Question: why is Distinct not supported?

[Vlandmist]

Distinct() method is not supported, not in the original @NArnott project (now disappeared.. why?) and not in this fork. what's the reason behind it? is it too difficult to implement?

[StefH]

I'll copy the logic from https://github.com/kahanu/System.Linq.Dynamic/blob/master/Src/System.Linq.Dynamic/DynamicLinq.cs#L129

[Vlandmist]

ok! that's great, I'll try it asap

[StefH]

Code + tests have been added to Github.

Please take a look.
If you want a new NuGet, I can create this if needed.

[Vlandmist]

thank you, yes a new nuget would be great to test it

[StefH]

https://www.nuget.org/packages/System.Linq.Dynamic.Core/1.0.3.1

[StefH]

Are you able to verify ?

[Vlandmist]

I tried but I'm having problems, how can I retrieve a distinct list of Persons by name? (like the commented line). This is my code

            var lp = new List<Person>();
            lp.Add(new Person {Name = "John", Surname = "Lennon"});
            lp.Add(new Person {Name = "John", Surname = "Williams"});
            lp.Add(new Person {Name = "Paul", Surname = "MC"});
            lp.Add(new Person {Name = "Paul", Surname = "Allen"});
            lp.Add(new Person {Name = "Ringo", Surname = "Starr"});


             var qp = lp.AsQueryable().Select("(Name).Distinct");
            //var qp = lp.AsQueryable().Select(p => p.Name).Distinct();

            foreach (var n in qp)
            {
                Console.WriteLine(n.ToString());
            }

            Console.ReadKey();

gives a ParseException

An unhandled exception of type 'System.Linq.Dynamic.Core.Exceptions.ParseException' occurred in System.Linq.Dynamic.Core.dll
Additional information: No property or field 'Distinct' exists in type 'String'

[StefH]

You can do this:
var qp = lp.AsQueryable().Select("Name").Distinct();

Or is the method you are looking for like:
var qp = lp.AsQueryable().Distinct("Name");
However that method does not exists (yet). I'll take a look if I can add this one.

[Vlandmist]

Yes, your sample works, but I couldn't think of a dynamic use for it since Distinct is explicit. Also I can't seem to use Distinct inside a Where.("...") expression.

I'm working on a Linq to Entities (SQL Server) project using the original @NArnott library and some complex queries aren't easy to manage without support for Distinct and GroupBy methods.

[StefH]

Can you provide an example for the _Where_ statement ?

Because _Select_ is supported : var result = queryable.Select("abc.Distinct()");

[Vlandmist]

I'll answer with a sample asap. what is "abc" in your code sample?

[StefH]

A property from a user which is a list or iqueryable or ienumerable. See https://github.com/StefH/System.Linq.Dynamic.Core/blob/master/test/System.Linq.Dynamic.Core.Tests/QueryableTests.Distinct.cs#L35

[StefH]

Any luck?

[Vlandmist]

Sorry mate, didn't have time to check it in these days. I'll write here when I have a couple more samples. keep on the good work

[StefH]

Can you provide feedback ?

[Vlandmist]

Ok let's forget about Distinct() support in Where. My situation was an extreme case and it's not worth replicating.

About Distinct in select I can't seem to make it work for simple lists (not nested object like your code sample)

This is my code:

        var integerList = new List<int>();
        integerList.AddRange(new []{1,1,2,3,4,4,5});

        //This works
        var xx = integerList.AsQueryable().Select("it").Distinct();

        //exception
        //Additional information: No property or field 'Distinct' exists in type 'Int32'
        var xy = integerList.AsQueryable().Select("it.Distinct()");`

        //exception
        //Additional information: No property or field 'Distinct' exists in type 'Int32'
        var xyz = integerList.AsQueryable().Select("Distinct(it)");

Maybe it's me that is not undertanding how this works. is there any way to achieve what I'm trying to do? if not, and if it's not useful to implement it, we can close this issue.

[StefH]

Distinct does not solve your problem I think:

In your original question:

// real linq
var qp = lp.AsQueryable().Select(p => p.Name).Distinct();

// dynamic linq
var qpDynamic = lp.AsQueryable().Select("Name").Distinct();

The value from qp will be a List of Strings with the values : "John", "Paul" and "Ringo".

What you are trying do does not make sense I think, I have a feeling you want to use a groupby logic.


Note that real linq code like

var error = lp.AsQueryable().Select(p => p.Name.Distinct());

does also not compile.

[Vlandmist]

Ok, thanks, that makes perfect sense, I'll close the issue.