LinqDataSource and Could not find a row that matches the given keys in the original values stored in ViewState.

Today I was working on something quickly where I used a LinqDataSource, to keep things simple I attached a simple list view to it and had insert, update and delete operations.

Insert operations seemed to work OK however when I came to do an update to an existing item it crashed out

   Could not find a row that matches the given keys in the original values stored in ViewState.

Initially I thought that maybe some databinding had gone astray but this wasn't the issue. As I hadn't used design view but source view to create the ListView had missed off the DataKeyNames attribute, this meant that upon update the LinqDataSource was trying to match all my values against the database, of course it couldnt do this. By specifying the DataKeyName to match against, usually your Primary Key if using LinqtoSql, not only do you limit how much matching and work your LinqDataSource has to do, but also you'll find you can update :D

So quick tip, if you get "Could not find a row that matches the given keys in the original values stored in ViewState." double check your ListView, FormView, GridView etc has the DataKeyNames set.