Class variable could not detected after putting the application in virtual directory on IIS

By putting my app in virtual directory on IIS, the compiler did not recognise the class variable
for example, DetectSqlInjection key word does not recognise
 
bool bDetectSQLInjectionUname = DetectSqlInjection.CheckInput(txtUsername.Text);  
from
 
public class DetectSqlInjection { ... public static bool CheckInput(string parameter) { ...