When building a custom / web / composite control in Visual Studio 2008 SP1, once you have a custom property that you attempt to set in design mode, you will see an error similar to this in design view: There was an error rendering the control. [value] could not be set on property [property name] The way to duplicate this error is to set a property from the designer on your control, then right-click the control and choose "Refresh" or just close the hosted ASPX page and re-open in design view. Either way, you will see this error. After wasting a ton of time searching for a solution, it turns out this a bug in Sp1 of Visual Studio 2008. As of the end of January 2009, there is now a hot fix for this issue that Microsoft is providing. The Knowledge Base Article this issue references is: KB961864 You can get the HotFix here: http://code.msdn.microsoft.com/KB961864/Release/ProjectReleases.aspx?ReleaseId=2194 There were many posts on this topic that said this fix did not work on Vista but I have tested this fix on both Windows XP and Vista (32 bit) and both worked just fine. NOTE: Make sure to close all instances of Visual Studio before running the hot fix.