public static FieldDescription CreateIntegerField( 
   string name
)

// Create a Cities feature class with the following fields
//  GlobalID
//  ObjectID
//  Name (string)
//  Population (integer)

// This static helper routine creates a FieldDescription for a GlobalID field with default values
FieldDescription globalIDFieldDescription = FieldDescription.CreateGlobalIDField();

// This static helper routine creates a FieldDescription for an ObjectID field with default values
FieldDescription objectIDFieldDescription = FieldDescription.CreateObjectIDField();

// This static helper routine creates a FieldDescription for a string field
FieldDescription nameFieldDescription = FieldDescription.CreateStringField("Name", 255);

// This static helper routine creates a FieldDescription for an integer field
FieldDescription populationFieldDescription = FieldDescription.CreateIntegerField("Population");

// Assemble a list of all of our field descriptions
List<FieldDescription> fieldDescriptions = new List<FieldDescription>()
  { globalIDFieldDescription, objectIDFieldDescription, nameFieldDescription, populationFieldDescription };

// Create a ShapeDescription object
ShapeDescription shapeDescription = new ShapeDescription(GeometryType.Point, spatialReference);

// Alternatively, ShapeDescriptions can be created from another feature class.  In this case, the new feature class will inherit the same shape properties of the existing class
ShapeDescription alternativeShapeDescription = new ShapeDescription(existingFeatureClass.GetDefinition());

// Create a FeatureClassDescription object to describe the feature class to create
FeatureClassDescription featureClassDescription = 
  new FeatureClassDescription("Cities", fieldDescriptions, shapeDescription);

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

// Add the creation of the Cities feature class to our list of DDL tasks
schemaBuilder.Create(featureClassDescription);

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

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

// 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;
}