dllimport - Using C# dll in Windows phone 7 -

i've tried using c# dll in windows phone 7 occurs error after start debugging illustrated below.

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troubleshooding tips: if access level of method in class library has changed, recompile assemblies reference library. generral exception.

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this code..

-----------------windows phone 7-----------------------------------------------

using system; ... using system.runtime.interopservices;  namespace dllloadtest {     public partial class mainpage : phoneapplicationpage     {         // constructor         public mainpage()         {             initializecomponent();         }          [dllimport("mathlibrary.dll")]         public static extern int addinteger(int a, int b);          private void button1_click(object sender, routedeventargs e)         {             messagebox.show("test " + addinteger(3, 4));         }     } } 

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------------------------c# mathlibrary.dll----------------------------------

using system; using system.collections.generic; using system.linq; using system.text;  namespace mathlibrary {     public class add     {         public static long addinteger(long i, long j)         {             return + j;         }     } } 

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is there problem? if not, using c# dll windowsphone7 impossible? c# dll loaded in visualstudio2008 c#.

why trying use p/invoke on class library written in c#? add reference dll , use directly:

using mathlibrary; ...  private void button1_click(object sender, routedeventargs e) {     messagebox.show("test " + add.addinteger(3, 4)); } 

you can't use p/invoke in windows phone 7, can use class libraries (built windows phone 7).