High and low bits of 64-bit long and C#

I can't help you with C, but maybe you'll be able to convert this. Here's how I do it in Python:

def convert_to_64_bit(high, low):
    """ Converts two 32 bit signed integers to a 64-bit unsigned integer.
    """
    if high < 0:
        high = high + (1 << 32)
    if low < 0:
        low = low + (1 << 32)
    value = long((high << 32) | low)
    assert(value >= 0)
    return value