MongoDB and C#: Case insensitive search

I've just implemented this much simpler than any of the other suggestions. However I realise due to the age of this question, this functionality may not have been available at the time.

Use the options of the Bson Regular Expression constructor to pass in case insensitivity. I just had a look at the source code and found that 'i' is all you need. For example.

var regexFilter = Regex.Escape(filter);
var bsonRegex = new BsonRegularExpression(regexFilter, "i");

Query.Matches("MyField", bsonRegex);

You shouldn't have to keep records twice for searching.

try to use something like this:

Query.Matches("FieldName", BsonRegularExpression.Create(new Regex(searchKey, RegexOptions.IgnoreCase)))

You will probably have to store the field twice, once with its real value, and again in all lowercase. You can then query the lowercased version for case-insensitive search (don't forget to also lowercase the query string).

This approach works (or is necessary) for many database systems, and it should perform better than regular expression based techniques (at least for prefix or exact matching).

The simplest and safest way to do that is using Linq:

var names = namesCollection.AsQueryable().Where(name =>
    name.FirstName.ToLower().Contains("hamster"));

As explained in the tutorial ToLower, ToLowerInvariant, ToUpper and ToUpperInvariant all perform matches in a case insensitive way. After that you can use all the supported string methods like Contains or StartsWith.

This example will generate:

{
    "FirstName" : /hamster/is
}

The i option makes it case insensitive.