CodeQL library for C#
codeql/csharp-all 1.0.1 (changelog, source)

Primitive predicate string::indexOf

Returns the index of the n’th (starting at 0) occurrence of the given string within the receiver, starting at the given 0-based offset. Has no result if the string does not occur in the receiver.

int indexOf(string substringToFind, int n, int startAt)