The IDbCommandInterceptor (sorry, no official documentation) interface was introduced in Entity Framework 6 as part of the new interception and logging API, and allows the interception of the SQL and its parameters that are sent to the database as the result of DbContext CRUD operations.
It offers two methods for each of the basic ADO.NET operations, those defined in DbCommand, one called before the operation is executed, and the other called afterwards:
ExecuteNonQuery (UPDATEs, INSERTs, DELETEs) –> NonQueryExecuting, NonQueryExecuted;
ExecuteScalar (SELECTs returning a single value) –> ScalarExecuting, ScalarExecuted;
ExecuteReader (SELECTs returning rows) –> ReaderExecuting, ReaderExecuted.
As usual, the methods ending with “ing” are executed before and those ending with “ed” are executed afterwards, always synchronously.
One might naively assume that INSERTs would always trigger a NonQueryExecuting/NonQueryExecuted call, and indeed it is so, unless we have an IDENTITY primary key, in which case, Entity Framework will instead call ReaderExecuting/ReaderExecuted. It’s easy to understand why: when we use an IDENTITY, we need to retrieve the generated value immediately after the INSERT, hence Entity Framework will generate code like:
1: INSERT INTO [dbo].[SomeTable] (...)
2: VALUES (...)
3: SELECT [Id]
4: FROM [dbo].[SomeTable]
5: WHERE @@ROWCOUNT > 0 AND [Id] = SCOPE_IDENTITY()
The INSERT and SELECT are combined in the same command, which justifies the choice of ReaderExecuting/ReaderExecuted. Because in Entity Framework we always use IDENTITY – don’t we? –, it’s ReaderExecuting/ReaderExecuted that you should be implementing if you want to change the INSERT SQL or its parameters.