How to avoid a "object reference not set to an instance of an object" exception in XAML code at design time?

Question

I have a problem with a wpf usercontrol which is of my own devising. The problem is that I get a object reference not set to an instance of an object exception in XAML code at design time, when I implement the usercontrol in my program.

The designer showed me the following information:

at
Microsoft.Expression.Platform.InstanceBuilders.InstanceBuilderOperations.InstantiateType(Type
type, Boolean supportInternal)    at
Microsoft.Expression.Platform.InstanceBuilders.ClrObjectInstanceBuilder.InstantiateTargetType(IInstanceBuilderContext
context, ViewNode viewNode)    at
Microsoft.Expression.Platform.InstanceBuilders.ClrObjectInstanceBuilder.Instantiate(IInstanceBuilderContext
context, ViewNode viewNode)    at
Microsoft.Expression.WpfPlatform.InstanceBuilders.FrameworkElementInstanceBuilder.Instantiate(IInstanceBuilderContext
context, ViewNode viewNode)    at
Microsoft.Expression.WpfPlatform.InstanceBuilders.UserControlInstanceBuilder.Instantiate(IInstanceBuilderContext
context, ViewNode viewNode)    at
Microsoft.Expression.Platform.InstanceBuilders.ViewNodeManager.CreateInstance(IInstanceBuilder
builder, ViewNode viewNode)

but I think these messages are not really helpful...

How can I fix or suppress this exception?

Answer 1

If you have 'Object reference not set to an instance of an object' in XAML, but your application compiles and runs fine, you will usually find out that its cause is something in a constructor that can't be resolved at design time.

You can just click the "Disable project code" button located on the bottom of your designer view and Visual Studio designer will stop trying to construct an instance to provide design time data view.

See here for detailed information and screenshots.