How to set .net framework trust level to Low for C#.net application

I have a C# app running in fully trusted zone "My computer". I have checked trust level of application under "Microsoft .net Framework configuration tool" and it is showing as "unrestricted" which means fully trusted.  Now I want to set this trust level to low for application. How can I achieve this ? It would be better if this can be achieved without code changes of application. Can we mention these settings in config file of application ?  Assembly is not signed.
I have tried to modify trust level from "Microsoft .net Framework configuration tool -> Runtime Security Policy -> Trust assembly"  but getting error message as "Hash for the assembly cannot be generated".
I think this error because of my assembly is not signed.
Can we have a application with trust level set to low, running in zone with trust level as Fully trusted.  

Thanks,
Sagar