Combine/Merge 3 lists of objects based on condition

Question

public class Derp
{
    public Derp
    {
        listOfStrings = new List<string>();
    }
    public string strName;
    public List<string> listOfStrings;
    public int unrequiredInt;
    public bool unrequiredBool;
}

List<Derp> derp1 = ... //generate data assume strName is unique in list, but not across lists;
List<Derp> derp2 = ... //generate data;
List<Derp> derp3 = ... //generate data;

List<Derp> mergedDerp = new List<Derp>();

I need to merge derp1 and derp2 and derp3 with the condition derp1[x].strName == derp2[y].strName == derp3[z].strName. The merged list should have all Derps but merge derp1,2,3 into one derp based on the condition above (unrequiredInt and unrequiredBool's content doesn't matter). I know it can be done in LINQ but I'm quite at a loss. Something like ...

mergedDerp = derp1.Join(derp2, d1 => derp1, d2 => derp2, (d1,d2) => new { ... ;
//and the next derp would be (i assume)
mergedDerp = mergedDerp.Join(derp3, md => mergedDerp, ...;

But i'm not getting it.

The result should contain a list of unique Derps by their strName, and if any Derps were merged, the listOfStrings should all be appended into the new Derp.

Fung · Accepted Answer · 2014-03-11 03:56:50Z

1

Using GroupBy instead of Join seems more suitable in your case:

var mergedDerp = derp1.Union(derp2).Union(derp3).GroupBy(x => x.strName)
    .Select(x => new Derp
        {
            strName = x.Key,
            // I guess you want to merge the list of strings as well?
            listOfStrings = x.SelectMany(d => d.listOfStrings).ToList()
            // Leave unrequired fields as default or just use the first derp's value
            // unrequiredInt = x.First().unrequiredInt,
            // unrequiredBool = x.First().unrequiredBool,
        })
    .ToList();

answered Mar 11, 2014 at 3:56

Fung

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1 Comment

Natalie Tay Over a year ago

THANKS. :O Initially tested your code with John's extra Equals and GetHashCode within Derp so it didn't work. Now that I've removed it, it works! This is amazing. Thanks again.

John Koerner · Accepted Answer · 2014-03-11 03:54:50Z

1

It sounds like you want to determine equality based on the strName value. If so, simply implement the Equals and GetHashCode methods on the object:

public class Derp
{
    public Derp()
    {
        listOfStrings = new List<string>();
    }

    public string strName;
    public List<string> listOfStrings;
    public int unrequiredInt;
    public bool unrequiredBool;


    public override bool Equals(object obj)
    {
        return ((Derp) obj).strName.Equals(strName);
    }

    public override int GetHashCode()
    {
        return strName.GetHashCode();
    }
}

Then when you combine them, you can just use Union and Distinct:

var derp1 = new List<Derp>();
derp1.Add(new Derp() {strName = "X"});
derp1.Add(new Derp() { strName = "Y" });
derp1.Add(new Derp() { strName = "Z" });

var derp2 = new List<Derp>();
derp2.Add(new Derp() {strName = "A"});
derp2.Add(new Derp() { strName = "B" });
derp2.Add(new Derp() { strName = "X" });


var derp3 = new List<Derp>();
derp3.Add(new Derp() { strName = "J" });
derp3.Add(new Derp() { strName = "B" });
derp3.Add(new Derp() { strName = "X" });


var merged = derp1.Union(derp2.Union(derp3)).Distinct();

Console.WriteLine(merged.Count());   // Returns 6: X, Y, Z, A, B, J

answered Mar 11, 2014 at 3:54

John Koerner

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1 Comment

Natalie Tay Over a year ago

Hi John, thanks for your prompt and detailed response. This would be good if the List<string> listOfStrings didn't matter, however I'd need for the listOfStrings to be appended along with the union. I'd reckon there could be methods I could implement in the Derp class, but I was thinking if there could be any LINQ ways (so I can level up a little)

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