string to unique long

I have a string made up of 2 parts (see code)

I want to know the UnknownDeterministicFunction which retus a Long, which can deterministically produce the same long for a given string.

private void MyProgram()
{
    string resultStr = "XXX"+"12345678";
    //1st part is a string of characters (the "XXX")
    //2nd part is a string of numbers (the "12345678")

    long resultLng = UnknownDeterministicFunction(myString);
}

private long UnknownDeterministicFunction(string inputStr)
{
    // ???
}

Is this possible in C#?