C# Expand Flat List<T> to Dictionary<T,ICollection<int>>

Question

I've got a List<MyClass>:

public int MyClass
{
   public int One { get; set; }
   public int Two { get; set; }
}

Now, the data can (and does) look like this:

One: 1, Two: 9

One: 2, Two: 9

One: 1, Two: 8

One: 3, Two: 7

See how "One" appears twice? I want to project this flat sequence into a grouped Dictionary<int,ICollection<int>>:

KeyValuePairOne: { Key: 1, Value: { 9, 8 }}

KeyValuePairTwo: { Key: 2, Value: { 9 }}

KeyValuePairThree: { Key: 3, Value: { 7 }}

I'm guessing i need to do a combination of .GroupBy and .ToDictionary?

Answer 1

This is what ToLookup extension method is for. Where ToDictionary will throw if the key shows up twice, ToLookup instead is fine with it and does what you're looking for.