如何获取动态列数据linq

Question

我有以下代码，

public List<MemberDto> GetMembers(out int rowCount,int pageIndex,int pageSize,  string seachColumn = "", string searchTerm = "", string sortBy = "", string sortDiection = "")
{
    var members = (from m in context.Members
                   where (string.IsNullOrEmpty(searchTerm) || m.MemberNumber.Equals(searchTerm))
                         || (string.IsNullOrEmpty(searchTerm) || m.LastName.Equals(searchTerm))
                   select m).AsEnumerable();
                   
    if (!string.IsNullOrEmpty(sortBy))
    {
        PropertyDescriptor prop = TypeDescriptor.GetProperties(typeof(EFModel.ClientData.Member)).Find(sortBy, true);
        members = (sortDiection.ToLower() == "descnding") ? members.OrderByDescending(x => prop.GetValue(x)).ToList() : members.OrderBy(x => prop.GetValue(x)).ToList();
    }

    rowCount =  (!string.IsNullOrEmpty(searchTerm)) ? members.Count() : rowCount = context.Members.Count() ;
    
    members = members.Skip(pageIndex).Take(pageSize).ToList();

    List<MemberDto> memberDtos = new List<MemberDto>();
    mapper.Map(members, memberDtos);
    return memberDtos;
}

在上面的代码中，我的seachColumn值可以是("a","b",or "")。当seachColumn = "a" 时，我需要根据searchTerm 值按列MemberNumber 搜索表数据

当seachColumn = "b" 时，我需要按LastName 列搜索表数据。基于searchTerm值

为了实现这一点，我编写了以下代码。

if(seachBy == "a")
{
    var sa = (from m in context.Members
              where (string.IsNullOrEmpty(searchTerm) || m.MemberNumber.Equals(searchTerm))
              select m).AsEnumerable();
}
else if (seachBy == "b")
{
    var sa = (from m in context.Members
              where (string.IsNullOrEmpty(searchTerm) || m.LastName.Equals(searchTerm))
              select m).AsEnumerable();
}

我知道，我试过的代码有点傻。有什么合适的方法吗？

Answer 1

是的，有更好的方法来做到这一点。首先，您想以IQueryable 的身份完成这一切 - 为什么？ - 因为只要您执行.AsEnumerable() 或.ToList()，查询就会在数据库服务器上执行，并且数据会加载到内存中。

因此，在您的代码中 - 因为您调用了 .AsEnumerable()，它已将所有 context.Members.Where condition is true 加载到内存中：

var members = (from m in context.Members
                   where (string.IsNullOrEmpty(searchTerm) || m.MemberNumber.Equals(searchTerm))
                         || (string.IsNullOrEmpty(searchTerm) || m.LastName.Equals(searchTerm))
                   select m)
.AsEnumerable(); 

您的第二部分代码是分页。我们通常做的是写一个IQueryable 扩展方法。

因此，将以下具有 2 个扩展方法的类添加到一个公共位置。

public static class IQueryableExtensions
{
    public static IQueryable<T> ApplyPagination<T>(this IQueryable<T> source, string sortDirection, string sortBy, int pageNumber, int pageSize)
    {
        var sortDirectionInternal = sortDirection == "asc" ? "OrderBy" : "OrderByDescending";

        var orderBy = sortBy;

        if (pageSize != -1)  // -1 is for All - I don't apply pagination if pageSize == -1.
        {
            return source.OrderBy(orderBy, sortDirectionInternal)
                    .Skip((pageNumber - 1) * pageSize)
                    .Take(pageSize);
        }

        return source.OrderBy(orderBy, sortDirection);
    }

    public static IQueryable<T> OrderBy<T>(this IQueryable<T> source, string ordering, string sortDirection)
    {
        var type = typeof(T);

        // Get Property to Sort By
        var property = type.GetProperty(ordering);

        // If Property is NULL (not found) - Just use the first Property (Default) to ORDER BY - as this will prevent Exception
        if (property == null)
        {
            property = type.GetProperties().First();
        }

        var parameter = Expression.Parameter(type, "p");
        var propertyAccess = Expression.MakeMemberAccess(parameter, property);
        var orderByExp = Expression.Lambda(propertyAccess, parameter);
        var resultExp = Expression.Call(typeof(Queryable), sortDirection, new[] { type, property.PropertyType }, source.Expression, Expression.Quote(orderByExp));
        return source.Provider.CreateQuery<T>(resultExp);
    }
}

所以现在你上面的代码看起来像这样 - 我没有使用 VS 所以可能有一些 sytnax 错误：

// NOTE: you will need to include the namespace for the new IQueryableExtensions class

var members = (from m in context.Members
               where (string.IsNullOrEmpty(searchTerm) || m.MemberNumber.Equals(searchTerm))
                     || (string.IsNullOrEmpty(searchTerm) || m.LastName.Equals(searchTerm))
               select m);

// retrieve count only if you need to of total members that match the above criteria
rowCount = members.Count();

// This is all you need to do! - ApplyPagination(params) - 
members = members.ApplyPagination(sortDiection, sortBy, pageIndex, pageSize);

return mapper.Map<List<MemberDto>>(members);