LINQ to Entities调用外部appendstr方法失败的解决方案求助

我明白你遇到的问题了——LINQ to Entities确实无法识别自定义的外部方法调用，因为它需要把整个查询表达式转换成数据库能执行的SQL语句，而像appendstr这样的自定义方法没办法被EF解析成对应的SQL操作。下面给你几个可行的解决方案，按推荐程度排序：

方案一：将PostedDate改为DateTime类型（强烈推荐） #

首先，把数据库中存储为字符串的PostedDate改成DateTime类型是最合理的做法——日期用字符串存储不仅容易出错，还会导致查询效率低下。修改后，你可以直接用日期范围比较，完全避免字符串处理的麻烦：

[HttpGet]
public ActionResult SearchResult(int? page, string searchTitle = null, string searchLocation = null, string last24 = "")
{
    ViewBag.searchTitle = searchTitle;
    ViewBag.searchLocation = searchLocation;
    ViewBag.page = page;
    ViewBag.last24 = last24;
    setUpApi(searchTitle, searchLocation);
    var result = new List<AllJobModel>().AsQueryable();
    if (!string.IsNullOrEmpty(searchTitle) || !string.IsNullOrEmpty(searchLocation) || !string.IsNullOrEmpty(last24))
    {
        setUpApi(searchTitle, searchLocation);
        DateTime now = DateTime.Now;
        DateTime cutoffDate = now.AddHours(-24);
        // 直接比较日期范围，EF能完美转换为SQL
        result = db.AllJobModel.Where(a => 
            a.JobTitle.Contains(searchTitle) && 
            a.locationName.Contains(searchLocation) &&
            a.PostedDate >= cutoffDate && a.PostedDate <= now);
    }
    else
    {
        result = from app in db.AllJobModel select app;
    }
    return View(result.ToList().ToPagedList(page ?? 1, 5));
}

方案二：用EF支持的字符串操作模拟appendstr逻辑 #

如果因为某些限制无法修改数据库字段类型，你需要把appendstr的逻辑用EF能识别的内置字符串方法重写（这些方法可以被转换成对应的SQL函数）。

假设你的PostedDate格式是类似"Mon 05 19 2024"（前缀+月+日+年），你想要提取出MM-dd-yyyy格式的字符串和目标日期对比，那么可以这样写：

[HttpGet]
public ActionResult SearchResult(int? page, string searchTitle = null, string searchLocation = null, string last24 = "")
{
    ViewBag.searchTitle = searchTitle;
    ViewBag.searchLocation = searchLocation;
    ViewBag.page = page;
    ViewBag.last24 = last24;
    setUpApi(searchTitle, searchLocation);
    var result = new List<AllJobModel>().AsQueryable();
    if (!string.IsNullOrEmpty(searchTitle) || !string.IsNullOrEmpty(searchLocation) || !string.IsNullOrEmpty(last24))
    {
        setUpApi(searchTitle, searchLocation);
        DateTime now = DateTime.Now;
        string targetDate = now.AddHours(-24).ToString("MM-dd-yyyy");
        // 用EF支持的Substring、IndexOf模拟appendstr的逻辑
        result = db.AllJobModel.Where(a => 
            a.JobTitle.Contains(searchTitle) && 
            a.locationName.Contains(searchLocation) &&
            string.Concat(
                a.PostedDate.Substring(a.PostedDate.IndexOf(' ') + 1, 2), "-",
                a.PostedDate.Substring(a.PostedDate.IndexOf(' ') + 4, 2), "-",
                a.PostedDate.Substring(a.PostedDate.IndexOf(' ') + 7, 4)
            ) == targetDate);
    }
    else
    {
        result = from app in db.AllJobModel select app;
    }
    return View(result.ToList().ToPagedList(page ?? 1, 5));
}

注意：你需要根据PostedDate的实际格式调整Substring的参数，确保能正确提取月、日、年部分。如果使用EF Core，还可以借助EF.Functions提供的更多字符串操作方法。

方案三：先加载数据到内存再过滤（仅小数据量可用） #

如果你的数据量很小，也可以先把符合标题和地点条件的数据加载到内存，再用appendstr方法处理过滤。但这种方法会把部分数据加载到内存，大数据量时性能极差，不推荐：

[HttpGet]
public ActionResult SearchResult(int? page, string searchTitle = null, string searchLocation = null, string last24 = "")
{
    ViewBag.searchTitle = searchTitle;
    ViewBag.searchLocation = searchLocation;
    ViewBag.page = page;
    ViewBag.last24 = last24;
    setUpApi(searchTitle, searchLocation);
    var result = new List<AllJobModel>().AsQueryable();
    if (!string.IsNullOrEmpty(searchTitle) || !string.IsNullOrEmpty(searchLocation) || !string.IsNullOrEmpty(last24))
    {
        setUpApi(searchTitle, searchLocation);
        DateTime now = DateTime.Now;
        string targetDate = now.AddHours(-24).ToString("MM-dd-yyyy");
        // 先调用AsEnumerable()把数据加载到内存，再用自定义方法过滤
        result = db.AllJobModel.Where(a => 
            a.JobTitle.Contains(searchTitle) && 
            a.locationName.Contains(searchLocation))
            .AsEnumerable()
            .Where(a => appendstr(a.PostedDate).Equals(targetDate))
            .AsQueryable();
    }
    else
    {
        result = from app in db.AllJobModel select app;
    }
    return View(result.ToList().ToPagedList(page ?? 1, 5));
}

内容的提问来源于stack exchange，提问作者moe