Addition component to web service

[Goodmannewz]

I am going to implement a program on the web service side. The program will
work with the web services so that it can do some check on the data before
invoking any web method. The program will do some general checks, and does
not know exactly what kinds of web methods behind.

I think I could write a dll, then let the web services to call this dll
first before the web methods are invoked. But I do not how to do exactly.

Is there any way to integrate the program into the web services? Please
give me a guide in details.

Thanks.