LINQ Object Referance not set - linq

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
}

Answer 2

Is a null at any point?
toRemove = (from a in ctx.MedicineInventories where (a != null && a.MedicineInventoryId == Mid) select a).Single();

Answer 3

I think your problem is happening because you're creating a new MedicineInventory.
Replace this:
MedicineInventory toRemove = new MedicineInventory();
With this:
var toRemove = ctx.MedicineInventories.Single(mi => mi.MedicineInventoryId == Mid);
Update:
When it retuns the error message "Sequence contains no elements" it's because EF could not find a row in the database with the same Guid you're using in the where clause. In this case and to avoid an exception, you can try this line of code:
var toRemove = ctx.MedicineInventories.SingleOrDefault(
mi => mi.MedicineInventoryId == Mid);
then use an if to delete if it's not NULL:
if(toRemove != null)
{
ctx.MedicineInventories.Remove(toRemove);
ctx.SubmitChanges();
}
else
{
// Only you know what to do! :-)
}
SingleOrDefault returns the only element of a sequence, or a default
value (NULL in this case) if the sequence is empty; this method
throws an exception if there is more than one element in the sequence.
Note: the way you're comparing the Guids is correct because == is overloaded on Guid so you don't need to compare the string representations.
See http://msdn.microsoft.com/en-us/library/system.guid.op_equality%28v=vs.110%29.aspx#Y474