public void Modify( 
   FeatureClassDescription featureClassDescription
)

// 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;
}