public void Modify( 
   FeatureClassDescription featureClassDescription
)

public void AddFieldsInFeatureClassSnippet(Geodatabase geodatabase)
{
  // Adding following fields to the 'Parcels' FeatureClass
  // Global ID
  // Parcel_ID
  // Tax_Code
  // Parcel_Address

  // The FeatureClass to add fields
  string featureClassName = "Parcels";

  FeatureClassDefinition originalFeatureClassDefinition = geodatabase.GetDefinition<FeatureClassDefinition>(featureClassName);
  FeatureClassDescription originalFeatureClassDescription = new FeatureClassDescription(originalFeatureClassDefinition);

  // The four new fields to add on the 'Parcels' FeatureClass
  FieldDescription globalIdField = FieldDescription.CreateGlobalIDField();
  FieldDescription parcelIdDescription = new FieldDescription("Parcel_ID", FieldType.GUID);
  FieldDescription taxCodeDescription = FieldDescription.CreateIntegerField("Tax_Code");
  FieldDescription addressDescription = FieldDescription.CreateStringField("Parcel_Address", 150);

  List<FieldDescription> fieldsToAdd = new List<FieldDescription> { globalIdField, parcelIdDescription,
    taxCodeDescription, addressDescription };

  // Add new fields on the new FieldDescription list
  List<FieldDescription> modifiedFieldDescriptions = new List<FieldDescription>(originalFeatureClassDescription.FieldDescriptions);
  modifiedFieldDescriptions.AddRange(fieldsToAdd);

  // The new FeatureClassDescription with additional fields
  FeatureClassDescription modifiedFeatureClassDescription = new FeatureClassDescription(originalFeatureClassDescription.Name,
    modifiedFieldDescriptions, originalFeatureClassDescription.ShapeDescription);

  SchemaBuilder schemaBuilder = new SchemaBuilder(geodatabase);

  // Update the 'Parcels' FeatureClass with newly added fields
  schemaBuilder.Modify(modifiedFeatureClassDescription);
  bool modifyStatus = schemaBuilder.Build();

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