Adding addin for excel in C#

See more:

Visual-Studio

MS-Excel

C#6.0

, +

I'm trying to make an C# Excel addin in order to automate the calculus of some cells value.What the addin should do is when a box is checked it should multiply the value of a cell with other cells and paste the result in different cells.The error that I get is "Cannot convert null to int because it is not a non nullable value type".When the Excel opens I put in numbers and after I check the box I get this error.From what I understand this line of code gets a null from Excel:

int cellValue = sheet.Cells[1, 12].Value2;

I tried in the first variant of my code to open a specific Excel workbook but it didn't work.It opens a new Excel empty file.Eliminating

var excelApp = new Excel.Application() as Excel._Application;

also brings up a new error:

Quote:

An object reference is required for the non-static field method or property.

What I have tried:

I Tried this 2 variants.I get the same error.

public void fc(string[] args)
        {
           

            string dataFilePath = new FileInfo(args[0]).FullName;
          
            string excelTemplateFilePath = Path.Combine(
               Path.GetDirectoryName(dataFilePath),
               "informations.xlsx");
            var excelApp = new Excel.Application() as Excel._Application;
            Excel.Workbook workbook = excelApp.Workbooks.Open(excelTemplateFilePath);
            Excel.Worksheet worksheet = workbook.Sheets[1];
            var dataRange = worksheet.get_Range("A20");
            dataRange.Value2 = 0.5;
            string baseFileName = Path.Combine(
              Path.GetDirectoryName(dataFilePath),
              Path.GetFileNameWithoutExtension(dataFilePath));
            string excelFilePath = baseFileName + ".xlsx";
            for (int i = 0; i <= worksheet.Cells.Count; i++)
            {
                for (int j = 0; j <= worksheet.Cells.Count; j++)
                {
                    while (worksheet.Cells[i+1, 7] != null)
                    {
                        (worksheet.Cells[i+1, 8] as Excel.Range).Value2 = worksheet.Cells[i+1, 7] * dataRange.Value2;
                    }
                }
            }
            System.Diagnostics.Process.Start(excelFilePath);
        }

if (((RibbonCheckBox)sender).Checked)
        {
            try
            {
                Excel.Application app = new Excel.Application();
                app.Workbooks.Add();
                Excel.Worksheet sheet = app.ActiveSheet;
                int cellValue = sheet.Cells[1, 12].Value2;
                for (int i = 0; i <= sheet.Cells.Count; i++)
                {
                    for (int j = 0; j <= sheet.Cells.Count; j++)
                    {
                        while (sheet.Cells[i + 1, 7] != null)
                        {
                            sheet.Cells[i + 1, 8].Value2 = sheet.Cells[i + 1, 7] * cellValue;
                        }
                    }
                }
            }
            catch(Exception ex)
            {

                MessageBox.Show("Exceptie:" + ex);

            }