动态构建lambda表达式

Question

I am trying to sort a object collection dynamically. Ideally I would want it to do the below functionality where I could specify the criteria at runtime.

_group.OrderByDescending(rec => "rec.CalculatedRecord.GL.PropertyValue").Take(Convert.ToInt16(_filters.Value))

I tried dynamic linq which didn't work.

var query = (from enumerable in _group
                           orderby "rec.CalculatedRecord.GL.PropertyValue").  descending 
                          select enumerable).Take(5);

Answer 1

The OrderByDescending method takes a parameter of Func<TSource, TKey> . While you can insert a lambda here, as is the most common use case, you can also pass in a variable. The variable can be dynamically assigned based on whatever you logic might be.

I have had to infer your class structure, but here's a basic example:

    public class Record
    {
        public CalculatedRecord CalculatedRecord { get; set; }
    }

    public class CalculatedRecord
    {
        public GL GL { get; set; }
    }

    public class GL
    {
        public PropertyValue PropertyValue1 { get; set; }
        public PropertyValue PropertyValue2 { get; set; }
    }

    public class PropertyValue { }

Then you can create the following logic:

    public class TestHarness
    {
        public void Test()
        {
            IEnumerable<Record> Group = new List<Record>();

            //Simple flag
            bool someCriteria = true;

            //Define orderByFunc
            Func<Record, PropertyValue> orderByFunc;

            //Assign the order by function based on someCriteria 
            if (someCriteria)
            {
                orderByFunc = record => 
                              record.CalculatedRecord.GL.PropertyValue1;
            }
            else
            {
                orderByFunc = record => 
                              record.CalculatedRecord.GL.PropertyValue2;
            }

            //Execute OrderBy
            var updated = Group.OrderByDescending(orderByFunc).Take(5);

        }
    }

However, this doesn't allow you to generate the Func dynamically at runtime. In order to do that you will need to build an Expression using expression trees and then compile it. You can follow the steps at Expression.Lambda and query generation at runtime, simplest "Where" example

Answer 2

A note on why the second one shouldn't work: your object of enumeration is called enumerable yet in your string you are still using rec as in the lambda earlier. changing the string to use enumerable instead of rec might alleviate your issues.