Expression function: Generic type T predicate

Question

Is it possible to parse generic type T from predicate expression and convert it back to another type and then create new expression by using converted type?

I'm doing the conversion from T to DTO.Company and after that I should do the mapping from DTO.Company to DAL.Company. In below code example I'm trying to do conversion but first I need to know that is it even possible and secondly I need to know how to convert DTO.Company to DAL.Company inside the predicate parameters before setting the bar variable.

    // This works!!!! But I want to make it generic as possible!
    public T Fetch<T>(Expression<Func<DAL.Company, bool>> predicate) where T : class
    {
        var typeOfT = typeof(T);
        if (typeOfT != typeof(DTO.Company))
        {
            return null;
        }           
        var companies = _dbContext.Companies.Where(predicate);     

        var ret = new Collection<DTO.Company>();
        foreach (var company in companies)
        {
            ret.Add(company.ConvertToDataTransferObject());
        }
        return (T)Convert.ChangeType(ret, typeof(T));
    }

    // This does not work!!!
    public T Fetch<T>(Expression<Func<T, bool>> predicate) where T : class
    {
        var typeOfT = typeof(T);
        if (typeOfT != typeof(DTO.Company))
        {
            return null;
        }
        var foo = Expression.Convert(predicate.Body, typeof(bool));


    //Below I'm getting Exception thrown: 'System.ArgumentException' in System.Core.dll
    //Additional information: ParameterExpression of type 'DTO.Company' cannot be used for delegate parameter of type 'DAL.Company'

        var bar = Expression.Lambda<Func<DAL.Company, bool>>(foo, predicate.Parameters);
        var companies = _dbContext.Companies.Where(bar);

        var ret = new Collection<DTO.Company>();
        foreach (var company in companies)
        {
            ret.Add(company.ConvertToDataTransferObject());
        }
        return (T)Convert.ChangeType(ret, typeof(T));
    }

There was this question: http://stackoverflow.com/questions/42902164/how-to-map-expressionfunctentity-bool-to-expressionfunctdbentity-bool/42904029#42904029 Take a look if it is enough for you — xanatos, Mar 28 '17 at 12:54

score 1 · Answer 1 · answered Apr 03 '17 at 05:43

Ok, I changed logic a little bit.

First logic was:

Service layer takes only DTO parameters and returns only DTO objects.
Repository layer takes only DTO parameters and predicate parameters are only DTO and returns only DTO pbjects.

Now it's:

Service layer takes only DTO parameters and returns only DTO objects.
Repository layer takes only DTO paramaters and predicate parameters are only DAL and returns only DTO objects.

Expression function: Generic type T predicate

1 Answers1