public object Item( 
   string fieldName
) {get; set;}

// get the currently selected features in the map
var selectedFeatures = ArcGIS.Desktop.Mapping.MapView.Active.Map.GetSelection();
// get the first layer and its corresponding selected feature OIDs
var firstSelectionSet = selectedFeatures.ToDictionary().First();

// create an instance of the inspector class
var inspector = new ArcGIS.Desktop.Editing.Attributes.Inspector();
// load the selected features into the inspector using a list of object IDs
await inspector.LoadAsync(firstSelectionSet.Key, firstSelectionSet.Value);

// assign the new attribute value to the field "Description"
// if more than one features are loaded, the change applies to all features
inspector["Description"] = "The new value.";
// apply the changes as an edit operation 
await inspector.ApplyAsync();