The ResourceExpressionEditorSheet.ResourceKey property is required for a resource expression; it indicates the name of the resource. The name of the resource is used at both run time and design time to look up a resource and assign it to a control property.

For example, if the System.Web.UI.WebControls.Label.Text property for a System.Web.UI.WebControls.Label control is assigned to the resource expression <%$ Resources:Strings, labelText $>, the resource expression is evaluated and the System.Web.UI.WebControls.Label.Text field is assigned to the labelText resource value in the Strings class.