C # takes advantage of String class indexof, LastIndexOf, substring to intercept strings

First, String.IndexOf

String.IndexOf method (Char, Int32, Int32)
Reports the index of the first occurrence of the specified character in this instance ( starting at 0 ). The search starts at the specified character position and checks the specified number of character positions.
String.IndexOf (value, StartIndex, Count)

Parameters
Value: The Unicode character to find.
StartIndex: Search start location.
Count: The number of character positions to check.
return value (INT32):
The index position of value if the character is found, or 1 if not found.

Second, String.LastIndexOf

String.LastIndexOf method
Reports the index position of the last occurrence of the specified Unicode character or String in this instance.

Third, string.substring

String.substring method
Retrieves a substring from this instance.

Name	Description
String.substring (Int32)	Retrieves a substring from this instance. The substring starts at the specified character position.
String.substring (Int32, Int32)	Retrieves a substring from this instance. The substring starts at the specified character position and has the specified length.

Example: File contents

1$aid_700e5984dba96744
2$aid_b5f0d8ca79ae856e#aid_700e5984dba96744
3$aid_2f0b6558766df9b6#aid_b5f0d8ca79ae856e

C # takes advantage of String class indexof, LastIndexOf, substring to intercept strings