CommentsController.cs

private const string TableName = "comments";
private readonly IAmazonDynamoDB _amazonDynamoDb;

public CommentsController(IAmazonDynamoDB amazonDynammoDb)
{
    _amazonDynamoDb = amazonDynammoDb;
}

[HttpGet]
public async Task<ScanResponse> GetComments()
{
    var request = new ScanRequest
    {
            TableName = TableName
    };

    var response = await _amazonDynamoDb.ScanAsync(request);
    return response;
}

[HttpGet("{url}")]
public async Task<ActionResult<string>> Get(string url)
{
    var request = new GetItemRequest
    {
        TableName = TableName,
        Key = new Dictionary<string, AttributeValue>
        {
            {
                "postId",
                new AttributeValue
                {
                    S = url
                }
            }
        }
    };

    var response = await _amazonDynamoDb.GetItemAsync(request);

    return response.Item["username"].S;
}