Question

I have created a Extension method for IQueryable class to convert list of a Generic source class to list of other generic destination class,(.net3.5)
I’m using reflection to get properties from source object value and assign it to destinvation object.

The source code of the class is as follows:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
using System.Text.RegularExpressions;

namespace System.Linq
{
    public static class QueryableExtensions
    {
        public static IQueryable<TDest> ToDTO<TDest>(this IQueryable<TSource> source)//getting error on this line
        {
            return DTOTranslator<TDest>.ConvertToDTO<TDest>(source);
        }
    }

    public class DTOTranslator<TSource>
    {
        public static IQueryable<TDest> ConvertToDTO<TDest>(IQueryable<TSource> source)
        {
            List<TDest> destinationList = new List<TDest>();
            List<TSource> sourceList = source.ToList<TSource>();

            var sourceType = typeof(TSource);
            var destType = typeof(TDest);
            foreach (TSource sourceElement in sourceList)
            {
                TDest destElement = Activator.CreateInstance<TDest>();
                //Get all properties from the object 
                PropertyInfo[] sourceProperties = typeof(TSource).GetProperties();
                foreach (PropertyInfo sourceProperty in sourceProperties)
                {
                    //and assign value to each propery according to property name.
                    PropertyInfo destProperty = destType.GetProperty(sourceProperty.Name);
                    destProperty.SetValue(destElement, sourceProperty.GetValue(sourceElement, null), null);
                    destinationList.Add(destElement);
                }
            }

            return destinationList.AsQueryable();
        }
    }
}

however at compile time I am getting the error on line 12:

The type or namespace name 'TSource' could not be found (are you missing a using directive or an assembly reference?)

UPDATE:

Thanks to all for reply.

Now I’ve updated my class as follows:

public static class QueryableExtensions
{
    public static IQueryable<TDest> ToDTO<TSource, TDest>(this IQueryable<TSource> source)
    {
        List<TDest> destinationList = new List<TDest>();
        List<TSource> sourceList = source.ToList<TSource>();

        var sourceType = typeof(TSource);
        var destType = typeof(TDest);
        foreach (TSource sourceElement in sourceList)
        {
            TDest destElement = Activator.CreateInstance<TDest>();
            //Get all properties from the object 
            PropertyInfo[] sourceProperties = typeof(TSource).GetProperties();
            foreach (PropertyInfo sourceProperty in sourceProperties)
            {
                //and assign value to each propery according to property name.
                PropertyInfo destProperty = destType.GetProperty(sourceProperty.Name);
                destProperty.SetValue(destElement, sourceProperty.GetValue(sourceElement, null), null);
                destinationList.Add(destElement);
            }
        }

        return destinationList.AsQueryable();
    }
}

This works fine,

now the only thing I wonder is that-

Is it possible to skip the <TSource> paramerter,

how to read it from the IQueryable’s element type and define its object?
I mean currently I call the method as

IQueryable<CATEGORY_DTO> dtos = (from p in new MyEntities().CATEGORY select p).ToDTO<CATEGORY, CATEGORY_DTO>();

I want to call it as

IQueryable<CATEGORY_DTO> dtos = (from p in new MyEntities().CATEGORY select p).ToDTO<CATEGORY_DTO>();

Thanks, NJ

Answer 1

Try this:

public static class QueryableExtensions
{
    public static IQueryable<TDest> ToDTO<TDest, TSource>(this IQueryable<TSource> source)
    {
        return DTOTranslator<TSource>.ConvertToDTO<TDest>(source);
    }
}

The addition being that the ToDTO method also references the TSource generic type (since otherwise how can you reference is as a parameter – this led to the original error), and this generic type is given in the constructor for DTOTranslator (since it is an error to use TDest in the constructor, as the definition of the generic type needed in the constructor itself is TSource, which is a different type from TDest).