public SpatialIndexDescription( 
   FeatureClassDescription featureClassDescription
)

public void AddingIndexes(SchemaBuilder schemaBuilder, FeatureClassDefinition featureClassDefinition)
{
  // Field names to add in the attribute index
  string fieldName = featureClassDefinition.GetFields().First(f => f.AliasName.Contains("Name")).Name;
  string fieldAddress = featureClassDefinition.GetFields().First(f => f.AliasName.Contains("Address")).Name;

  // Creating an attribute index with index name 'Idx' and two participating fields' name
  AttributeIndexDescription attributeIndexDescription = new AttributeIndexDescription("Idx", new TableDescription(featureClassDefinition), new List<string> { fieldName, fieldAddress });

  // Enqueue DDL operation for an attribute index creation 
  schemaBuilder.Create(attributeIndexDescription);

  // Creating the spatial index 
  SpatialIndexDescription spatialIndexDescription = new SpatialIndexDescription(new FeatureClassDescription(featureClassDefinition));

  // Enqueue DDL operation for the spatial index creation
  schemaBuilder.Create(spatialIndexDescription);

  // Execute build indexes operation
  bool isBuildSuccess = schemaBuilder.Build();

  if (!isBuildSuccess)
  {
    IReadOnlyList<string> errors = schemaBuilder.ErrorMessages;
    // Iterate and handle errors 
  }
}

public void RemoveSpatialIndex(SchemaBuilder schemaBuilder, FeatureClassDefinition featureClassDefinition)
{
  // Create a spatial description  
  SpatialIndexDescription spatialIndexDescription = new SpatialIndexDescription(new FeatureClassDescription(featureClassDefinition));

  // Enqueue the DDL operation to remove index 
  schemaBuilder.Delete(spatialIndexDescription);

  // Execute the delete index operation
  bool isDeleteIndexSuccess = schemaBuilder.Build();
}