Thank you for the contribution. Since there already multiple tests that verifies that _dbConnectionFactory works when called correctly, then I think only these tests are interesting:

• DbConnectionFactoryNullTest - Might not compile if enabling nullable for the unit-test-project.
• DbConnectionFactoryReturnsNullTest - Might not compile if enabling nullable for the unit-test-project.
• DbConnectionFactoryThrowsTest - This is a good test.
• DbConnectionFactoryWithParametersWriteTest - Like this test as it combines both dbConnection-delegate and dbParameter-delegate.

I think these tests are already covered by existing test:

• DbConnectionFactoryKeepConnectionTest
• DbConnectionFactoryBatchedWriteTest
• DbConnectionFactorySimpleWriteTest

The 7 tests cover the DatabaseTarget(Func<IDbConnection>) constructor added in NLog/NLog#5878. Before this PR the user-provided factory variant has no direct tests; the existing suite covers the connection-string constructor only.

The 4 you want to keep cover defensive cases (null factory, factory-returns-null, factory-throws) and parameters-write. The 3 you want to drop (Simple, Batched, KeepConnection) give the factory constructor parity with the write-mode coverage the existing tests provide for the connection-string constructor.

Architecturally both constructors set _dbConnectionFactory and the single OpenConnection method invokes that field for every write (single and batched, KeepConnection on or off). There is no downstream branching by constructor. Under that invariant the contested 3 are redundant; they serve as regression coverage at the public API in case a future refactor introduces divergence. Will drop if you'd rather rely on the architecture invariant alone.

I don't see a need to future proof the unit-tests to handle any possible combination of code-changes in the future.

Hopefully any future changes to the code will include their own unit-tests. The goal for me is to verify that input-delegate is called when opening database-connection.

Agreed. Narrowing to the 4 you listed. Will drop the other 3 and push an update shortly.

Closing and opening pull-request to tickle the build-server

Thank you for the nice contribution, and making the last adjustments. Much appreciated.