asked on

LINQ help

Hi, I thought I understood LINQ.

I do not.

Below is some code I have written to try to get a distinct list of strings. It doesn't even compile. Can someone please help me get it right?

Thank you

John

using System;
using System.Linq;

namespace ConsoleApplication41
{
    public class Test0
    {
        public String[] Names { get; set; }
        public int id { get; set; }
    }

    public class Test1
    {
        public Test0[] Tests { get; set; }
        public int group { get; set; }
    }

    class Program
    {
        private void LinqTest()
        {
            Test0 test011 = new Test0 { Names = new String[] { "one", "two", "three" } };
            Test0 test012 = new Test0 { Names = new String[] { "three", "four" } };
            Test0 test013 = new Test0 { Names = new String[] { "ten", "one" } };
            Test1[] test1 = new Test1[] {new Test1 { Tests = new Test0[] { test011, test012, test013 } } };

            // Some magic LINQ to give me 
            // one, two, three, four, ten
            // from test1
            //
            // THE FOLLOWING DOESN'T WORKa
            var allTest1s = (
                                from y
                                    in (from x
                                           in test1
                                         select x.Tests
                                       )
                                select y.Names
                            ).ToArray<String>().Distinct<String>();
        }

        static void Main(string[] args)
        {
            Program program = new Program();
            program.LinqTest();
        }
    }
}

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p_davis

what is the error/errors you are getting, when compiling.... seems x.Tests should be casted??

John Bolter

ASKER

Hi, I struggled with that too.

I tried

            var allTest1s = (
                                from y
                                    in (from x
                                           in test1
                                         select ((Test0[])x.Tests)
                                       )
                                select y
                            ).ToArray<Test0>().Distinct<String>();

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but then I couldn't convert all the Test0's to their Names Strings so I'm sort of missing an inner loop or something.

p_davis

what errors are you getting?

John Bolter

ASKER

The error is it doesn't compile. This is what I think my LINQ query should look like but it is clearly wrong.

The compile error is

Error	1	Instance argument: cannot convert from 'System.Collections.Generic.IEnumerable<ConsoleApplication41.Test0[]>' to 'System.Collections.Generic.IEnumerable<ConsoleApplication41.Test0>'	c:\users\johneb\documents\visual studio 2013\Projects\ConsoleApplication41\ConsoleApplication41\Program.cs	39	30	ConsoleApplication41
Error	2	'System.Collections.Generic.IEnumerable<ConsoleApplication41.Test0[]>' does not contain a definition for 'ToArray' and the best extension method overload 'System.Linq.Enumerable.ToArray<TSource>(System.Collections.Generic.IEnumerable<TSource>)' has some invalid arguments	c:\users\johneb\documents\visual studio 2013\Projects\ConsoleApplication41\ConsoleApplication41\Program.cs	39	30	ConsoleApplication41

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p_davis

what happens if, in the select, you get rid of the cast (Test0[])

John Bolter

ASKER

It still doesn't compile. I only added the cast because you suggested it above.

Try it. The whole program can be copy/pasted into Visual Studio

ASKER CERTIFIED SOLUTION

it_saige

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it_saige

Using:

using System;
using System.Linq;

namespace LinqExample
{
	class Test0
	{
		public string[] Names { get; set; }
		public int Id { get; set; }
	}

	class Test1
	{
		public Test0[] Tests { get; set; }
		public int Group { get; set; }
	}

	class Program
	{
		static void Main(string[] args)
		{
			Test0 test011 = new Test0() { Names = new string[] { "one", "two", "three" } };
			Test0 test012 = new Test0() { Names = new string[] { "three", "four" } };
			Test0 test013 = new Test0() { Names = new string[] { "ten", "one" } };
			Test1[] test1 = new Test1[] { new Test1() { Tests = new Test0[] { test011, test012, test013 } } };

			var allTest1s = (from testGroup in test1
						  from test in testGroup.Tests
						  from name in test.Names
						  select name).Distinct();

			foreach (var test in allTest1s)
				Console.WriteLine(test);
			Console.ReadLine();
		}
	}
}

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Produces the following output:

-saige-

John Bolter

ASKER

I see how you've done it now, I didn't realise you could have two "from" like that. I was thinking in a sort of SQL way where that isn't a valid syntax.
Thank you

it_saige

When you really think about it it makes sense.

test1 represents an array of Test1, so you enumerate over each Test1.
Inside of each Test1 you have an array of Test0's, represented as Tests, so you have to enumerate over each Test0.
Then inside of each Test0, you have an array of strings, represented as Names, so you have to enumerate over each Name.

-saige-