For MongoDB 3.4+ the recommended way is to use indexes.
See https://jira.mongodb.org/browse/DOCS-11105?focusedCommentId=1859745&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-1859745
I am successfully searching with case insensitive by:
1. Creating an index with Collation for a locale (e.g: "en") and with a strength of 1 or 2. See https://docs.mongodb.com/manual/core/index-case-insensitive/ for further details
- Using the same Collation when performing searches on the MongoDb collection.
As an example:
Create a collation with strength 1 or 2 for case insensitive
private readonly Collation _caseInsensitiveCollation = new Collation("en", strength: CollationStrength.Primary);
Create an index. In my case I index several fields:
private void CreateIndex()
{
var indexOptions = new CreateIndexOptions {Collation = _caseInsensitiveCollation};
var indexDefinition
= Builders<MyDto>.IndexKeys.Combine(
Builders<MyDto>.IndexKeys.Ascending(x => x.Foo),
Builders<MyDto>.IndexKeys.Ascending(x => x.Bar));
_myCollection.Indexes.CreateOne(indexDefinition, indexOptions);
}
When querying make sure you use the same Collation:
public IEnumerable<MyDto> GetItems()
{
var anyFilter = GetQueryFilter();
var anySort = sortBuilder.Descending(x => x.StartsOn);
var findOptions = new FindOptions {Collation = _caseInsensitiveCollation};
var result = _salesFeeRules
.Find(anyFilter, findOptions)
.Sort(anySort)
.ToList();
return result;
}