Dynamic Linq & ICompare

问题

I'm trying to make use of James McCormack's guide to using a custom IComparer with Dynamic Linq. - see http://zootfroot.blogspot.co.uk/2009/10/dynamic-linq-orderby.html?showComment=1347276236930#c11348033278810583

I use a query to pull back an Enumerable of string values:

.Select("fieldname").Distinct()

Then try to use

.OrderBy(item=>item.GetReflectedPropertyValue("fieldname"),new myComparer()) 

GetReflectedPropertyValue is a helper method defined by James as

public static string GetReflectedPropertyValue(this object subject, string field)
{
object reflectedValue = subject.GetType().GetProperty(field).GetValue(subject, null);
return reflectedValue != null ? reflectedValue.ToString() : "";
}

But get the error "'System.Collections.Generic.IEnumerable' does not contain a definition for 'OrderBy' and the best extension method overload 'System.Linq.Dynamic.DynamicQueryable.OrderBy(System.Linq.IQueryable, string, params object[])' has some invalid arguments"

Any ideas? I'm new to this and just trying to get something working before I get the time to actually go through and learn it properly.

回答1:

it's hard to tell based only on what you've posted because the resolution of OrderBy will depend on the implementation of myConverter. But, Since GetReflectedPropertyValue returns string, myConverter must implement IConverter<String> to be used with OrderBy. If it doesn't, and it implements something like IComparer<MyItem> then compiler is really looking for a method OrderBy(this IEnumerable<object>, Func<object,string>, IComparer<MyItem>) which doesn't exist and spits out that error.

If that isn't clear, provide the details of myConverter and I can be more specific.

If you're using something that doesn't implement IComparer<T>, then you can wrap the IComparer with something that implements IComparable<string> so that it is compatible with Orderby. For example:

public class MyStringComparer : IComparer<String>
{
    readonly IComparer comparer = new MyComparer();
    public int Compare(string x, string y)
    {
        return comparer.Compare(x, y);
    }
}

来源：https://stackoverflow.com/questions/12350965/dynamic-linq-icompare