我使用实体框架,ASP.NET和C#3.5 我借下面的代码,使使用的SortExpression从GridView控件,而不是实体的属性排序可能:如何从实际类型推断TResult?
public static IEnumerable<T> Sort<T>(this IEnumerable<T> source, string sortExpression)
{
string[] sortParts = sortExpression.Split(' ');
var param = Expression.Parameter(typeof(T), string.Empty);
var property = Expression.Property(param, sortParts[0]);
var sortLambda = Expression.Lambda<Func<T, object>>(Expression.Convert(property, typeof(object)), param);
if (sortParts.Length > 1 && sortParts[1].Equals("desc", StringComparison.OrdinalIgnoreCase))
{
return source.AsQueryable<T>().OrderByDescending(sortLambda);
}
return source.AsQueryable<T>().OrderBy(sortLambda);
}
的问题是LINQ to Entities不支持将对象转换为对象。而不是Func,我需要实际的返回类型而不是对象。我计算出如何做到这一点:
public static IEnumerable<T> Sort<T>(this IEnumerable<T> source, string sortExpression)
{
string[] sortParts = sortExpression.Split(' ');
var param = Expression.Parameter(typeof(T), string.Empty);
var property = Expression.Property(param, sortParts[0]);
// NEW CODE HERE
Type propertyType = property.Type;
Type lambdaType = typeof(Func<,>).MakeGenericType(typeof(T), propertyType);
var sortLambda = Expression.Lambda<Func<T, object>>(Expression.Convert(property, propertyType), param);
//var sortLambda = Expression.Lambda<Func<T, object>>(Expression.Convert(property, typeof(object)), param);
if (sortParts.Length > 1 && sortParts[1].Equals("desc", StringComparison.OrdinalIgnoreCase))
{
return source.AsQueryable<T>().OrderByDescending(sortLambda);
}
return source.AsQueryable<T>().OrderBy(sortLambda);
}
现在的问题是,如果我有一个Int32,它不会转换为对象,这仍然是返回类型。我像这样解决了这个问题:
public static IEnumerable<T> Sort<T>(this IEnumerable<T> source, string sortExpression)
{
string[] sortParts = sortExpression.Split(' ');
var param = Expression.Parameter(typeof(T), string.Empty);
var property = Expression.Property(param, sortParts[0]);
// New code here
Type propertyType = property.Type;
Type lambdaType = typeof(Func<,>).MakeGenericType(typeof(T), propertyType);
// NEWEST CODE HERE
var sortLambda = Expression.Lambda(lambdaType, Expression.Convert(property, propertyType), param);
//var sortLambda = Expression.Lambda<Func<T, object>>(Expression.Convert(property, propertyType), param);
//var sortLambda = Expression.Lambda<Func<T, object>>(Expression.Convert(property, typeof(object)), param);
if (sortParts.Length > 1 && sortParts[1].Equals("desc", StringComparison.OrdinalIgnoreCase))
{
return source.AsQueryable<T>().OrderByDescending(sortLambda);
}
return source.AsQueryable<T>().OrderBy(sortLambda);
}
然而,这不能再编译了。错误是:
用于方法的类型参数“System.Linq.Enumerable.OrderByDescending(System.Collections.Generic.IEnumerable,System.Func)”不能从使用推断。尝试明确指定类型参数。
问题是我不想明确指定类型参数。
有谁知道如何解决这个或无论如何从“propertyType”推断TResult类型?
谢谢,正是我在找的! – Calin 2010-05-17 12:00:57