如何在HTTP GET请求中添加URL查询参数？

I am trying to add a query parameter to a HTTP GET request but somehow methods pointed out on SO (e.g. here) don't work.

I have the following piece of code:

package main

import (
    "fmt"
    "log"
    "net/http"
)

func main() {
    req, err := http.NewRequest("GET", "/callback", nil)
    req.URL.Query().Add("code", "0xdead 0xbeef")
    req.URL.Query().Set("code", "0xdead 0xbeef")
    // this doesn't help
    //req.URL.RawQuery = req.URL.Query().Encode()

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

    fmt.Printf("URL      %+v
", req.URL)
    fmt.Printf("RawQuery %+v
", req.URL.RawQuery)
    fmt.Printf("Query    %+v
", req.URL.Query())
}

which prints:

URL      /callback
RawQuery 
Query    map[]

Any suggestions on how to achieve this?

Playground example: https://play.golang.org/p/SYN4yNbCmo

Check the docs for req.URL.Query():

Query parses RawQuery and returns the corresponding values.

Since it "parses RawQuery and returns" the values what you get is just a copy of the URL query values, not a "live reference", so modifying that copy does nothing to the original query. In order to modify the original query you must assign to the original RawQuery.

q := req.URL.Query() // Get a copy of the query values.
q.Add("code", "0xdead 0xbeef") // Add a new value to the set.
req.URL.RawQuery = q.Encode() // Encode and assign back to the original query.

// URL      /callback?code=0xdead+0xbeef
// RawQuery code=0xdead+0xbeef
// Query    map[code:[0xdead 0xbeef]]

Note that your original attempt to do so didn't work because it simply parses the query values, encodes them, and assigns them right back to the URL:

req.URL.RawQuery = req.URL.Query().Encode()
// This is basically a noop!