os.FileInfo mock not matching

Question

in one of my golang Projects i went to mock the os.FileInfo for testcases.

I am not sure if I understand the interface handling of golang correctly. As far as I know the following piece of code should work, but I get an Compilererror saying that the interface does not match.

I modified this go-doc example a bit in case you want to test it out yourself.

package main

import (
    "fmt"
    "io/ioutil"
    "log"
)

type file interface{
    Name() string
} 

func readFiles() []file{
    files, err := ioutil.ReadDir(".")

    if err != nil {
        log.Fatal(err)
    }

    return files
}

func main() {
    files := readFiles()

    for _, file := range files {
        fmt.Println(file.Name())
    }
}

Following at the golang doc, the ioutil.ReadDir(".") should Return a slice of os.FileInfo which should be a specialisation of my selfwritten file interface.

Can anyone help me out of this hell of misconceptions and entanglements, please?

Thank you very much guys!

That's what https://golang.org/doc/faq#convert_slice_of_interface talks about. The problem is the slice and not the interface. — Volker, May 03 '17 at 21:08

David S · Answer 1 · 2017-05-03T22:15:41.870

0

os.FileInfo is interface and it can converted to file. But you try to convert []os.FileInfo to []file. Slice is not interface, and can not be asserted.

edited May 03 '17 at 22:15

answered May 03 '17 at 22:10

David S

19
4

score 0 · Answer 2 · answered Feb 19 '20 at 13:48

To elaborate on the other answer and comments, os.FileInfo does fulfill the file interface you've defined, but a slice of os.FileInfo does not implicitly convert to a slice of your file interface. A simple fix would be to have your readFiles do the conversion explicitly, e.g.

func readFiles() []file{
    fileInfos, err := ioutil.ReadDir(".")

    if err != nil {
        log.Fatal(err)
    }

    files := make([]file, len(fileInfos))
    for i, fi := range fileInfos {
        files[i] = fi
    }

    return files
}

os.FileInfo mock not matching

2 Answers2