public FeatureClassToken Create( 
   FeatureDatasetDescription featureDatasetDescription,
   FeatureClassDescription featureClassDescription
)

public void CreateFeatureDatasetWithFeatureClassSnippet(Geodatabase geodatabase)
{
  // Creating a FeatureDataset named as 'Parcel_Information' and a FeatureClass with name 'Parcels' in one operation

  string featureDatasetName = "Parcel_Information";
  string featureClassName = "Parcels";

  SchemaBuilder schemaBuilder = new SchemaBuilder(geodatabase);

  // Create a FeatureDataset token
  FeatureDatasetDescription featureDatasetDescription = new FeatureDatasetDescription(featureDatasetName, SpatialReferences.WGS84);
  FeatureDatasetToken featureDatasetToken = schemaBuilder.Create(featureDatasetDescription);

  // Create a FeatureClass description
  FeatureClassDescription featureClassDescription = new FeatureClassDescription(featureClassName,
    new List<FieldDescription>()
    {
      new FieldDescription("Id", FieldType.Integer),
      new FieldDescription("Address", FieldType.String)
    },
    new ShapeDescription(GeometryType.Point, SpatialReferences.WGS84));

  // Create a FeatureClass inside a FeatureDataset
  FeatureClassToken featureClassToken = schemaBuilder.Create(new FeatureDatasetDescription(featureDatasetToken), featureClassDescription);

  // Build status
  bool buildStatus = schemaBuilder.Build();

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

public void CreateFeatureClassInsideFeatureDatasetSnippet(Geodatabase geodatabase)
{
  // Creating a FeatureClass named as 'Tax_Jurisdiction' in existing FeatureDataset with name 'Parcels_Information'
  string featureDatasetName = "Parcels_Information";
  string featureClassName = "Tax_Jurisdiction";

  // Create a FeatureClass description
  FeatureClassDescription featureClassDescription = new FeatureClassDescription(featureClassName,
    new List<FieldDescription>()
    {
      new FieldDescription("Tax_Id", FieldType.Integer),
      new FieldDescription("Address", FieldType.String)
    },
    new ShapeDescription(GeometryType.Point, SpatialReferences.WGS84));

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

  SchemaBuilder schemaBuilder = new SchemaBuilder(geodatabase);

  // Create a FeatureClass inside a FeatureDataset using a FeatureDatasetDefinition
  schemaBuilder.Create(new FeatureDatasetDescription(featureDatasetDefinition), featureClassDescription);

  // Build status
  bool buildStatus = schemaBuilder.Build();

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