在LINQPad中使用数据上下文调用静态方法

Question

I am using LINQPad to develop a new data access method. In the method, which performs a LINQ-to-SQL query against a table in MySchemaContext, I am using a parameter for which I am loading a default value from the same DataContext object using a static method.

When I call this static function in LINQPad (using the "C# Program" setting) - one that works fine when called from with an application, and that is public - I get a null reference exception on the line that assigns the return value of the static function to a variable. If I change the logic of the function, it works if I do not reference MySchemaContext (for instance, if I assemble the List by hand and return that), but throws the exception if I do reference MySchemaContext.

My guess is that this has something to do with the way LINQPad is connecting to my database - that the DataContext is not valid when the library function is called from LINQPad, and that the null reference exception is being thrown by GetTable<MyClassTable> in the library. Is this a limitation of LINQPad, or is there something obvious I'm missing?

Here is a sample of the sort of function I'm having a problem with. The first part is the code in my library (which I am calling from the LINQPad query):

namespace MyLibrary
{
 public class MyClass : MyClassTable
 {
   //There are reasons not to just add stuff to the partial class MyClassTable 
   //that aren't shown in this generic example

  public int MyClassTemplateID {get; set;}
  public int MyInt {get; set;}
  public string MyString {get; set;}

  public MyClass() {}
  public MyClass(int myClassTemplateID, int myInt, string myString) 
  {
   MyClassTemplateID = myClassTemplateID;
   MyInt = myInt;
   MyString = myString;
  }
  public static List<MyClass> GetMyClassesTemplate(int myClassTemplateID)
  {
   //MySchemaContext is a LINQ-to-SQL DataContext object, i.e., inherits from System.Data.Linq.DataContext 
   using (MySchemaContext myContext = new MySchemaContext())
   {
    return (from mct in myContext.GetTable<MyClassTable>().AsQueryable() 
     where mct.MyClassTemplateID == myClassTemplateID 
     select new MyClass
     {
      MyClassTemplateID = mct.MyClassTemplateID,
      MyInt = mct.MyInt,
      MyString = mct.MyString
     }).ToList<MyClass>();
   }
  }
 }
}

The library function works well in my application. This second part is in my LINQPad query, trying to call this same function:

/*
    Call from LINQPad
    MyLibrary.dll is in "Additional References," and MyLibrary is in "Additional Namespace Imports."
*/

void Main()
{
 List<MyClass> myClassList = MyClass.GetMyClassesTemplate(1);
 //get error "NullReferenceException: Object reference not set to an instance of an object.
 //However, this exact line of code works fine in VisualStudio.
 //Have correct connection string for MySchemaContext in LINQPad.exe.config and lprun.exe.config
}

Answer 1

I suspect your issue is the use of the separate namespace. I was able to access a static method in LINQPad using the following code. Nptice in particular the mismatch of the opening and closing brackets.

    void Main()
    {
        tName.test.DoIt(); // Fully qualify the static method name
    }

} // Close the generated context's class

namespace tName
{
    public class test
    {
    // Define other methods and classes here
        public static void DoIt()
        {
            Console.WriteLine("Done");
        }
    }
// } Don't close the namespace. Let LinqPad do it.

With LinqPad, you are essentially injecting the code you type into a generated context. As a result, you need to fake it out thinking that the context's class is closed before injecting the new namespace/class. If you didn't use the namespace, you would be able to create a new class, but that class would be a child of the hosting class, not directly under the namespace in question. I discussed this further in this blog post .