Di necessary before audio interface?

[Sixfootdig]

I have heard conflict information

[BertKoor]

To connect what exactly?

If you have the option: try with and without, then draw your conclusion.
They need to be pretty expensive. A cheap DI is worse than no DI.

[CapnLockheed]

It also sometimes depends on what your input source is. Things like ribbon mics and basses with active pups can really benefit from a GOOD DI.

A cheap DI is worse than no DI.

Agree 100%. The same can be said of preamps.

[Tj Shredder]

Four factors: Is the source balanced?, level of the source, impedance of the source, length of the cable.
If its a synth or a preamp, the impedance matches already, and the level is set. If the output isn’t balanced a DI will help for long cables, if its balanced all is set already. Low level signals like those from mics should always be balanced for longer distances…

[bungle]

They need to be pretty expensive. A cheap DI is worse than no DI.

Typical uninformed nonsense, blankets statements like that are ridiculous, cost does not always equate to quality.
"A bad quality DI is worse than a clean signal with no DI"

[justin3am]

Four factors: Is the source balanced?, level of the source, impedance of the source, length of the cable.
If its a synth or a preamp, the impedance matches already, and the level is set. If the output isn’t balanced a DI will help for long cables, if its balanced all is set already. Low level signals like those from mics should always be balanced for longer distances…

This is a good response.
It depends on what you are connecting to the interface and also what type of inputs the interface has. If you are connecting a guitar and your interface has a 'hi-z' or 'Instrument' input, then you most likely won't need a DI. If your interface only has a mic input, then you will probably want an outboard DI for a guitar. If your interface only has line inputs, you might want an active DI or a preamp with a hi-z input to connect a guitar.

For synths, going straight into a line input is probably the most straight forward option but then again, I do run a few of my synths through active DIs, because the preamps and transformers inside impart some character.

[Tj Shredder]

And DI isn’t DI. If its passive, it needs to have very good transformers which have some nonlinearities and alter the signal (you may call it character). If its active, its probably pretty clean though you could still clip it…