To use CGO in Windows 64bit, you have to install gcc. The easiest way to get gcc which is eligible for CGO is to install MinGW-w64.

The official documentation also refers mingw-w64 as a gcc compiler for CGO.

Windows
In order to use cgo on Windows, you'll also need to first install a gcc compiler (for instance, mingw-w64) and have gcc.exe (etc.) in your PATH environment variable before compiling with cgo will work.

The CGO page of the go official wiki

Download MINGW-W64

Download a MinGW-W64 installer from MinGW-W64-builds.

Install MINGW-W64

Execute the downloaded installer, then change Architecture to x86_64 and Threads to win32.

Add path to mingw64/bin to PATH

Add the path to mingw64/bin to your system PATH environment variable. The path may look like C:\Program Files\mingw-w64\x86_64******\mingw64\bin.

There you go!

This is a tiny go library source.

// main.go
package main

import "fmt"
import "C"

// no space before export!
//export hello
func hello() {
    fmt.Printf("Hello!")
}

// you have to define the main function
func main() {
}

To build:

go build -buildmode=c-shared -o hello.dll

To export a header file:

go tool cgo main.go

It generates c source and header files into the _obj directory.

If you use Visual Studio, you can inspect the dll by dumpbin.exe.

dumpbin /exports hello.dll | grep hello

It'll be like this:

> dumpbin.exe /exports hello.dll | grep hello
Dump of file hello.dll
  Section contains the following exports for hello.dll
         15    E 000A7760 _cgoexp_7cdc2f248ed0_hello
         96   5F 000A77C0 hello
        302  12D 000A76C0 main._cgoexpwrap_7cdc2f248ed0_hello

You can call the function from C#:

using System.Runtime.InteropServices;

namespace Main
{
    class Program
    {
        [DllImport("hello.dll", EntryPoint = "hello")]
        static extern void Hello();

        static void Main(string[] args)
        {
            Hello(); // => Hello!
        }
    }
}