Creating a dictionary of strings and list of strings

Question

I have a List of objects, List<myObject> objectList, and each object contains a List of strings like so:

myObject: StringList( "A_1", "B_1", "C_1", "D_1", "E_1", "F_1" ) 
myObject: StringList( "A_2", "B_2", "C_1", "D_2", "E_2", "F_1" ) 
myObject: StringList( "A_2", "B_3", "C_1", "D_3", "E_2", "F_1" ) 

I'm trying to merge the lists into a dictionary of: Dictionary<string, List<string>>, where the final results will look like so:

Dictionary: { [A:1,2] [B:1,2,3] [C:1] [D:1,2,3] [E:1,2] [F:1] }

Here is what I have done, and it does work:

Dictionary<string, List<string>> dict = new Dictionary<string, List<string>>();
foreach (myObject result in objectList)
{
 foreach (var item in result.StringList)
 {
 string Key = item.Split('_')[0];
 string Value = item.Split('_')[1];
 List<string> sValue = new List<string>();
 bool exists = dict.TryGetValue(Key, out sValue);
 if (exists && !sValue.Contains(Value))
 {
 sValue.Add(Value);
 dict[Key] = sValue;
 }
 else if (!exists)
 {
 sValue = sValue ?? new List<string>();
 sValue.Add(Value);
 dict.Add(Key, sValue);
 }
 }
}

Is there a better way to do this, where I don't have to use two foreach loops? Is there a way to do it without using any foreach loops?

I have tried using lambda but this is as far as I got:

Dictionary<string, List<string>> dict = new Dictionary<string, List<string>>();
foreach (myObject result in objectList)
{
 dict = result.StringList.Select(x => x.Split('_'))
 .GroupBy(x => x[0])
 .ToDictionary(x => x.Key, x => x.Select(g => g[1]).ToList());
}

The problem is that the expression keeps overwriting any existing entries in the dictionary each time I iterate through the loop.

Is there a way to keep the existing entries, groupby the Key and add to the existing list of values but don't duplicate any values?

I think the answer lies in this part of the expression:

x => x.Select(g => g[1]).ToList()

but I'm not 100% sure.

• Mike Cheel: Did you try SelectMany?

Answer 1

Just get rid of the outer foreach as well:

dict = objectList.SelectMany(x => x.StringList)
 .Select(x => x.Split('_'))
 .GroupBy(x => x[0])
 .ToDictionary(x => x.Key, x => x.Select(g => g[1]).Distinct().ToList());

SelectMany retrieves all the StringLists and flattens them into one single list:

"A_1", "B_1", ..., "F_1", "A_2", "B_2", ...

• Chris: I think you want a distinct in there as well so that A is 1,2 rather than 1,2,2.
• Ahmed KRAIEM: x => x.Select(g => g[1]).Distinct().ToList()
• Tim S.: Note that Distinct destroys order, so it might be 1,2 or 2,1.
• user3042376: @Heinzi - Thanks so much! This works perfectly! Much simpler and more elegant. For the life of me, I couldn't figure this out...a million times thank you!
• Tim S.: @Groo Like a comment at my link says, "what you say could be true [that it preserves first-found order], but it would be a bad idea to rely on that behavior" (because it is documented as being an unordered sequence, you should treat it as such). Also, I don't know that objectList had it sorted to begin with, the example just happens to have it so (I think).