The index is zero-based and the first character in this secure string is at index position zero.

The value of the secure string, if any, is decrypted; c is inserted at the specified location; then the new value is encrypted. The SecureString.InsertAt(int, char) method yields the same results as the SecureString.AppendChar(char) method, which inserts a character at the end of a secure string, if the index parameter of SecureString.InsertAt(int, char) is set to the length of this instance.