C# Linq Find duplicates with multiple group by

Question

I have a list in c# as:

List<Data> uData = new List<uData>();

Where uData is populated from UI as:

{
   Id: 1,
   Name: "Smith",
   Input: "7,8",
   Output: "Output1",
   CreatedBy: "swallac",
   CreatedON: "12/01/2018"
},
{
   Id: 2,
   Name: "Austin",
   Input: "9,10",
   Output: "Output1",
   CreatedBy: "amanda",
   CreatedON: "12/03/2018"
},
{
   Id: 3,
   Name: "Smith",
   Input: "22,22",
   Output: "Output2",
   CreatedBy: "swallac",
   CreatedON: "12/01/2018"
},
{
   Id: 4,
   Name: "Smith",
   Input: "9,8",
   Output: "Output2",
   CreatedBy: "aaa",
   CreatedON: "12/01/2018"
},
{
   Id: 5,
   Name: "Peter",
   Input: "7,8",
   Output: "Output3",
   CreatedBy: "swallac",
   CreatedON: "12/02/2018"
}

What I want to do is search this list on "Output" key, and find out if there are in duplicates in the corresponding combination value of "Input" & CreatedBy key.

For example, in my above example list I have three Output: Output1,Output2, Output3. Now for lists with key of Output value as "Output1" & "Output3" the corresponding "Input" & "CreatedBy" key value is duplicate here. The value being "7,8"& "swallac" as combined value. This is what I want to highlight

For this I tried out the below query:

var myList = uData.GroupBy(l => l.Ouput)
                  .SelectMany(g => g.GroupBy(x => (x.Input, x.CreatedBy)).Where(x => x.Count() > 1))
                  .SelectMany(x => x);

This does not gives me any error but does not gives me desired result as it lists all the data. What am I missing here.

--Updated---

Earlier I wanted that the Input should not be repeated in one Output because of which I had the below query.

uData.GroupBy(l => l.Ouput)
    .Any(g => g.GroupBy(x => x.Input).Any(x => x.Count() > 1))

Now I want another query to check if the combination of Input and CreatedBy is repeated in the list.

I tried the above posted query and below query as per the suggestion:

uData.GroupBy(g=> new {g.CreatedBy,g.Input})
    .Where(w=>w.Count() > 1)

But this returns me all the list instead of the duplicate

Updated to add an example link:

https://dotnetfiddle.net/HWMYp6

I have created the example in above link.

In the example I want to mark the set with id 10 with output (output5) as the duplicate as such combination of Input and created by already existed before in id 1,2,3 (all of which belong to output1). So basically one combination of input and createby should not be repeated over another set. The reference key being Output. Sorry if my initial post was not very clear. I tried.

Answer 1

it seems like you want to group by the "created by" and "input" only in which case a slight modification to your current query should suffice:

var result = uData.GroupBy(x => (x.Input, x.CreatedBy))
                  .Where(x => x.Count() > 1)
                  .SelectMany(x => x);

I've simply removed the GroupBy for the Output field.

• GroupBy reads as "group by Input and CreatedBy"
• Where reads as "retain the groups where there are two or more items"
• SelectMany collapse the nested sequences into a IEnumerable<Data>

Update:

Given your edit, you're looking for:

var myList = uData.GroupBy(x => new {x.Input, x.CreatedBy})
                  .SelectMany(x => x.GroupBy(z => z.Output).Skip(1))
                  .SelectMany(x => x);

Answer 2

You want to know for any Output if there is a matching Input & CreatedBy so you are grouping by Input & CreatedBy with just a count of the results being greater than 1.

var myList = uData.GroupBy(g=> new {g.CreatedBy,g.Input})
    .Where(w=>w.Count() > 1)