// Register DI services...
services.RegisterAppleReceiptParser();
...
// ... and resolve the service later.
IAppleReceiptParserService parserService;
...
// Get your base64 Apple Receipt
const string appleAppReceipt = "{receipt_base64_string}";
// Convert to Bytes
byte[] data = Convert.FromBase64String(appleAppReceipt);
// Get parsed receipt
AppleAppReceipt receipt = parserService.GetAppleReceiptFromBytes(data);

// (Optional) You can create implementation of custom validation process:
services.AddScoped<IAppleReceiptCustomVerificatorService, AppleReceiptCustomVerificatorService>();
...
// Fill settings:
services.RegisterAppleReceiptVerificator(x =>
{
    x.VerifyReceiptSharedSecret = "XXXX"; // Apple Shared Secret Key
    x.VerificationType = AppleReceiptVerificationType.Sandbox; // Verification Type: Sandbox / Production
    x.AllowedBundleIds = new[] {"com.mbaasy.ios.demo"}; // Array with allowed bundle ids
});
...
// ... and resolve the service later.
IAppleReceiptVerificatorService verificator;
...
AppleReceiptVerificationResult result = await verificator.VerifyAppleReceiptAsync(appleAppReceipt);