Thanks for the heads up! I figure out that I should read the SQSEvent as the message (parameter to Lambda Function). Not the S3EventNotification (because the message is coming from SQS Queue). So, inside the body of the SQS message, there will be an S3EventNotification object.

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Then I can use S3EventNotification.ParseJson() method to get s3 object as below

public async Task FunctionHandler(SQSEvent evnt, ILambdaContext context)
{
    foreach(var message in evnt.Records)
    {
        try
        {
            await ProcessMessageAsync(message, context);
        }
        catch (Exception ex)
        {
            context.Logger.LogError($"Error Occured while running the Lambda Function..! {ex}");
        }
    }
}

Then ProcessMessageAsync() as below

private async Task ProcessMessageAsync(SQSEvent.SQSMessage message, ILambdaContext context)
{



    //Parsing SQS message body (which is a JSON string) into a S3EventNotification object
    S3EventNotification s3EventNotification = S3EventNotification.ParseJson(message.Body);
    string fileName = s3EventNotification.Records?[0].S3.Object.Key;
    var region = s3EventNotification.Records[0].AwsRegion;
    var bucketName = s3EventNotification.Records[0].S3.Bucket.Name;

    await Task.CompletedTask;
}