问题描述

public static IQueryable<TResult> ApplySortFilter<T, TResult>(this IQueryable<T> query, string columnName)
  where T : EntityObject
{
  var param = Expression.Parameter(typeof(T), "o");
  var body = Expression.PropertyOrField(param,columnName);

  var sortExpression = Expression.Lambda(body, param);
  return query.OrderBy(sortExpression);
}

因为 OrderBy 的类型不是从 sortExpression 推断出来的，所以我需要在运行时指定类似这样的内容:

Because the type for OrderBy is not inferred from sortExpression I need to specify it something like this at run time:

var sortExpression = Expression.Lambda<T, TSortColumn>(body, param);

或者

return query.OrderBy<T, TSortColumn>(sortExpression);

但我认为这是不可能的，因为 TSortColumn 只能在运行时确定.

I don't think this is possible however as TSortColumn can only be determined during runtime.

有没有办法解决这个问题?

Is there a way around this?

推荐答案

我们在一个 LINQ to SQL 项目中做了类似的事情(不是 100% 相同，而是类似).代码如下:

We did something similar (not 100% the same, but similar) in a LINQ to SQL project. Here's the code:

public static IQueryable<T> OrderBy<T>(this IQueryable<T> source, string ordering, params object[] values) {
    var type = typeof(T);
    var property = type.GetProperty(ordering);
    var parameter = Expression.Parameter(type, "p");
    var propertyAccess = Expression.MakeMemberAccess(parameter, property);
    var orderByExp = Expression.Lambda(propertyAccess, parameter);
    MethodCallExpression resultExp = Expression.Call(typeof(Queryable), "OrderBy", new Type[] { type, property.PropertyType }, source.Expression, Expression.Quote(orderByExp));
    return source.Provider.CreateQuery<T>(resultExp);
}

我们实际上并没有使用泛型，我们有一个已知的类，但它应该适用于泛型(我已将泛型占位符放在它应该在的位置).

We didn't actually use a generic, we had a known class, but it should work on a generic (I've put the generic placeholder where it should be).

对于降序，传入 OrderByDescending 而不是OrderBy":

For descending order, pass in OrderByDescending instead of "OrderBy":

MethodCallExpression resultExp = Expression.Call(typeof(Queryable), "OrderByDescending", new Type[] { type, property.PropertyType }, source.Expression, Expression.Quote(orderByExp));

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