VS 2010 Attempted to read or write protected memory.

If OpenFileDialog1.ShowDialog = Windows.Forms.DialogResult.OK Then
xlApp = New Excel.Application
xlWorkBook = xlApp.Workbooks.Open(OpenFileDialog1.FileName)
xlWorkSheet = xlWorkBook.Worksheets("sheet1")

.
.
.

end if

i am getting the error : Attempted to read or write protected memory.

@ line 1

please help