How does InvokeMember know about the HighPart property?

I want to use the System.Reflection library and not ActiveDs. I found this code on the web that parses the LargeInteger into HighPart and LowPart.

I don't understand it completely, particularly where is the method 'HighPart' and 'LowPart' defined? Is that within the Object class or do I have to define it?

Below is the code that parses the largeInteger:

de = new DirectoryEntry(curDomain,adUser,adPwd);        
object largeInteger = de.Properties["maxPwdAge"].Value;
System.Type type = largeInteger.GetType();
int high = (int)type.InvokeMember("HighPart", BindingFlags.GetProperty, null, largeInteger, null);
int low = (int)type.InvokeMember("LowPart", BindingFlags.GetProperty, null, largeInteger, null);

Thanks!