dll in windows form C#

You need to add a reference to the class library. Right click on References, click Add References..., click on the project tab, and add the library. This is assuming the class library is in the same solution. Also I am using 2008 express, so it might be slightly different.

That is ok,bt what do i write on the click event of the button

That depends on what namespace it is in and what you are doing, but you would call it the same way as if it was in the same project.

I dont knw hw to call tht class library,my nanespace is ExcelReadWrite

using this in your project,

using ExcelReadWrite;

then use your classes in that dll as usual , depending on the type of your classes.

Prof. Mendl