// 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;
...

// Usage option 1. Apple recommends you use that behaviour: https://developer.apple.com/documentation/storekit/in-app_purchase/validating_receipts_with_the_app_store
// Like 'Check on prod and in case of 21004 check on sandbox'. 
// BUT I CANNOT RECOMMEND THAT WAY, because Production Server cannot switch to Sandbox based on Apple Response.
// Intruder would be able to send Sandbox data to your Server and get the Success response.
// I Recommend the second/third options.
AppleReceiptVerificationResult result = await verificator.VerifyAppleProductionReceiptAsync(appleAppReceipt).ConfigureAwait(false);
if (result.Status == IAPVerificationResponseStatus.TestReceiptOnProd)
{
    result = await verificator.VerifyAppleSandBoxReceiptAsync(appleAppReceipt).ConfigureAwait(false);
}

// Usage option 2. Determine if the Server was requested from Preview environment
// Or App belongs to Not published apps (based on version for example).
var isPreviewEnvironmentOrAppIsBelongsToUnpublishedBasedOnSomePattern = true;
result = isPreviewEnvironmentOrAppIsBelongsToUnpublishedBasedOnSomePattern
    ? await verificator.VerifyAppleSandBoxReceiptAsync(appleAppReceipt).ConfigureAwait(false)
    : await verificator.VerifyAppleProductionReceiptAsync(appleAppReceipt).ConfigureAwait(false);

// Usage option 3. Btw, you still has previous option to setup usage in the configuration during a Server Init step.
result = await verificator.VerifyAppleReceiptAsync(appleAppReceipt).ConfigureAwait(false);

// OBSOLETE USAGE (Just for Backward Compatibity):
var verificationStatus = verificationResult.Status;
var verificationReceipt = verificationResult.Receipt;
var verificationMessage = verificationResult.Message;

// USAGE (Full Apple Response Info):
var verificationStatus = verificationResult.AppleVerificationResponse.StatusCode;
var verificationReceipt = verificationResult.AppleVerificationResponse.Receipt;
var verificationLatestReceiptInfo = verificationResult.AppleVerificationResponse.LatestReceiptInfo;
var verificationPendingRenewalInfo = verificationResult.AppleVerificationResponse.PendingRenewalInfo;
var verificationMessage = verificationResult.Message;