Go: Property exists but Go compiler says it doesn't?

filelogger.go

package logger

import (
	"io"
)

type FileLogger struct{
	File io.Writer
}

func NewFileLogger(file io.Writer) *FileLogger{
	return &FileLogger{file}
}

func (this *FileLogger) Log(message string) error {
	_, err := this.File.Write([]byte(appendNewLine(message)))

	return err
}

filelogger_test.go:

package logger

import (
	"testing"

	"github.com/stretchr/testify/assert"
)

type WriterMock struct{
	data []byte
}

func (this WriterMock) Write(b []byte) (n int, err error) {
	this.data = append(this.data, b ...)

	return len(this.data), nil
}

func NewMockedFileLogger() *FileLogger{
	writer := WriterMock{}

	fileLogger := FileLogger{writer}

	return &fileLogger
}

func TestLog(t *testing.T) {
	fileLogger := NewMockedFileLogger()

	fileLogger.Log("Hello World!")

	assert.Equal(t, "Hello World!", string(fileLogger.File.data))
}

My problem:

I'm getting this error message when running go test:

fileLogger.File.data undefined (type io.Writer has no field or method data)

file.Logger.File is indeed of type io.Writer, but this field data exists, I know Go is a strongly typed language, that's why it's not accepting this.

How can solve this?

The writer File in a FileLogger is an interface (io.Writer), not a struct.

You would need a type assertion in order to access data of WriterMock:

fileLooger.File.(*WriterMock).data

(Note: that would fail if File was not a *WriterMock: more on that below)

See this simplified example:

package main

import "fmt"
import "io"

type WriterMock struct {
	data []byte
}

func (this WriterMock) Write(b []byte) (n int, err error) {
	this.data = append(this.data, b...)

	return len(this.data), nil
}

func main() {
	var w io.Writer = &WriterMock{}
	fmt.Printf("Hello, playground '%+v'", w.(*WriterMock).data)
}

Output:

Hello, playground '[]'

Since a type asertion car error, and you should always check the error, consider the section "Interface conversions and type assertions":

> But if it turns out that the value does not contain a string, the program will crash with a run-time error.
To guard against that, use the "comma, ok" idiom to test, safely, whether the value is a string:

str, ok := value.(string)
if ok {
    fmt.Printf("string value is: %q\n", str)
} else {
    fmt.Printf("value is not a string\n")
}