public void AddFeatureClass( 
   FeatureDatasetDescription featureDatasetDescription,
   FeatureClassDescription featureClassDescription
)

public void AddFeatureClassToFeatureDatasetSnippet(Geodatabase geodatabase)
{
  // Adding a FeatureClass with name 'Tax_Jurisdiction' into a FeatureDataset named as 'Parcels_Information'

  string featureDatasetName = "Parcels_Information";
  string featureClassNameToAdd = "Tax_Jurisdiction";

  FeatureDatasetDefinition featureDatasetDefinition = geodatabase.GetDefinition<FeatureDatasetDefinition>(featureDatasetName);
  FeatureDatasetDescription featureDatasetDescription = new FeatureDatasetDescription(featureDatasetDefinition);

  FeatureClassDefinition featureClassDefinition = geodatabase.GetDefinition<FeatureClassDefinition>(featureClassNameToAdd);
  FeatureClassDescription featureClassDescription = new FeatureClassDescription(featureClassDefinition);

  SchemaBuilder schemaBuilder = new SchemaBuilder(geodatabase);

  // Add the 'Tax_Jurisdiction' FeatureClass to the 'Parcels_Information' FeatureDataset 
  schemaBuilder.AddFeatureClass(featureDatasetDescription, featureClassDescription);
  bool addStatus = schemaBuilder.Build();

  if (!addStatus)
  {
    IReadOnlyList<string> errors = schemaBuilder.ErrorMessages;
  }
}