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Hoe: Windows Forms-besturingselementen koppelen aan DBNull-databasewaarden

  • Artikel

Wanneer u Besturingselementen van Windows Forms koppelt aan een gegevensbron en de gegevensbron een DBNull waarde retourneert, kunt u een geschikte waarde vervangen zonder gebeurtenissen te verwerken, op te maken of te parseren. De eigenschap NullValue converteert DBNull naar een opgegeven object bij het opmaken of parseren van de gegevensbronwaarden.

Voorbeeld

In het volgende voorbeeld ziet u hoe u een DBNull waarde in twee verschillende situaties bindt. De eerste laat zien hoe u de NullValue instelt voor een tekenreekseigenschap; de tweede laat zien hoe u de NullValue instelt voor een afbeeldingseigenschap.


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

De typen van de gebonden eigenschap en de eigenschap NullValue moeten hetzelfde zijn, anders ontstaat er een fout, en worden verdere NullValue-waarden niet verwerkt. In deze situatie wordt er geen uitzondering gegenereerd.

De code compileren

Voor dit voorbeeld is het volgende vereist:

  • Verwijzingen naar de assembly's System, System.Data, System.Drawing en System.Windows.Forms.

Zie ook