Question

Given

Dictionary<string, List<string>> myDict = new Dictionary<string, List<string>>()
{
    "Apples", new List<string>() { "Green", "Red" },
    "Grapefruits", new List<string>() { "Sweet", "Tart" },
}

I wish to create a mapping from the child to the parent, e.g.

“Green” => “Apples”

In my particular use case, the child strings will be globally unique (e.g. no Green Grapefruits to worry about), so the mapping could be to a Dictionary<string,string>.

It’s fairly straightforward to accomplish by iterating myDict conventionally.

Dictionary<string, string> map = new Dictionary<string,string>();
foreach (KeyValuePair<string, List<string>> kvp in myDict)
{
    foreach (string name in kvp.Value)
    {
        map.Add(name, kvp.Key);
    }
}

Can this be done with Linq?

There is a very similar question about just flattening the same data structure

Flatten a C# Dictionary of Lists with Linq

However that does not maintain the relationship to the dictionary key.

I reviewed a nice visual tutorial on SelectMany (the approach used in the related question) but see no way to relate the key.

Answer 1

Sounds like you want:

var query = myDict.SelectMany(pair => pair.Value,
                              (pair, v) => new { Key = v, Value = pair.Key })
                  .ToDictionary(pair => pair.Key, pair => pair.Value);

Note that the second argument to SelectMany here looks a little odd here, because the original key becomes the value in the final dictionary, and vice versa.