public static FieldDescription CreateIntegerField( 
   string name
)

public void CreateFeatureClassSnippet(Geodatabase geodatabase, FeatureClass existingFeatureClass, SpatialReference spatialReference)
{
  // 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.
  }
}

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;
  }
}