The SqlNotificationSource enumeration is referenced by an instance of the System.Data.SqlClient.SqlNotificationEventArgs class.
Query notifications are supported only for SELECT statements that meet a list of specific requirements. The following table provides links to the Service Broker and Query Notifications documentation in SQL Server Books Online.
SQL Server Books Online
A client-initiated notification occurred, such as a client-side time-out or as a result of attempting to add a command to a dependency that has already fired.
Data has changed; for example, an insert, update, delete, or truncate operation occurred.
The database state changed; for example, the database related to the query was dropped or detached.
The run-time environment was not compatible with notifications; for example, the isolation level was set to snapshot, or one or more SET options are not compatible.
A run-time error occurred during execution.
A database object changed; for example, an underlying object related to the query was dropped or modified.
Internal only; not intended to be used in your code.
The Transact-SQL statement is not valid for notifications; for example, a SELECT statement that could not be notified or a non-SELECT statement was executed.
A system-related event occurred. For example, there was an internal error, the server was restarted, or resource pressure caused the invalidation.
The subscription time-out expired.
Used when the source option sent by the server was not recognized by the client.