error including dll file in asp.net

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C#

ASP.NET

i have created a dll file in c#.For ex as follows..
namespace abc
{
public class class1
{
public void def()
{

}
}
}

i want to include this dll in my asp.net web app.I have added reference to it by add reference method,also written "using abc" in code behind.I am even able get the class name of dll.But still its giving the error while compiling
"namespace "abc" could not be found(are you missing a using directive or an assembly reference)".Is there any other method to include dll file? Do i need to copy the dll file to my apps bin folder?.How can i do this?help me..