As of R1 2019, the RadExpressionEditor control allows users to expand complex objects and access their sub-properties. This functionality is controlled by the new IsFieldsDrillDownEnabled property.
Example 1: Enabling drill-down in XAML
<telerik:RadExpressionEditor IsFieldsDrillDownEnabled="True" />
Example 2: Enabling drill-down in code-behind
this.radExpressionEditor.IsFieldsDrillDownEnabled = true;
Me.radExpressionEditor.IsFieldsDrillDownEnabled = True
Figure 1 illustrates the difference between the functionality being disabled and enabled.