Gewusst wie: Binden von Windows Forms-Steuerelementen an DBNull-DatenbankwerteHow to: Bind Windows Forms Controls to DBNull Database Values

Wenn Sie Windows Forms-Steuerelemente an eine Datenquelle gebunden haben und die Datenquelle einen DBNull-Wert zurückgibt, können Sie einen entsprechenden Wert ersetzen, ohne Ereignisse behandeln, formatieren oder analysieren zu müssen.When you bind Windows Forms controls to a data source and the data source returns a DBNull value, you can substitute an appropriate value without handling, formatting, or parsing events. Die NullValue-Eigenschaft konvertiert DBNull beim Formatieren oder Analysieren der Datenquellenwerte in ein angegebenes Objekt.The NullValue property will convert DBNull to a specified object when formatting or parsing the data source values.

BeispielExample

Im folgenden Beispiel wird veranschaulicht, wie ein DBNull-Wert in zwei verschiedenen Situationen gebunden wird.The following example demonstrates how to bind a DBNull value in two different situations. In der ersten Situation wird dargestellt, wie NullValue für eine Zeichenfolgeneigenschaft festgelegt wird, und in der zweiten Situation wird dargestellt, wie NullValue für eine Bildeigenschaft festgelegt wird.The first demonstrates how to set the NullValue for a string property; the second demonstrates how to set the NullValue for an image property.


using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Data.SqlClient;
using System.Windows.Forms;

namespace DBNullCS
{
    public class Form1 : Form
    {
        public Form1()
        {		
            this.Load += new EventHandler(Form1_Load);
        }

        // The controls and components we need for the form.
        private Button button1;
        private PictureBox pictureBox1;
        private BindingSource bindingSource1;
        private TextBox textBox1;
        private TextBox textBox2;
        
        // Data table to hold the database data.
        DataTable employeeTable = new DataTable();

        void Form1_Load(object sender, EventArgs e)
        {
            // Basic form setup.
            this.pictureBox1 = new PictureBox();
            this.bindingSource1 = new BindingSource();
            this.textBox1 = new TextBox();
            this.textBox2 = new TextBox();
            this.button1 = new Button();
            this.pictureBox1.Location = new System.Drawing.Point(20, 20);
            this.pictureBox1.Size = new System.Drawing.Size(174, 179);
            this.textBox1.Location = new System.Drawing.Point(25, 215);
            this.textBox1.ReadOnly = true;
            this.textBox2.Location = new System.Drawing.Point(25, 241);
            this.textBox2.ReadOnly = true;
            this.button1.Location = new System.Drawing.Point(200, 103);
            this.button1.Text = "Move Next";
            this.button1.Click += new System.EventHandler(this.button1_Click);
            this.ClientSize = new System.Drawing.Size(292, 273);
            this.Controls.Add(this.button1);
            this.Controls.Add(this.textBox2);
            this.Controls.Add(this.textBox1);
            this.Controls.Add(this.pictureBox1);
            this.ResumeLayout(false);
            this.PerformLayout();
           
            // Create the connection string and populate the data table
            // with data.
            string connectionString = "Integrated Security=SSPI;" +
                "Persist Security Info = False;Initial Catalog=Northwind;" +
                "Data Source = localhost";
            SqlConnection connection = new SqlConnection();
            connection.ConnectionString = connectionString;
            SqlDataAdapter employeeAdapter = 
                new SqlDataAdapter(new SqlCommand("Select * from Employees", connection));
            connection.Open();
            employeeAdapter.Fill(employeeTable);
            
            // Set the DataSource property of the BindingSource to the employee table.
            bindingSource1.DataSource = employeeTable;

           // Set up the binding to the ReportsTo column.
            Binding reportsToBinding = textBox2.DataBindings.Add("Text", bindingSource1, 
                "ReportsTo", true);

            // Set the NullValue property for this binding.
            reportsToBinding.NullValue = "No Manager";

            // Set up the binding for the PictureBox using the Add method, setting
            // the null value in method call.
            pictureBox1.DataBindings.Add("Image", bindingSource1, "Photo", true, 
                DataSourceUpdateMode.Never, new Bitmap(typeof(Button), "Button.bmp"));

            // Set up the remaining binding.
            textBox1.DataBindings.Add("Text", bindingSource1, "LastName", true);
        }

        // Move through the data when the button is clicked.
        private void button1_Click(object sender, EventArgs e)
        {
            bindingSource1.MoveNext();
        }

        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.Run(new Form1());
        }
    }
}
Imports System.Collections.Generic
Imports System.ComponentModel
Imports System.Data
Imports System.Drawing
Imports System.Text
Imports System.Data.SqlClient
Imports System.Windows.Forms



Public Class Form1
    Inherits Form
    
    Public Sub New() 
    End Sub
    
    ' The controls and components we need for the form.
    Private WithEvents button1 As Button
    Private pictureBox1 As PictureBox
    Private bindingSource1 As BindingSource
    Private textBox1 As TextBox
    Private textBox2 As TextBox
    
    ' Data table to hold the database data.
    Private employeeTable As New DataTable()
    
    
    Private Sub Form1_Load(ByVal sender As Object, ByVal e As EventArgs) _
        Handles Me.Load

        ' Basic form setup.
        Me.pictureBox1 = New PictureBox()
        Me.bindingSource1 = New BindingSource()
        Me.textBox1 = New TextBox()
        Me.textBox2 = New TextBox()
        Me.button1 = New Button()
        Me.pictureBox1.Location = New System.Drawing.Point(20, 20)
        Me.pictureBox1.Size = New System.Drawing.Size(174, 179)
        Me.textBox1.Location = New System.Drawing.Point(25, 215)
        Me.textBox1.ReadOnly = True
        Me.textBox2.Location = New System.Drawing.Point(25, 241)
        Me.textBox2.ReadOnly = True
        Me.button1.Location = New System.Drawing.Point(200, 103)
        Me.button1.Text = "Move Next"
        Me.ClientSize = New System.Drawing.Size(292, 273)
        Me.Controls.Add(Me.button1)
        Me.Controls.Add(Me.textBox2)
        Me.Controls.Add(Me.textBox1)
        Me.Controls.Add(Me.pictureBox1)
        Me.ResumeLayout(False)
        Me.PerformLayout()

        ' Create the connection string and populate the data table
        ' with data.
        Dim connectionString As String = "Integrated Security=SSPI;" & _
            "Persist Security Info = False;Initial Catalog=Northwind;" _
            & "Data Source = localhost"
        Dim connection As New SqlConnection()
        connection.ConnectionString = connectionString
        Dim employeeAdapter As New SqlDataAdapter _
            (New SqlCommand("Select * from Employees", connection))
        connection.Open()
        employeeAdapter.Fill(employeeTable)

        ' Set the DataSource property of the BindingSource to the employee table.
        bindingSource1.DataSource = employeeTable

        ' Set up the binding to the ReportsTo column.
        Dim reportsToBinding As Binding = _
            textBox2.DataBindings.Add("Text", bindingSource1, "ReportsTo", _
                True)

        ' Set the NullValue property for this binding.
        reportsToBinding.NullValue = "No Manager"

        ' Set up the binding for the PictureBox using the Add method, setting
        ' the null value in method call.
        pictureBox1.DataBindings.Add("Image", bindingSource1, "Photo", _
            True, DataSourceUpdateMode.Never, _
            New Bitmap(GetType(Button), "Button.bmp"))

        ' Set up the remaining binding.
        textBox1.DataBindings.Add("Text", bindingSource1, "LastName", True)

    End Sub
    
    
    ' Move through the data when the button is clicked.
    Private Sub button1_Click(ByVal sender As Object, _
        ByVal e As EventArgs) Handles button1.Click

        bindingSource1.MoveNext()

    End Sub
    
    
    <STAThread()>  _
    Shared Sub Main() 
        Application.EnableVisualStyles()
        Application.Run(New Form1())
    
    End Sub
End Class

Der Typ der gebundenen Eigenschaft muss mit dem Typ der NullValue-Eigenschaft übereinstimmen. Andernfalls wird ein Fehler ausgegeben, und es werden keine weiteren NullValue-Werte verarbeitet .The types of the bound property and the NullValue property must be the same or an error will result, and no further NullValue values will be processed. In diesem Fall wird keine Ausnahme ausgelöst.In this situation, an exception will not be thrown.

Kompilieren des CodesCompiling the Code

Dieses Beispiel erfordert Folgendes:This example requires:

  • Verweise auf die Assemblys "System", "System.Data", "System.Drawing" und "System.Windows.Forms".References to the System, System.Data, System.Drawing and System.Windows.Forms assemblies.

Weitere InformationenSee also