Question

I am getting an “Object Reference not set to an instance of an object” error when searching for an item (on Guid) that is selected from a datagrid. I have checked that the item does return the Guid correctly (by writing it to a label on the page), however in my linq query (i assume) i am comparing incorrectly.

ctx is the domaindatasource, I know the element im trying to remove exists.

      private void medItemRemove_Click(object sender, RoutedEventArgs e)
    {

            MedicineInventory M = (MedicineInventory)medicineInventoryDataGrid.SelectedItem;
            Guid Mid = M.MedicineInventoryId;
            MedicineInventory toRemove = new MedicineInventory();
            toRemove = (from a in ctx.MedicineInventories where (a.MedicineInventoryId == Mid) select a).Single();
            ctx.MedicineInventories.Remove(toRemove);
            ctx.SubmitChanges();

        }

Answer 1

Rewrite your code as follows:

private void medItemRemove_Click(object sender, RoutedEventArgs e)
{

    MedicineInventory M = (MedicineInventory)medicineInventoryDataGrid.SelectedItem;
    Guid Mid = M.MedicineInventoryId;
    MedicineInventory toRemove = (from a in ctx.MedicineInventories where (a != null && a.MedicineInventoryId == Mid) select a).SingleOrDefault();
    if (toRemove != null){
       ctx.MedicineInventories.Remove(toRemove);
       ctx.SubmitChanges();
    }
    else { .... } // code if toRemove is null

 }