在ASP.NET Core 3.1中实现Datatables服务器端处理的方法如下：

1. 首先，安装所需的NuGet包。使用Package Manager Console或者NuGet包管理器界面，添加以下NuGet包：

   • Microsoft.AspNetCore.Mvc.NewtonsoftJson
   • Microsoft.EntityFrameworkCore
   • Microsoft.EntityFrameworkCore.SqlServer

2. 在Startup.cs文件中，添加以下代码：

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllersWithViews().AddNewtonsoftJson();
    services.AddDbContext(options =>
        options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
}

3. 创建一个新的控制器，如DataController.cs，添加以下代码：

public class DataController : Controller
{
    private readonly ApplicationDbContext _context;

    public DataController(ApplicationDbContext context)
    {
        _context = context;
    }

    [HttpPost]
    public IActionResult LoadData([FromBody] DTParameters param)
    {
        var searchValue = param.Search.Value;
        var sortColumnName = param.Columns[param.Order[0].Column].Data;
        var sortDirection = param.Order[0].Dir;

        var query = _context.YourEntity.AsQueryable();

        // 搜索过滤
        if (!string.IsNullOrEmpty(searchValue))
        {
            query = query.Where(x => x.Column1.Contains(searchValue) || x.Column2.Contains(searchValue));
        }

        // 排序
        switch (sortColumnName)
        {
            case "Column1":
                query = (sortDirection == "asc") ? query.OrderBy(x => x.Column1) : query.OrderByDescending(x => x.Column1);
                break;
            case "Column2":
                query = (sortDirection == "asc") ? query.OrderBy(x => x.Column2) : query.OrderByDescending(x => x.Column2);
                break;
            // 添加其他列的排序逻辑
        }

        // 分页
        var result = new DTResult
        {
            draw = param.Draw,
            recordsTotal = query.Count(),
            recordsFiltered = query.Count(),
            data = query.Skip(param.Start).Take(param.Length).ToList()
        };

        return Json(result);
    }
}

4. 创建一个名为DTParameters的类，用于解析Datatables发送的参数：

public class DTParameters
{
    public int Draw { get; set; }
    public int Start { get; set; }
    public int Length { get; set; }
    public DTColumn[] Columns { get; set; }
    public DTOrder[] Order { get; set; }
    public DTSearch Search { get; set; }
}

public class DTColumn
{
    public string Data { get; set; }
    public string Name { get; set; }
    public bool Searchable { get; set; }
    public bool Orderable { get; set; }
    public DTSearch Search { get; set; }
}

public class DTOrder
{
    public int Column { get; set; }
    public string Dir { get; set; }
}

public class DTSearch
{
    public string Value { get; set; }
    public bool Regex { get; set; }
}

5. 创建一个名为DTResult的类，用于返回Datatables所需的结果：

public class DTResult
{
    public int draw { get; set; }
    public int recordsTotal { get; set; }
    public int recordsFiltered { get; set; }
    public List data { get; set; }
}

6. 在前端页面中，使用Datatables的ajax选项，将请求发送到DataController的LoadData动作：

$(document).ready(function() {
    $('#dataTable').DataTable({
        "processing": true,
        "serverSide": true,
        "ajax": {
            "url": "/Data/LoadData",
            "type": "POST"
        },
        "columns": [
            { "data": "column1" },
            { "data": "column2" },
            // 添加其他列的配置
        ]
    });
});

注意：上述代码示例假定您已经创建了一个名为YourEntity的实体类，其中包含Column1和Column2等属性。请根据您的实际需求进行相应的修改和调整。