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Asked: 2026-05-11T03:09:44+00:00

I’d like to create a derived control from System.Windows.Forms.ComboBox that is bound to a

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I’d like to create a derived control from System.Windows.Forms.ComboBox that is bound to a list of objects that I retrieve from the database. Idea is other developers can just drop this control on their form without having to worry about the datasource, binding, unless they want to.

I have tried to extend combobox and then set the DataSource, DisplayMember, and ValueMember in the constructor. 

public class CustomComboBox : ComboBox {     public CustomComboBox()      {         this.DataSource = MyDAL.GetItems(); // Returns List<MyItem>         this.DisplayMember = 'Name';         this.ValueMember = 'ItemID';     } }

Works when I run, but throws a lot of errors in Visual Studio’s once it’s added to any form. The error I get is:

‘Code generation for property ‘Items’ failed. Error was: ‘Object reference not set to an instance of an object’

What’s the correct way to accomplish this (C#, Winforms, .NET 2.0+)?

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1 Answer

  1. DesignMode property doesn’t work in a constructor. From googling for a while, found this LicenseManager thing.

    if (LicenseManager.UsageMode != LicenseUsageMode.Designtime) {     // Do your database/IO/remote call }

    However LicenseManager only works in constructors. For eventhandlers use DesignMode.

    Source: http://dotnetfacts.blogspot.com/2009/01/identifying-run-time-and-design-mode.html

    Another reference: http://weblogs.asp.net/fmarguerie/archive/2005/03/23/395658.aspx

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