.net项目多个目标架构

refs:

1）https://docs.microsoft.com/en-us/dotnet/core/tutorials/libraries

2）https://docs.microsoft.com/en-us/dotnet/core/tutorials/libraries#how-to-multitarget

3）https://weblog.west-wind.com/posts/2017/Sep/18/Conditional-TargetFrameworks-for-MultiTargeted-NET-SDK-Projects-on-CrossPlatform-Builds

4）https://stackoverflow.com/questions/42747977/how-do-you-multi-target-a-net-core-class-library-with-csproj

需要手工编辑项目文件。

注意点:

1.项目编辑中

TargetFramework  ==> TargetFrameworks

各平台的简写见上面引用文档，可以根据架构不同做预编译处理：
using System;
using System.Text.RegularExpressions;
#if NET40
// This only compiles for the .NET Framework 4 targets
using System.Net;
#else
 // This compiles for all other targets
using System.Net.Http;
using System.Threading.Tasks;
#endif

namespace MultitargetLib
{
    public class Library
    {
#if NET40
        private readonly WebClient _client = new WebClient();
        private readonly object _locker = new object();
#else
        private readonly HttpClient _client = new HttpClient();
#endif

#if NET40
        // .NET Framework 4.0 does not have async/await
        public string GetDotNetCount()
        {
            string url = "https://www.dotnetfoundation.org/";

            var uri = new Uri(url);

            string result = "";

            // Lock here to provide thread-safety.
            lock(_locker)
            {
                result = _client.DownloadString(uri);
            }

            int dotNetCount = Regex.Matches(result, ".NET").Count;

            return $"Dotnet Foundation mentions .NET {dotNetCount} times!";
        }
#else
        // .NET 4.5+ can use async/await!
        public async Task<string> GetDotNetCountAsync()
        {
            string url = "https://www.dotnetfoundation.org/";

            // HttpClient is thread-safe, so no need to explicitly lock here
            var result = await _client.GetStringAsync(url);

            int dotNetCount = Regex.Matches(result, ".NET").Count;

            return $"dotnetfoundation.org mentions .NET {dotNetCount} times in its HTML!";
        }
#endif
    }
}