Opening Excel-Workbook works with .Net3.5 but not with .Net4

Question

I am trying to convert an "old" .Net-3.5-Project to .Net-4.0. By now everything works fine, but the Excel-Interop.

What I am having problems with is the opening of an workbook. I already tried an all new project and compiled it once with .Net-3.5 and once with .Net-4.0. With the "old" Framework it works as excpected, but with 4.0 I only get null as a result?

My code in my testapp is this:

using System;
using System.Windows.Forms;
using Excel = Microsoft.Office.Interop.Excel;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        private Excel.Workbook test;

        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            OpenWithInterop();
        }

        private void OpenWithInterop()
        {
            Microsoft.Office.Interop.Excel.Application excel = new Microsoft.Office.Interop.Excel.Application();
            excel.WorkbookOpen += new Excel.AppEvents_WorkbookOpenEventHandler(excel_WorkbookOpen);
            test = excel.Workbooks.Open(@"C:/Test/test.xlsx");

            excel.Quit();
        }

        void excel_WorkbookOpen(Excel.Workbook Wb)
        {
            if (test.Name.Equals(Wb.Name)) // Here there will be an null-exception with .net-4 but not with .net-3.5
            {
                Console.WriteLine("done it right");
            }
        }
    }
}

The testapp contains only a WinForm-Form (from the template) and I added just one button that has the above shown behaviour.

Is there anything I am missing here? Did anything change in the way to work with Excel in 4.0?

UPDATE: To answer your questions:

It's Office 2010 (x86) on Windows 7 Enterprise (x64)
I just added it in "references" in Visual Studio and called it as "using" in my Main-Class. (Will update my code to show it full)
That's now the real minimal code to show you the behaviour - strange isn't it?

UPDATE 2: I found out some "new" stuff:

It's also happening on Windows XP (x86)
You don't need to have the project, in which Excel.Interop is referenced, in .net4, it's enough that it is called by some .net4-project. Example:

BaseProject (.net-4) --> DataLayer (.net-3.5) references Excel.Interop --> Presentation (.net-4)

In this example the error will happen. Even when DataLayer is .net-3.5. BaseProject has to be .net-4, because it's calling Presentation which is .net-4 (and needs to be...)

UPDATE 3:

Just found out, that everything would work fine, but you run in really trouble if you are using one of these to events in .net4

excel.WorkbookBeforeClose += new Excel.AppEvents_WorkbookBeforeCloseEventHandler(this.HandleWorkbookClosed);
excel.WorkbookOpen += new Excel.AppEvents_WorkbookOpenEventHandler(this.HandleWorkbookOpen);

Are there any aquivalent events in Excel._Application like in Excel.ApplicationClass?

UPDATE 4:

To answer the question of the comments (thanks Will!) I extended the example a little bit. The main-problem is in the event. Why is there this difference in .net-4 or is somewhere documented? And how to avoid it?

That works for me, btw you can leave off all the System.Type.Missings as they are optional arguments. How did you add the reference to the interop? — Dan, May 04 '12 at 06:37
It's not clear - which variable is coming back as null? `excel` or `test`? — Will, May 04 '12 at 11:13
It's interesting that this question `http://stackoverflow.com/questions/1922851/c-sharp-excel-interoperability` says we should use ApplicationClass, not Application. Does this help? — Will, May 04 '12 at 11:15
@Will: I found out, that it's actually not coming back as null but as some strange type with Exceptions as values. I am talking about "test". The actual problem is described in Update-3. In the WorkbookOpen-event is a paramter Workbook given (the one that got opened). If you now compare this event-given-workbook with the one in "test" you will get strange exceptions. — basti, May 04 '12 at 12:37
@Will: Your cited question has as toprated answer that you should change from ApplicationClass to Application — basti, May 04 '12 at 12:37
Check for target framework whether you are using .net Framework 4.0 client profile or .net Framework 4.0 in project properties. — Romil Kumar Jain, May 04 '12 at 13:12
Romil: I am using .net-Framework-4.0 - not the client-profile. — basti, May 04 '12 at 13:40

score 0 · Accepted Answer · answered May 07 '12 at 12:30

I have a kind of a solution by now:

I changed my 2 event-handlers from

    void excel_WorkbookOpen(Excel.Workbook Wb)
    {
        if (test.Name.Equals(Wb.Name)) // Here there will be an null-exception with .net-4 but not with .net-3.5
        {
            Console.WriteLine("done it right");
        }
    }

to this changed logic

    void excel_WorkbookOpen(Excel.Workbook wb)
    {
        if (!wb.FullName.Equals(pathToExcelFile)) // pathToExcelFile is class-wide visible
        {
            return;
        }

        Console.WriteLine("done it right");
    }

So this is a workaround to my previous logic.

As this is not a explanation why it works with 3.5 and not with 4 and it actually doesn't solve the problem I am not willing to accept my own answer as solution.

For future searches / problems of other people this thread might be helpful, so I will let it stay here open. If someone finds a explanation for the behaviour I would be absolute willing to accept it as an answer :)

Opening Excel-Workbook works with .Net3.5 but not with .Net4

1 Answers1