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var pressedKeys = keyboardState.PressedKeys();
var inputActions = pressedKeys.Select(GetInputAction).ToList();
var downActions = inputActions.Select(ia => ia.Down);

This is what's painted you into the corner you're in. You're not showing your GameActions enum (a better name would be GameAction, or even better, MoveDirection..actually, Direction suffices, since all there is to it is a bunch of values that each represent a direction - the point is, enum type names should not be plural), but I presume it looks something like this:

public enum GameActions
{
 MoveLeft,
 MoveUpLeft,
 MoveUp,
 MoveUpRight,
 MoveRight,
 MoveDownRight,
 MoveDown,
 MoveDownLeft
}

The problem is that MoveUpLeft is a combination of MoveUp and MoveLeft, and since the enum type itself doesn't account for it, your code has to.

I'd suggest using a [Flags] attribute:

[Flags]
public enum Direction
{
 Left = 1,
 Top = 2,
 Right = 4,
 Down = 8
}

Then, instead of downActions being an IEnumerable<GameActions> and using LINQ IEnumearble<T>.Contains extension, you can add the values of all "down actions", and pass ActionRequested an enum value that represents the sum of all pressed directions, be it Left+Top, Down+Right or whatever. The [Flags] attribute merely serves as a visual cue to remind you that these values can be combined and, using bitwise AND operations or simply, the .HasFlag() method, the handler can determine whether the received value 3 contains Direction.Left:

void ActionRequested(Direction direction)
{
 if (direction.HasFlag(Direction.Left))
 {
 // direction contains Direction.Left
 }
 if (direction.HasFlag(Direction.Right))
 {
 // direction contains Direction.Right
 }
 if (direction.HasFlag(Direction.Top))
 {
 // direction contains Direction.Top
 }
 if (direction.HasFlag(Direction.Bottom))
 {
 // direction contains Direction.Bottom
 }
}

It's not clear from your post exactly what GetInputAction does and what downActions means, so it's hard to recommend a change that will fit into your code, but here's some inspiration:

class Program
{
 [Flags]
 public enum Direction
 {
 Left = 1,
 Top = 2,
 Right = 4,
 Bottom = 8
 }
 static void Main(string[] args)
 {
 var direction = (Direction)3;
 Console.WriteLine("Direction: {0}", direction);
 Console.WriteLine("Left: {0}", direction.HasFlag(Direction.Left);
 Console.WriteLine("Top: {0}", direction.HasFlag(Direction.Top);
 Console.WriteLine("Right: {0}", direction.HasFlag(Direction.Right);
 Console.WriteLine("Bottom: {0}", direction.HasFlag(Direction.Bottom);
 Console.ReadLine();
 }
}

Output:

enter image description here