[英]Why there is no SelectToArray method in System.Linq.Enumerable
[英]Invoking OrderBy (System.Linq.Enumerable) with Reflection
我正在使用開源系統,並且已被要求根據術語的相關性對一組實體進行排序。
首先需要對集合進行精確匹配,然后再對“ startswith”進行匹配,然后對“ contains”進行匹配。
我想出的lambda表達式是:
var results = products.OrderBy(p => p.Name.StartsWith(searchTerm) ? Convert.ToString((char)0) : p.Name);
當前,預編寫系統的工作方式是,它使用帶有反射的lambda表達式的實例調用OrderBy方法,以便對結果進行排序。
為了盡量減少在此任務上花費的時間,我希望遵守此約定。 我試圖創建一個lambda表達式的實例,並將其作為參數傳遞給MethodInfo.Invoke(),但由於其中包含變量,因此會引發異常。
該系統的sort方法的代碼為:
private IOrderedQueryable<ReferenceProduct> OrderProducts(IQueryable<ReferenceProduct> filteredProducts, String sortExpression, String sortDirection, out String checkedSortExpression, out String checkedSortedDescending)
{
ProductListManager.SortExpression correspondingSortExpression = this.SortExpressions.OrderByDescending(se => String.Equals(se.Code, sortExpression, StringComparison.OrdinalIgnoreCase))
.DefaultIfEmpty(new SortExpression<Guid> { Code = String.Empty, Expression = rp => rp.ProductId })
.FirstOrDefault();
checkedSortExpression = correspondingSortExpression.Code;
checkedSortedDescending = String.Equals("desc", sortDirection, StringComparison.OrdinalIgnoreCase) ? "desc" : "asc";
MethodInfo orderMethod = (checkedSortedDescending.Equals("desc", StringComparison.OrdinalIgnoreCase) ? (Queryable.OrderByDescending) : new Func<IQueryable<Object>, Expression<Func<Object, Object>>, IOrderedQueryable<Object>>(Queryable.OrderBy)).Method.GetGenericMethodDefinition().MakeGenericMethod(typeof(ReferenceProduct), correspondingSortExpression.Expression.ReturnType);
IOrderedQueryable<ReferenceProduct> orderedProducts = orderMethod.Invoke(null, new Object[] { filteredProducts, correspondingSortExpression.Expression }) as IOrderedQueryable<ReferenceProduct>;
return orderedProducts;
}
有人知道嗎?
提前致謝。
瑞安
這差不多是我能做到的,它使用dynamic
來完成一些晦澀的方法解析(這是一個邪惡的技巧,但在這里非常值得-手動進行操作並沒有增加任何用處)。 我不是依靠具有searchTerm
的“測試類”的未定義范圍,而是將其提升到了最高級別-但是您可能需要一些不同的東西(因此,我在有關searchTerm
范圍的注釋中繼續提出疑問):
class ReferenceProduct {
public string Name { get; set; }
static readonly Dictionary<string, Func<string, LambdaExpression>> knownSortFactories =
new Dictionary<string, Func<string,LambdaExpression>> {
{"name", searchTerm =>(Expression<Func<ReferenceProduct, string>>)(p => p.Name.StartsWith(searchTerm) ? Convert.ToString((char)0) : p.Name) }
// ... more here
};
public static IOrderedQueryable<ReferenceProduct> OrderProducts(IQueryable<ReferenceProduct> filteredProducts, string sortExpression, string sortDirection, string queryTerm)
{
Func<string, LambdaExpression> factory;
if (!knownSortFactories.TryGetValue(sortExpression, out factory))
throw new InvalidOperationException("Unknown sort expression: " + sortExpression);
dynamic lambda = factory(queryTerm); // evil happens here ;p
switch(sortDirection)
{
case "asc":
return Queryable.OrderBy(filteredProducts, lambda);
case "desc":
return Queryable.OrderByDescending(filteredProducts, lambda);
default:
throw new InvalidOperationException("Unknown sort direction: " + sortDirection);
}
}
}
使用示例用法(此處使用LINQ-to-Objects作為外觀):
static void Main()
{
var source = new[] {
new ReferenceProduct { Name = "def" },
new ReferenceProduct { Name = "fooghi" },
new ReferenceProduct { Name = "abc" }
}.AsQueryable();
var sorted = ReferenceProduct.OrderProducts(source, "name", "asc", "foo");
var arr = sorted.ToArray();
foreach(var item in arr) {
Console.WriteLine(item.Name);
}
}
輸出:
fooghi
abc
def
完全不需要反射調用就可以實現您想要的目標。
代替
var orderedProducts = orderMethod.Invoke(null, new Object[] { filteredProducts, correspondingSortExpression.Expression }) as IOrderedQueryable<ReferenceProduct>;
只需使用查詢提供程序:
var orderedProducts = filteresProducts.Provider.CreateQuery<ReferenceProduct>(Expression.Call(null, orderMethod, Expression.Constant(correspondingSortExpression.Expression)));
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