Update requires a valid UpdateCommand when passed DataRow collection with modified rows

This message will also be displayed caused when you do not have a primary key defined on the table you are updating.

I ran into the same problem as Sam. I had working code that just suddenly was no longer working. I didn't know when I wrote it, but it must have been automatically inferring the update command, and then stopped doing it. Perhaps a service pack from MS in between versions that we never noticed. Anyway, the solution I came across is using a (in my case for oracle) a OracleCommandBuilder which takes the DataAdapter (after calling fill) as a parameter to the constructor and then calling GetUpdateCommand() and assigning that to the UpdateCommand on the DataAdapter.

pseudocode:

DataAdapter da = new DataAdapter(...)
...
da.Fill();
da.UpdateCommand = new OracleCommandBuilder(da).GetUpdateCommand();
...
da.Update();

The error is quite literal: The Adapter needs a valid SQL Update statement. Dataset designers and CommandBuilders will generate these for you, but there is nothing wrong with hand-crafting a bit of SQL either.

Anyway, you'll have to verify (debugger) that the Update statement is still configured and what it actually is. It could be more of a SQL than a C# problem.

Edit: the Command Builder tools will only handle straight, single table, Select statements. Use a Join or anything fancy and you're on your own.