using public static class

Hi,
I was used to (in Windows Forms) to define methods and properties in public static class, to access them anywhere in solution code files.
In WebForms I have problems with this scenario.
If I set Build Action for cs file to Content (like default) , I cannot access the static methods or properties. 
If I set the Build Action for cs file to Compile I have compilation error:
The type 'UsingWebControls.Utility' in 'D:\ProjectsVS\UsingWebControls\UsingWebControls\App_Code\Class\Utility.cs' conflicts with the imported type 'UsingWebControls.Utility' in 'D:\ProjectsVS\UsingWebControls\UsingWebControls\'. Using the type defined in 'D:\ProjectsVS\UsingWebControls\UsingWebControls\App_Code\Class\Utility.cs'. D:\ProjectsVS\UsingWebControls\UsingWebControls\App_Code\Class\ExtensionMethods.cs 19 4 UsingWebControls

So I think it's not the way I should go...
Then , what I need to do to have same result: some methods which ...