CompareFacesRequestSample.cs

var image1 = await File.ReadAllBytesAsync("path\\to\\source\\image");
var image2 = await File.ReadAllBytesAsync("path\\to\\target\\image");

const float SIMILARITY_THRESHOLD = 75;
const float CONFIDENCE_LEVEL = 95;

using(var source = new MemoryStream(image1))
using(var target = new MemoryStream(image2))
{
    var request = new CompareFacesRequest
    { 
      SourceImage = new Image { Bytes = source }, 
      TargetImage = new Image { Bytes = target }, 
      SimilarityThreshold = SIMILARITY_THRESHOLD 
    };

    var reponse = await amazonRekognitionClient.CompareFacesAsync(request);

    Console.WriteLine(HaveSamePerson(reponse.FaceMatches, similarity:SIMILARITY_THRESHOLD, confidence: CONFIDENCE_LEVEL)
            ? "The person in the source image is in the target image"
            : "The person in the source image isn't in the target image");
}