CodeQL library for C#
codeql/csharp-all 0.7.5 ( 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 )