Can not deal with IEnumerable in generic

Question

I have a method to concatenate strings provided by int?.

public string ConcatenateNumber(IEnumerable<int?> myList)
{
    return myList
        .Distinct()
        .Aggregate(
            new StringBuilder(), 
            (current, next) => current.Append("'").Append(next))
        .ToString()
        .Substring(1);
}

Now I want to do unit test.

[TestMethod]
public void InputIntegers_Should_Be_Concatenated_When_Consider_Distinct()
{
    var myList = CreateEnumerable(1, 2, 2, 3);
    var logic = new MainLogic();
    var result = logic.ConcatenateNumber(myList);
    Assert.AreEqual("1'2'3", result);
}

public IEnumerable<T> CreateEnumerable<T>(params T[] items)
{
    if (items == null)
        yield break;

    foreach (T mitem in items)
        yield return mitem;
}

However I have the compile error.

C#: Unknown method ConcatenateNumber(System.Collections.Generic.IEnumerable) of ....

I think it is caused by nullable integer int?. But I am not sure how to fix it.

Just specify the type like this `CreateEnumerable(1,2,2,3)`. — juharr, Feb 15 '17 at 17:02
`IEnumerable` is not `IEnumerable`... and I dont think you are going to want that `.Substring(1)` — Matthew Whited, Feb 15 '17 at 17:02
Without Substring(1), the result would be '1'2'3. But I want 1'2'3. — Bigeyes, Feb 15 '17 at 17:03
@MatthewWhited. I don't know the performance of string.join if it is a long sequence. — Bigeyes, Feb 15 '17 at 17:05
"Unknown method" sounds to me like you haven't referenced it properly or are calling it wrong. — Sami, Feb 15 '17 at 17:05
Better than the performance of your method. `string.Join` uses `StringBuilder` already. — Matthew Whited, Feb 15 '17 at 17:06
As others already mentioned, `IEnumerable` != `IEnumerable`. You can change your generic method to return `Nullable`, like this: `public static IEnumerable CreateEnumerable(params T[] items) where T : struct`, but this whole design is questionable, as others also mentioned. — PoweredByOrange, Feb 15 '17 at 17:06

score 2 · Accepted Answer · answered Feb 15 '17 at 17:05

2

Explicitly pass the type as a nullable int.

var myList = CreateEnumerable<int?>(1, 2, 2, 3);

For example:

using System;
using System.Linq;              
using System.Collections.Generic;

public class Program
{
    public static void Main()
    {
        var p = new Program();

        var list = p.CreateEnumerable<int?>(1, 2, 3, 4);
        p.DoWork(list);         
    }

    public void DoWork(IEnumerable<int?> enumerable)
    {
        enumerable.ToList().ForEach(x => 
        {
            Console.WriteLine(x);
        });
    }

    public IEnumerable<T> CreateEnumerable<T>(params T[] items)
    {
        if (items == null)
            yield break;

        foreach (T mitem in items)
            yield return mitem;
    }
}

answered Feb 15 '17 at 17:05

Shaun Luttin

107,550
65
332
414

Do not use `.ToList()` just to call `.ForEach(...)` ... this causes the set to be looped twice is provides no benefits. – Matthew Whited Feb 15 '17 at 17:16
@MatthewWhited `ForEach` does not exist on `IEnumerable`. – Shaun Luttin Feb 15 '17 at 17:17
@MatthewWhited `foreach` and `ToList().ForEach()` appear to make the same number of loops: https://dotnetfiddle.net/V7iExb – Shaun Luttin Feb 15 '17 at 17:29
2

Only because you're only counting half of the loops in that fiddle, you aren't counting the loops internal to `ToList()`. It's also wasting memory as well as time. – Jon Hanna Feb 15 '17 at 17:50
That's because your code is wrong. In the second set you are incrementing the set twice for each `.MoveNext()`. In the first you are counting the values twice in succession. – Matthew Whited Feb 15 '17 at 17:53
check out what really happens with your code here... you will see why I said it is passed twice. https://dotnetfiddle.net/mC31k5 – Matthew Whited Feb 15 '17 at 18:01
1

No problem ... I see people do this all the time and they just don't get why I try to tell them to do it differently :) – Matthew Whited Feb 15 '17 at 18:07

score 2 · Answer 2 · answered Feb 15 '17 at 17:14

2

public void InputIntegers_Should_Be_Concatenated_When_Consider_Distinct()
{
    var myList = CreateEnumerable(1, 2, 2, 3);
    var logic = new MainLogic();
    var result = logic.ConcatenateNumber(myList);
}

public IEnumerable<T> CreateEnumerable<T>(params T[] items)
{
    return items ?? Enumerable.Empty<T>();
}

public class MainLogic
{

    public string ConcatenateNumber<T>(IEnumerable<T> myList)
    {
        // do this if you want to remove nulls
        return string.Join("'", myList.Where(i => i != null).Distinct()); 

        //return string.Join("'", myList.Distinct()); // otherwise do this
    }
}

answered Feb 15 '17 at 17:14

Matthew Whited

21,248
2
48
67

Thank you. You mentioned string.Join uses StringBuilder already. I can't find the source. Have you looked [disscussion](http://stackoverflow.com/questions/585860/string-join-vs-stringbuilder-which-is-faster)? – Bigeyes Feb 15 '17 at 18:33
@Bigeyes https://referencesource.microsoft.com/#mscorlib/system/string.cs,fa961ce440526a4d – Alexei Levenkov Feb 15 '17 at 18:46
You are going a single join not a set of joins. – Matthew Whited Feb 15 '17 at 18:46

Can not deal with IEnumerable in generic

2 Answers2