How to convert this hex string into a long?

I don't know of any function to do it, but I think you can do it quite simply by splitting the hex string and passing each part through Convert.ToInt32():

int part1 = Convert.ToInt32("E9", 16)
int part2 = Convert.ToInt32("4C827CEB", 16) //the last 4 bytes
long result = part1 * 4294967296 + part2  //4294967296 being 2^32. Result = 1002011000043

Kick it old-school and roll your your own. This is not exactly rocket science here:

public ulong HexLiteral2Unsigned( string hex )
{
    if ( string.IsNullOrEmpty(hex) ) throw new ArgumentException("hex") ;

    int i = hex.Length > 1 && hex[0]=='0' && (hex[1]=='x'||hex[1]=='X') ? 2 : 0 ;
    ulong value = 0 ;

    while ( i < hex.Length )
    {
        uint x = hex[i++] ;

        if      ( x >= '0' && x <= '9' ) x =   x - '0' ;
        else if ( x >= 'A' && x <= 'F' ) x = ( x - 'A' ) + 10 ;
        else if ( x >= 'a' && x <= 'f' ) x = ( x - 'a' ) + 10 ;
        else throw new ArgumentOutOfRangeException("hex") ;

        value = 16*value + x ;

    }

    return value ;
}

For those of you looking for the answer using the full .NET framework for pc.

long answer = Convert.ToInt64("E94C827CEB",16);

see: MSDN Documentation