BUG: The ListBox control or the ComboBox control copies list items several times in Visual Basic .NET or in Visual C# .NET (820636)


The information in this article applies to:

  • Microsoft Visual Basic .NET (2003)
  • Microsoft Visual Basic .NET (2002)
  • Microsoft Visual C# .NET (2003)
  • Microsoft Visual C# .NET (2002)

SYMPTOMS

You have a Microsoft Windows form with a ListBox control or a ComboBox control. When you bind the ListBox or the ComboBox to a data source, the control copies the items in the list several times.

CAUSE

When you create the ListBox or when you create the ComboBox, the form does not have a BindingContext property. The ListBox or the ComboBox must have a BindingContext to copy the data. Therefore, the control requests that the form send the BindingContext. The form creates the BindingContext, and then the form fires notification to the control. The ListBox or the ComboBox receives the notification, and then the control tries to update the DataManager property. Therefore, the ListBox or the ComboBox creates the first copy of the list. When the form creates the BindingContext, the ListBox or the ComboBox creates the second copy of the list.

RESOLUTION

To resolve this problem, use the following procedures:
  • Set the BindingContext property of the control before you set any data bindings.
  • Set the DisplayMember property of the control before you set the DataSource property.
To use these procedures, follow these steps:
  1. Replace the existing code in the Form f constructor with the following code.

    Microsoft Visual Basic .NET Code
    Dim pt As ProductTypes = New ProductTypes
'Add a ComboBox control to the form.
Dim cb As ComboBox = New ComboBox
Controls.Add(cb)
'Invoke the binding context for the ComboBox.
Dim bc As New BindingContext
cb.BindingContext = bc
'Set the DataSource property and set the DisplayMember property of the ComboBox.
cb.DisplayMember = "Description"
cb.DataSource = pt
    Microsoft Visual C# .NET Code
    ProductTypes pt = new ProductTypes();
//Add a ComboBox control to the form.
ComboBox cb = new ComboBox();
Controls.Add(cb);
//Invoke the binding context for the ComboBox.
BindingContext bc=new BindingContext();
cb.BindingContext =bc;   
//Set the DataSource property and set the DisplayMember property of the ComboBox.
cb.DisplayMember = "Description";
cb.DataSource =pt;
    Note You can also use this code for a ListBox control by replacing ComboBox with ListBox.
  2. On the Debug menu, click Start.

    The CopyTo method is called only one time.

STATUS

Microsoft has confirmed that this is a bug in the Microsoft products that are listed in the "Applies to" section.

MORE INFORMATION

Steps to Reproduce the Behavior


  1. In Microsoft Visual Studio .NET, start a new Windows application by using Visual Basic .NET or Visual C# .NET.

    By default, Form1 is created.
  2. Replace the existing code in Form1 with the following code.

    Visual Basic .NET Code
    Imports System.Windows.Forms

Class f
    Inherits Form
    Shared Sub Main()
        Application.Run(New f)
    End Sub

    Private Sub New()
        Dim pt As ProductTypes = New ProductTypes
        'Add a ComboBox control to the form.
        Dim cb As ComboBox = New ComboBox
        Controls.Add(cb)
        'Set the DataSource property and set the DisplayMember property of the ComboBox.
        cb.DataSource = pt
        cb.DisplayMember = "Description"
    End Sub
End Class
    Visual C# .NET Code
    using System.Windows.Forms;
class f : Form 
{
    static void Main() 
    {
        Application.Run(new f());
    }

    f() 
    {
        ProductTypes pt = new ProductTypes();
        ComboBox cb = new ComboBox();
        //Add a ComboBox control to the form.
        Controls.Add(cb);
        //Set the DataSource property and set the DisplayMember property of the ComboBox.
        cb.DataSource = pt;
        cb.DisplayMember = "Description";
    }
}
    Note You can also use this code for a ListBox control by replacing ComboBox with ListBox.
  3. On the Project menu, click Add Class, and then click Open.
  4. Replace the existing code in Class1 with the following code.

    Visual Basic .NET Code
    Imports System
Imports System.ComponentModel
Imports System.Collections
Imports System.Data
Imports System.Data.SqlClient

Public Class ProductTypes
    Implements IList
    Implements IEnumerator
    Private position As Integer = -1
    'List of items
    Private _Cache As Hashtable
    'List of ordinal pointers to the items
    Private _CacheOrdinal As Hashtable

    Public Sub New()
        _Cache = New Hashtable
        _CacheOrdinal = New Hashtable
        FillCache()
    End Sub

    Public Function GetEnumerator() As ProductTypes
        position = -1
        Console.WriteLine("In type GetEnum..." + Environment.StackTrace)
        Return (Me)
    End Function

    Private Function IGetEnumerator() As IEnumerator Implements IEnumerable.GetEnumerator
        position = -1
        Console.WriteLine("In IEnum GetEnum..." + Environment.StackTrace)
        Return CType((Me), IEnumerator)
    End Function

    ' IList
    Public ReadOnly Property IsFixedSize() As Boolean Implements IList.IsFixedSize
        Get
            Return (False)
        End Get
    End Property

    ' IList
    Public ReadOnly Property IsReadOnly() As Boolean Implements IList.IsReadOnly
        Get
            Return (False)
        End Get
    End Property
    Public Property IItem(ByVal index As Integer) As Object Implements IList.Item
        Get
            Console.WriteLine("In IList this...")
            Return (CType(_Cache(_CacheOrdinal(index)), ProductType))
        End Get
        Set(ByVal Value As Object)

        End Set
    End Property

    Default Property Item(ByVal pos As Integer) As ProductType
        Get
            Console.WriteLine("In type this...")
            Return (CType(_Cache(_CacheOrdinal(pos)), ProductType))
        End Get
        Set(ByVal Value As ProductType)

        End Set
    End Property

    ' ICollection
    Public ReadOnly Property Count() As Integer Implements ICollection.Count
        Get
            Console.WriteLine("In Count()")
            Return (_Cache.Count)
        End Get
    End Property

    ' Declare the Reset method that IEnumerator requires:
    Public Sub Reset() Implements IEnumerator.Reset
        Console.WriteLine("Called reset")
        position = -1
    End Sub

    ' Declare the MoveNext method that IEnumerator requires:
    Public Function MoveNext() As Boolean Implements IEnumerator.MoveNext
        If position < _Cache.Count - 1 Then
            position = position + 1
            Return True
        Else
            Return False
        End If
    End Function

    Public Function Add(ByVal o As Object) As Integer Implements IList.Add
        Return 1
    End Function

    Public Sub Clear() Implements IList.Clear
    End Sub

    Public Sub RemoveAt(ByVal i As Integer) Implements IList.RemoveAt
    End Sub

    Public Sub Remove(ByVal o As Object) Implements IList.Remove
    End Sub

    Public Function IndexOf(ByVal o As Object) As Integer Implements IList.IndexOf
        Return 0
    End Function

    Public Function Contains(ByVal o As Object) As Boolean Implements IList.Contains
        Return False
    End Function

    Public ReadOnly Property IsSynchronized() As Boolean Implements ICollection.IsSynchronized
        Get
            Return (False)
        End Get
    End Property

    Public ReadOnly Property SyncRoot() As Object Implements ICollection.SyncRoot
        Get
            Return (Nothing)
        End Get
    End Property

    Public Sub Insert(ByVal i As Integer, ByVal o As Object) Implements IList.Insert
    End Sub


    Public ReadOnly Property Current() As ProductType
        Get
            Return CType(_Cache(_CacheOrdinal(position)), ProductType)
        End Get
    End Property

    ' Declare the Current property that IEnumerator requires:
    Public ReadOnly Property ICurrent() As Object Implements IEnumerator.Current
        Get
            Return _Cache(_CacheOrdinal(position))
        End Get

    End Property

    Default Property Item(ByVal productTypeId As String) As ProductType
        Get
            Return CType((_Cache(productTypeId)), ProductType)
        End Get
        Set(ByVal Value As ProductType)

        End Set
    End Property

    Public Sub CopyTo(ByVal arr As System.Array, ByVal i As Integer) Implements ICollection.CopyTo
        _Cache.CopyTo(arr, i)
        Console.WriteLine("Calling copy..." + Environment.StackTrace)
    End Sub

    Private Sub FillCache()
        Dim ordinal As Integer = 0

        Dim pType As ProductType = New ProductType
        pType.Description = "eric"
        pType.ProductTypeId = System.Guid.NewGuid()
        _Cache.Add(pType.ProductTypeId, pType)
        _CacheOrdinal.Add(ordinal, pType.ProductTypeId)
        ordinal = ordinal + 1
        Dim pType1 As ProductType = New ProductType
        pType1.Description = "nancy"
        pType1.ProductTypeId = System.Guid.NewGuid()
        _Cache.Add(pType1.ProductTypeId, pType1)
        _CacheOrdinal.Add(ordinal, pType1.ProductTypeId)
        ordinal = ordinal + 1
    End Sub

End Class

Public Class ProductType
    Private _ProductTypeId As Guid
    Private _Description As String

    Public Sub New()
        _ProductTypeId = System.Guid.NewGuid()
        _Description = Nothing
    End Sub

    Public Property ProductTypeId() As Guid
        Get
            Return (_ProductTypeId)
        End Get
        Set(ByVal Value As Guid)
            _ProductTypeId = Value
        End Set
    End Property

    Public Property Description() As String
        Get
            Return (_Description)
        End Get
        Set(ByVal Value As String)
            _Description = Value
        End Set
    End Property
End Class
    Visual C# .NET Code
    using System;
using System.ComponentModel;
using System.Collections;
using System.Data;
using System.Data.SqlClient;

    public class ProductTypes : IList, IEnumerator
    {
        private int position = -1;
        //List of items
        private Hashtable _Cache;
        //List of ordinal pointers to the items
        private Hashtable _CacheOrdinal;

        public ProductTypes()
        {
            _Cache = new Hashtable();
            _CacheOrdinal = new Hashtable();
            FillCache();
        }
	
        public   ProductTypes GetEnumerator()
        {
            position = -1;
            Console.WriteLine("In type GetEnum..." + Environment.StackTrace);
            return (this);
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            position = -1;
            Console.WriteLine("In IEnum GetEnum..." + Environment.StackTrace);
            return (IEnumerator) (this);
        }

        // IList
        public bool IsFixedSize
        {
            get
            {
                return (false);
            }
        }

        // IList
        public bool IsReadOnly
        {
            get
            {
                return (false);
            }
        }
		
        // IList
        object IList.this[int pos]
        {
            get 
            {
                Console.WriteLine("In IList this...");
                return (_Cache[_CacheOrdinal[pos]]);
            }
            set
            {
            }
        }
		
        public ProductType this[int pos]
        {
            get 
            {
                Console.WriteLine("In type this...");
                return ((ProductType)_Cache[_CacheOrdinal[pos]]);
            }
            set
            {
            }
        }
		
        // ICollection
        public   int  Count
        {
            get
            {
                Console.WriteLine("In Count()");
                return (_Cache.Count);
            }
        }
	
        // Declare the Reset method that IEnumerator requires:
        public void Reset()
        {
            Console.WriteLine("Called reset");
            position = -1;
        }

        // Declare the MoveNext method that IEnumerator requires:
        {
            if (position < _Cache.Count - 1)
            {
                position++;
                return true;
            }
            else
            {
                return false;
            }
        }

        public int Add(object o)
        {
            return 1;
        }

        public void Clear()
        {}

        public void RemoveAt(int i)
        {}
		
        public void Remove(object o)
        {}

        public int IndexOf(object o)
        {
            return 0;
        }

        public bool Contains(object o)
        {
            return false;
        }

        public bool IsSynchronized
        {
            get
            {
                return (false);
            }
        }

        public object SyncRoot
        {
            get
            {
                return (null);
            }
        }

        public void Insert(int i, object o)
        {}
		

        public ProductType Current
        {
            get
            {
                return (ProductType)_Cache[_CacheOrdinal[position]];
            }
        }

        // Declare the Current property that IEnumerator requires:
		
        object IEnumerator.Current 
        {
            get
            {
                return _Cache[_CacheOrdinal[position]];
            }
        }
			
        public ProductType this[string productTypeId]
        {
            get
            {
                return (ProductType)(_Cache[productTypeId]);
            }
        }	

        public void CopyTo(System.Array arr, int i)
        {
            _Cache.CopyTo(arr, i);
            Console.WriteLine("Calling copy..." + Environment.StackTrace);
        }		
		
        private void FillCache()
        {
            int ordinal = 0;
			
            ProductType pType = new ProductType();
            pType.Description = "eric";
            pType.ProductTypeId = System.Guid.NewGuid();
            _Cache.Add(pType.ProductTypeId, pType);
            _CacheOrdinal.Add(ordinal++, pType.ProductTypeId);

            ProductType pType1 = new ProductType();
            pType1.Description = "nancy";
            pType1.ProductTypeId = System.Guid.NewGuid();
            _Cache.Add(pType1.ProductTypeId, pType1);
            _CacheOrdinal.Add(ordinal++, pType1.ProductTypeId);
        }
    }

    public  class ProductType 
    {
        private Guid					_ProductTypeId;
        private string					_Description;

        public ProductType()
        {
            _ProductTypeId = System.Guid.NewGuid();
            _Description = null;
        }

        public Guid  ProductTypeId
        {
            get
            {
                return (_ProductTypeId);
            }
            set
            {
                _ProductTypeId = value;
            }
        }

        public string Description
        {
            get
            {
                return (_Description);
            }
            set
            {
                _Description = value;
            }
        }

    }
  5. Right-click WindowsApplication1, and then click Properties.
  6. In the Output type list, click Console Application.
  7. In the Startup object list, click f, and then click OK.
  8. On the Build menu, click Build Solution.
  9. On the Debug menu, click Start.

    Observe the output in the console. The CopyTo method and the GetEnumerator method are called many times, even though you may expect the GetEnumerator method to be called two times and the CopyTo method to be called only one time.

REFERENCES

For more information, visit the following link in Microsoft Developer Network (MSDN) Web site:

http://msdn.microsoft.com/library/en-us/vbcon/html/vbtskdatabindingcomboboxcheckedlistboxorlistboxcontrol.asp

Modification Type:MinorLast Reviewed:2/3/2006
Keywords:kbvs2005swept kbvs2005doesnotapply kbvs2002sp1sweep kbDataBinding kbListBox kbCtrl kbControl kbComboBox kbbug KB820636 kbAudDeveloper
©2004 Microsoft Corporation. All rights reserved.