OpenFileItem.cs: win32api所需结构体
StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto) 不可少，指定非托管对象必须

using System;
using System.Runtime.InteropServices;

namespace Assets.Editor.rayfalling {
    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
    public class OpenFileItem {
        public int structSize = 0;
        public IntPtr dlgOwner = IntPtr.Zero;
        public IntPtr instance = IntPtr.Zero;
        public String filter = null;
        public String customFilter = null;
        public int maxCustFilter = 0;
        public int filterIndex = 0;
        public String file = null;
        public int maxFile = 0;
        public String fileTitle = null;
        public int maxFileTitle = 0;
        public String initialDir = null;
        public String title = null;
        public int flags = 0;
        public short fileOffset = 0;
        public short fileExtension = 0;
        public String defExt = null;
        public IntPtr custData = IntPtr.Zero;
        public IntPtr hook = IntPtr.Zero;
        public String templateName = null;
        public IntPtr reservedPtr = IntPtr.Zero;
        public int reservedInt = 0;
        public int flagsEx = 0;
    }
}

OpenFileDialog.cs: 调用系统dll

using System.Runtime.InteropServices;

namespace Assets.Editor.rayfalling {
    public class OpenFileDialog {
        //链接指定系统函数       打开文件对话框
        [DllImport("Comdlg32.dll", SetLastError = true, ThrowOnUnmappableChar = true, CharSet = CharSet.Auto)]
        public static extern bool GetOpenFileName([In, Out] OpenFileItem ofn);

        //链接指定系统函数        另存为对话框
        [DllImport("Comdlg32.dll", SetLastError = true, ThrowOnUnmappableChar = true, CharSet = CharSet.Auto)]
        public static extern bool GetSaveFileName([In, Out] OpenFileItem ofn);
    }
}

测试代码：

using System.Runtime.InteropServices;
using Assets.Editor.rayfalling;
using UnityEditor;
using UnityEngine;
using OpenFileDialog = Assets.Editor.rayfalling.OpenFileDialog;

namespace Assets.Editor {
    public class ExternalDownload : MonoBehaviour {
        [UnityEditor.MenuItem("External Download/Import")]
        static void Import() {
            var openFileName = new OpenFileItem();
            openFileName.structSize = Marshal.SizeOf(openFileName);
            openFileName.filter = "unitypackage(*.unitypackage)\0*.*|\0*.*";
            openFileName.file = new string(new char[256]);
            openFileName.maxFile = openFileName.file.Length;
            openFileName.fileTitle = new string(new char[64]);
            openFileName.maxFileTitle = openFileName.fileTitle.Length;
            openFileName.initialDir = UnityEngine.Application.streamingAssetsPath.Replace('/', '\\'); //默认路径
            openFileName.title = "请选择路径或者文件";
            openFileName.flags = 0x00080000 | 0x00001000 | 0x00000800 | 0x00000008;

            if (!OpenFileDialog.GetSaveFileName(openFileName)) return;
            var fileName = openFileName.file;
            AssetDatabase.ImportPackage(packagePath: fileName, false);
        }
    }
}

参考来源: 简书