Jamieson, Fausset and Brown on Isaiah 45:19

19. not ... secret--not like the heathen oracles which gave their responses from dark caverns, with studied obscurity (Is 48:16). Christ plainly quotes these words, thereby identifying Himself with Jehovah (Joh 18:20).

I said not ... Seek ... in vain--When I commanded you to seek Me (Jehovah did so, Is 45:11, "Ask Me," &c.), it was not in order that ye might be sent empty away (De 32:47). Especially in Israel's time of trial, God's interposition, in behalf of Zion hereafter, is expressly stated as about to be the answer to prayer (Is 62:6, 7-10; Psa 102:13-17, 19-21). So in the case of all believers, the spiritual Israel.

righteousness--that which is veracious: not in the equivocal terms of heathen responses, fitly symbolized by the "dark places" from which they were uttered.

right--true (see on Is 41:26).