controllo impostando la proprietà text e usando il metodo per popolare l'oggetto AddRangeComboBox. Illustra anche la gestione dell'evento DropDown . Per eseguire l'esempio, incollare il codice seguente in un modulo e chiamare il InitializeComboBox metodo nel costruttore o Load nell'evento del modulo.

internal:
   // Declare ComboBox1
   System::Windows::Forms::ComboBox^ ComboBox1;

private:
   // Initialize ComboBox1.
   void InitializeComboBox()
   {
      this->ComboBox1 = gcnew ComboBox;
      this->ComboBox1->Location = System::Drawing::Point( 128, 48 );
      this->ComboBox1->Name = "ComboBox1";
      this->ComboBox1->Size = System::Drawing::Size( 100, 21 );
      this->ComboBox1->TabIndex = 0;
      this->ComboBox1->Text = "Typical";
      array<String^>^ installs = {"Typical","Compact","Custom"};
      ComboBox1->Items->AddRange( installs );
      this->Controls->Add( this->ComboBox1 );
      
      // Hook up the event handler.
      this->ComboBox1->DropDown += gcnew System::EventHandler(
         this, &Form1::ComboBox1_DropDown );
   }

   // Handles the ComboBox1 DropDown event. If the user expands the  
   // drop-down box, a message box will appear, recommending the
   // typical installation.
   void ComboBox1_DropDown( Object^ sender, System::EventArgs^ e )
   {
      MessageBox::Show( "Typical installation is strongly recommended.",
         "Install information", MessageBoxButtons::OK,
         MessageBoxIcon::Information );
   }

// Declare ComboBox1.
internal System.Windows.Forms.ComboBox ComboBox1;

// Initialize ComboBox1.
private void InitializeComboBox()
{
    this.ComboBox1 = new ComboBox();
    this.ComboBox1.Location = new System.Drawing.Point(128, 48);
    this.ComboBox1.Name = "ComboBox1";
    this.ComboBox1.Size = new System.Drawing.Size(100, 21);
    this.ComboBox1.TabIndex = 0;
    this.ComboBox1.Text	= "Typical";
    string[] installs = new string[]{"Typical", "Compact", "Custom"};
    ComboBox1.Items.AddRange(installs);
    this.Controls.Add(this.ComboBox1);
    
    // Hook up the event handler.
    this.ComboBox1.DropDown +=  
        new System.EventHandler(ComboBox1_DropDown);
}

// Handles the ComboBox1 DropDown event. If the user expands the  
// drop-down box, a message box will appear, recommending the
// typical installation.
private void ComboBox1_DropDown(object sender, System.EventArgs e)
{
    MessageBox.Show("Typical installation is strongly recommended.", 
    "Install information", MessageBoxButtons.OK, 
        MessageBoxIcon.Information);
}

' Declare ComboBox1.
Friend WithEvents ComboBox1 As System.Windows.Forms.ComboBox

' Initialize ComboBox1.
Private Sub InitializeComboBox()
    Me.ComboBox1 = New ComboBox
    Me.ComboBox1.Location = New System.Drawing.Point(128, 48)
    Me.ComboBox1.Name = "ComboBox1"
    Me.ComboBox1.Size = New System.Drawing.Size(100, 21)
    Me.ComboBox1.TabIndex = 0
    Me.ComboBox1.Text = "Typical"
    Dim installs() As String = New String() _
        {"Typical", "Compact", "Custom"}
    ComboBox1.Items.AddRange(installs)
    Me.Controls.Add(Me.ComboBox1)
End Sub

' Handles the ComboBox1 DropDown event. If the user expands the  
' drop-down box, a message box will appear, recommending the
' typical installation.
Private Sub ComboBox1_DropDown _ 
    (ByVal sender As Object, ByVal e As System.EventArgs) _ 
    Handles ComboBox1.DropDown
    MessageBox.Show("Typical installation is strongly recommended.", _
    "Install information", MessageBoxButtons.OK, MessageBoxIcon.Information)
End Sub

Commenti

Per ulteriori informazioni sulla gestione degli eventi, consultare gestione e generazione di eventi.

Vedi anche

• DropDownClosed