WPF DataBinding specifying array index in Binding not working

I am facing a problem with DataBinding in WPF::::

The below code is working:

<TextBox Text="{Binding ProcessStepBlock.ProcessStep[2].ProcessDescription}"></TextBox>
 <TextBox Text="{Binding ProcessStepBlock.SelectedIndex}" ></TextBox>

The below code is not work开发者_StackOverflowing:It gives me a binding error BindingExpression path error: '[ ]' property not found on 'object' ''ObservableCollectionEx`1'

 <TextBox Text="{Binding ProcessStepBlock.ProcessStep[ProcessStepBlock.SelectedIndex].ProcessDescription}"></TextBox>
 <TextBox Text="{Binding ProcessStepBlock.SelectedIndex}" ></TextBox>

Please help!!!

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The XAML parser can't resolve properties to provide values inside a PropertyPath. To get what you're looking for use a MultiBinding that takes both the ProcessStep collection and the SelectedIndex property and create an IMultiValueConverter that produces the ProcessDescription by doing the indexing in code.