Releasing Excel Interop COM Objects..

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C#

C#4.0

public void button1_Click(object sender, EventArgs e)
        {
            openFileDialog1.Filter = "Excel File|*.xls|Excel File|*.xlsx";
            
            openFileDialog1.ShowDialog();
            
            string path = openFileDialog1.FileName;
            
            comboBox1.Items.Clear();

            Microsoft.Office.Interop.Excel.Application xlr = new            
            Microsoft.Office.Interop.Excel.Application();

            Workbook wb = xlr.Workbooks.Open(path, 0, false, 5, "", "", false, 
            XlPlatform.xlWindows, "", true, false, 0, true, false, false);

            foreach (Worksheet w in xlr.Worksheets)
            {
                comboBox1.Items.Add(w.Name);
            }

            xlr.Visible = true;

            wb.Close();
            xlr.Quit();

            System.Runtime.InteropServices.Marshal.ReleaseComObject(wb);
            System.Runtime.InteropServices.Marshal.ReleaseComObject(xlr);

        }

How can i safely dispose off/Release Excel COM objects in the above code??
Even after releasing in the above fashion i see that excel is running in the task manager under processes ...?