public AttributeIndexDescription( 
   Index attributeIndex,
   TableDescription tableDescription
)

public void CreatingTableWithIndex(SchemaBuilder schemaBuilder)
{
    FieldDescription nameFieldDescription = FieldDescription.CreateStringField("Name", 50);
    FieldDescription addressFieldDescription = FieldDescription.CreateStringField("Address", 200);

    // Creating a feature class, 'Buildings' with two fields
    TableDescription tableDescription = new TableDescription("Buildings", new List<FieldDescription>() { nameFieldDescription, addressFieldDescription });

    // Enqueue DDL operation to create a table
    TableToken tableToken = schemaBuilder.Create(tableDescription);

    // Creating an attribute index named as 'Idx'
    AttributeIndexDescription attributeIndexDescription = new AttributeIndexDescription("Idx", new TableDescription(tableToken),
      new List<string> { nameFieldDescription.Name, addressFieldDescription.Name });

    // Enqueue DDL operation to create an attribute index
    schemaBuilder.Create(attributeIndexDescription);

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

public void RemoveAttributeIndex(SchemaBuilder schemaBuilder, FeatureClassDefinition featureClassDefinition, string attributeIndexName)
{
    // Find a index to be removed 
    ArcGIS.Core.Data.Index indexToRemove = featureClassDefinition.GetIndexes().First(f => f.GetName().Equals(attributeIndexName));

    // Index description of the index to be removed 
    AttributeIndexDescription indexDescriptionToRemove = new AttributeIndexDescription(indexToRemove, new TableDescription(featureClassDefinition));

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

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