实体框架核心3动态顺序不起作用

public class Branch
{
[Sortable(OrderBy = "BranchId")]
public long BranchId { get; set; }
public string Name { get; set; }
public string Code { get; set; }
public string Type { get; set; }
}


这是我的Model类，我还创建了一个自定义属性

public class SortableAttribute : Attribute
{
public string OrderBy { get; set; }
}


现在我用orderby降序创建分页，但是此代码不起作用

public static async Task> ToPagedListAsync(this IQueryable source,GeneralPagingRequest pagingRequest,int indexFrom = 0,CancellationToken cancellatiOnToken= default(CancellationToken))
{
if (indexFrom > pagingRequest.PageNumber)
{
throw new ArgumentException(
$"indexFrom: {indexFrom} > pageNumber: {pagingRequest.PageNumber},must indexFrom <= pageNumber");
}
var count = await source.CountAsync(cancellationToken).ConfigureAwait(false);
var items = source.Skip(((pagingRequest.PageNumber - 1) - indexFrom) * pagingRequest.PageSize)
.Take(pagingRequest.PageSize);
var props = typeof(T).GetProperties();
PropertyInfo orderByProperty;
orderByProperty =
props.FirstOrDefault(x=>x.getcustomAttributes(typeof(SortableAttribute),true).Length != 0);
if (pagingRequest.OrderBy == "desc")
{
items = items.OrderBy(x => orderByProperty.Getvalue(x));
}
else
{
items = items.OrderBy(x => orderByProperty.Getvalue(x));
}
var result = await items.ToListAsync(cancellationToken).ConfigureAwait(false);
var pagedList = new PagedList
{
PageNumber = pagingRequest.PageNumber,PageSize = pagingRequest.PageSize,IndexFrom = indexFrom,TotalCount = count,Items = result,TotalPages = (int) Math.Ceiling(count / (double) pagingRequest.PageSize)
};
return pagedList;
}


但结果变量创建异常

.OrderBy()需要一个委托，该委托将告诉它 HOW （选择），而不是键值本身。因此，您正在这里查看一些元编程。

自然地，您将看到构建动态的LINQ表达式树，该树将为您获取属性：

// your code up above
PropertyInfo orderByProperty = props.FirstOrDefault(x => x.GetCustomAttributes(typeof(SortableAttribute),true).Length != 0);
var p = Expression.Parameter(typeof(T),"x"); // you define your delegate parameter here
var accessor = Expression.Property(p,orderByProperty.GetMethod); // this basically becomes your `x => x.BranchId` construct
var predicate = Expression.Lambda(accessor,p).Compile(); // here's our problem: as we don't know resulting type at compile time we can't go `Expression.Lambda(accessor,p)` here
if (pagingRequest.OrderBy == "desc")
{
items = items.OrderByDescending(x => predicate(x)); // passing a Delegate here will not work as OrderBy requires Func
}
else
{
items = items.OrderBy(x => predicate(x)); // passing a Delegate here will not work as OrderBy requires Func
}
var result = await items.ToListAsync(cancellationToken).ConfigureAwait(false);
// your code down below


上面的代码存在问题-您不预先知道TKey。因此，我们将不得不更深入，并动态地构建整个items.OrderBy(x => x.BranchId)表达式。信念的最大飞跃将是OrderBy是一种扩展方法，它实际上驻留在IQueryable类型上。在获得通用方法参考之后，您将需要在知道属性类型时构建特定的委托类型。这样您的方法就变成这样：

public static class ExtToPagedListAsync
{
private static readonly MethodInfo OrderByMethod = typeof(Queryable).GetMethods().Single(method => method.Name == "OrderBy" && method.GetParameters().Length == 2); // you need your method reference,might as well find it once
private static readonly MethodInfo OrderByDescendingMethod = typeof(Queryable).GetMethods().Single(method => method.Name == "OrderByDescending" && method.GetParameters().Length == 2); // you need your method reference,might as well find it once
public static async Task> ToPagedListAsync(this IQueryable source,GeneralPagingRequest pagingRequest,int indexFrom = 0,CancellationToken cancellatiOnToken= default(CancellationToken))
{
if (indexFrom > pagingRequest.PageNumber)
{
throw new ArgumentException(
$"indexFrom: {indexFrom} > pageNumber: {pagingRequest.PageNumber},must indexFrom <= pageNumber");
}
var count = await source.CountAsync(cancellationToken).ConfigureAwait(false);
var items = source.Skip(((pagingRequest.PageNumber - 1) - indexFrom) * pagingRequest.PageSize)
.Take(pagingRequest.PageSize);
var props = typeof(T).GetProperties();
PropertyInfo orderByProperty = props.FirstOrDefault(x => x.GetCustomAttributes(typeof(SortableAttribute),true).Length != 0);
var p = Expression.Parameter(typeof(T),"x");
var accessor = Expression.Property(p,orderByProperty.GetMethod);
var predicate = Expression.Lambda(accessor,p); // notice,we're not yet compiling the predicate. we still want an Expression here
// grab the correct method depending on your condition
MethodInfo genericMethod = (pagingRequest.OrderBy == "desc") ? OrderByDescendingMethod.MakeGenericMethod(typeof(T),orderByProperty.PropertyType)
:OrderByMethod.MakeGenericMethod(typeof(T),orderByProperty.PropertyType);
object ret = genericMethod.Invoke(null,new object[] { items,predicate });
items = (IQueryable)ret; // finally cast it back to Queryable with your known T
var result = await items.ToListAsync(cancellationToken).ConfigureAwait(false);
var pagedList = new PagedList
{
PageNumber = pagingRequest.PageNumber,PageSize = pagingRequest.PageSize,IndexFrom = indexFrom,TotalCount = count,Items = result,TotalPages = (int)Math.Ceiling(count / (double)pagingRequest.PageSize)
};
return pagedList;
}
}


我必须透露我确实从this answer here得到了一些启发，因此请检查一下以供进一步阅读。