Genesis 42:7 And Joseph saw his brethren, and he knew them, but made himself strange unto them, and spake roughly unto them; and he said unto them, Whence come ye? And they said, From the land of Canaan to buy food. - Free Bible Online

      7, 8. Joseph saw his brethren, and he knew them, . . . but they knew not him--This is not strange. They were full-grown men--he was but a lad at parting. They were in their usual garb--he was in his official robes. They never dreamt of him as governor of Egypt, while he had been expecting them. They had but one face; he had ten persons to judge by.
      made himself strange unto them, and spake roughly--It would be an injustice to Joseph's character to suppose that this stern manner was prompted by any vindictive feelings--he never indulged any resentment against others who had injured him. But he spoke in the authoritative tone of the governor in order to elicit some much-longed-for information respecting the state of his father's family, as well as to bring his brethren, by their own humiliation and distress, to a sense of the evils they had done to him.

JFB.