Use a library compiled in Go in c# (.net)

Program.cs

using System;
using System.Runtime.InteropServices;
using System.Text;

namespace GoSharedDLL
{
    class Program
    {
        [DllImport("shared.dll", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.StdCall)]
        private static extern IntPtr ReturnReversedString(byte[] input);

        static void Main(string[] args)
        {
            var input = Encoding.UTF8.GetBytes("Hello World!");
            var resultbytes = ReturnReversedString(input);
            var output = Marshal.PtrToStringAnsi(resultbytes);
            Console.WriteLine(output);
            Console.WriteLine("Press enter to continue...");
            Console.ReadLine();
        }
    }
}

shared.go

package main

import "C"

//export ReturnReversedString
func ReturnReversedString(input *C.char) *C.char {
	str := C.GoString(input)
	runes := []rune(str)
	for i, j := 0, len(runes)-1; i < j; i, j = i+1, j-1 {
		runes[i], runes[j] = runes[j], runes[i]
	}
	retval := string(runes)
	return C.CString(retval)
}

func main() {}

// compile using go build -o shared.dll -buildmode=c-shared

You should mark exporting method with comment above
//export [Some name]
No space after //