public class FeatureDatasetDescription : Description 

// Create a Cities annotation feature class inside Places feature dataset using existing annotation feature class 

// Feature dataset name
string featureDatasetName = "Places";

// Annotation feature class name
string annotationFeatureClassName = "CitiesAnnotation";

// Create a SchemaBuilder object
SchemaBuilder schemaBuilder = new SchemaBuilder(geodatabase);

// Open existing annotation feature class name
using (AnnotationFeatureClass existingAnnotationFeatureClass = geodatabase.OpenDataset<AnnotationFeatureClass>("ExistingAnnotationFeatureClass"))
{

  // Create Feature dataset description
  FeatureDatasetDescription featureDatasetDescription = 
    new FeatureDatasetDescription(featureDatasetName, existingAnnotationFeatureClass.GetDefinition().GetSpatialReference());

  // Add the creation of the Places dataset to DDL task
  FeatureDatasetToken featureDatasetToken = schemaBuilder.Create(featureDatasetDescription);

  // Create an annotation feature class description using existing annotation feature class
  AnnotationFeatureClassDescription annotationFeatureClassDescription = new AnnotationFeatureClassDescription(annotationFeatureClassName, existingAnnotationFeatureClass.GetDefinition())
  {
    IsAutoCreate = true,
    IsSymbolIDRequired = false,
    IsUpdatedOnShapeChange = true
  };

  // Add the creation of the Cities annotation feature class inside Places feature dataset
  schemaBuilder.Create(new FeatureDatasetDescription(featureDatasetToken), annotationFeatureClassDescription);

  // Execute the DDL
  bool success = schemaBuilder.Build();

  // Inspect error messages
  if (!success)
  {
    IReadOnlyList<string> errorMessages = schemaBuilder.ErrorMessages;
    //etc.
  }
}