Unexpected result while generating string with Aggregate() in linq

Question

Utility Function:

  public static string GetProperties<T>(string alias="")
    {
       if (alias.Length>0)
       {
          return typeof(T).GetProperties().Select(x => x.Name).Aggregate((x, y) => 
            alias + " = " + alias + "." + x + "," + Environment.NewLine + alias + " = " + alias + "." + y + ",");
       }
       else
       {
           return typeof(T).GetProperties().Select(x => x.Name).Aggregate((x, y) => x + Environment.NewLine + y);
       }
    }

Code:

    public class ContainerInLog
    {

        public int ContainerInLogID { get; set; }
        public int ContainerInID { get; set; }
        public int CargoID { get; set; }      
        public int LoadStatus { get; set; }    

    }

    public static void Main()
    {
         string list = GetProperties<ContainerInLog>("y");
        Console.WriteLine(list);
    }

Result:

y = y.y = y.y = y.ContainerInLogID,
y = y.ContainerInID,,
y = y.CargoID,,
y = y.LoadStatus,

Expected Result:

ContainerInLogID = y.ContainerInLogID,
ContainerInID = y.ContainerInID,
CargoID = y.CargoID,
LoadStatus = y.LoadStatus,

Don't use Aggregate with strings. You generate a lot of intermediate strings which are then thrown away, making it expensive. Use string.Join / a StringBuilder as appropriate. — canton7
– canton7, Commented May 16, 2019 at 11:29

DavidG · Accepted Answer · 2019-05-16 11:36:20Z

5

If you are really stuck on returning the entire concatenated string instead of returning an enumerable of them, I wouldn't use Aggregate here, just use string.Join. Also, you can simplify the statement by crafting the string inside the Select. For example:

return string.Join(
    Environment.NewLine,
    typeof(T)
        .GetProperties()
        .Select(x => $"{x.Name} = {alias}.{x.Name},"));

Bonus: If you change the separator to $",{Environment.NewLine}" you can remove the inline comma and you won't get the final comma on the end of your string (example fiddle).

edited May 16, 2019 at 11:36

answered May 16, 2019 at 11:30

DavidG

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6 Comments

SamuraiJack Over a year ago

That is working just fine. But could you tell me what was causing the initial y.y = y.y

DavidG Over a year ago

Well what changed? What is different? Are you using the latest version of C#?

SamuraiJack Over a year ago

nothing that I can think of. I am using vs2013, .net framework 4.5.1

DavidG Over a year ago

That means you're not using a modern version of the C# compiler, which means the $"...." notation doesn't work. Replace those with string.Format.

SamuraiJack Over a year ago

that's strange, because it worked the first few times.

|

w5l · Accepted Answer · 2019-05-16 11:57:05Z

1

@DavidG has the nicer solution here, but whats going wrong with your aggregation is the following:

.Aggregate((x, y) => 
            alias + " = " + alias + "." + x + "," + Environment.NewLine + alias + " = " + alias + "." + y + ",");

The Aggregate selector function (your (x,y)) takes the following form:

Func<TAccumulate,TResult> resultSelector

That means in your case, x is the accumulated aggregate result already, eg "ContainerInLogID = y.ContainerInLogID".

But to make the next aggregate, you transform x again: alias + " = " + alias + "." + x, making "y = y.y = y.ContainerInLogID". And so on for each following property, each one adding another prefix of "y = y.".

edited May 16, 2019 at 11:57

answered May 16, 2019 at 11:47

w5l

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