"Cannot get Mail" The user name or password for imap.gmail.com is incorrect iOS

For iOS if you are using Apple configurator to setup gmail make sure you follow these steps.

https://discussions.apple.com/message/26827218#26827218

If you followed the steps above and still getting the incorrect username or password error make sure you have IMAP enabled on your gmail account.

Still if the issue persists then in your google/gmail security settings make sure you have enabled the Allow less secure apps flag.

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The this keyword in javascript explained

this keyword in Javascript is a source for confusion. The confusion is mostly due to the fact that the 'this' keyword is used differently in other languages compared to Javascript.

I was also confused of this (Well I still am sometimes) as I was coming from a C# background where the 'this' keyword meant the current instance of that class.

I hope the following will help someone to avoid the 'this' confusion altogether in Javascript.

Let's take an example,


function bar()
{
    console.log(this.age);
}


So what will be the value of 'this'?

Is it an object or is it referring to the function it self?

What if I told you that you cannot answer this question at all at compile time.

The trick is that the value of 'this' would depend on how the function is called.

e.g 1, (Run this on the browser JS console)

bar();

The output would be undefined.

So what was the 'this' object in the above example ? It was the global object (aka window object in the browser)

So naturally the window object did not have a property named 'age' thus the undefined output.

An easy way to think of this is who is calling the bar() function, if its no one then 'this' is going to be the global object.


e.g 2,

var x = { age : 10 , foo : bar };

x.foo();

The above code will actually call the bar function as foo is pointing to bar.

The output will be 10

So in this example bar is being called by x and thus the 'this' object is the x object.


e.g 3,
var y = { age : 15 };
bar.call(y);

The output will be 15

This is because the call() function explicitly sets the this object to the first argument passed into the call function.


e.g 4,

var z = new bar();

Now the 'this' object will be a brand new object and it will also get assigned to z.
This is because of the nature of the 'new' keyword in Javascript.

So the bottom line is the 'this' keyword will depend on how its containing function is called and look to the left of the function call site to determine 'this'


Hope this helps.

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ASP.NET Concurrent Ajax Requests and slow performance

While working on a client project I came across an interesting issue. That is when concurrent Ajax requests (originating from Jquery ajax) came in the asp.net web service (A set of aspx web methods in this case) were performing ridiculously slow. The methods it self were executing pretty fast. It turned out that when you have "[WebMethod(EnableSession = true)]" the requests are processed sequentially with in the same session.

The fix is to make the session read only, that is if your web methods are not modifying the session.

If any other page methods are modifying the session then the best approach is to move your web methods to a different aspx file and make the session read only for that page. Below given is the page directive for doing that,

<% @Page EnableSessionState="ReadOnly" %>

Hope this helps.

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cassandra vs rdbms data model

If you are coming from a relational database world like me then the initial terminology used in cassandra will be a bit confusing.
I'll state some keywords which are used in cassandra and try to draw parallels of them to the relational world.

Note: These parallels are just for ease of understanding and actually does not equal to each other at all.

Cassandra keyword	RDBMS keyword
Keyspace	Database
Column family	Table
Column	Table Column
Row	Table row

The outermost element of Cassandra is a cluster which can be one or more nodes set up as a ring which appears as a single instance to the end user.

A cluster usually contains one or more keyspaces (Databases).

A keyspace contains one or more column families  (Tables)

A column family contains one or more rows. (Slightly different from a Table which would contain columns )

A row contains one or more columns.

The biggest difference is that the number and type of columns in rows can  differ from one another thus making the Cassandra column family much more flexible compared with the rigid table structure in the rdbms world.

In other words each row in  a column family can have its own schema. This is an extremely useful option for storing dynamic data

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Cannot read remote side has closed Thrift Cassandra C#

I was seeing this error ("Cannot read remote side has closed") while trying to get Thrift API to work with Cassandra using Csharp. For the life of me I couldn't figure out what was wrong as even though the error is saying something like "Hey I think your Thrift server is down". That was not the case because I was connecting to My Cassandra server using other client libraries that was using Thrift under the hood. Eventually I figured out the error, the offending line was transport.Open(); When I replaced it with cassandraClient.InputProtocol.Transport.Open(); The error went away.

Here is the full snippet,

            TSocket socket = null;

            TTransport transport = null;

        

            socket = new TSocket("localhost", 9160);

          

          

            transport = new TFramedTransport(socket);

            TProtocol protocol = new TBinaryProtocol(transport);

            CassandraClient cassandraClient = new CassandraClient(protocol);

            cassandraClient.InputProtocol.Transport.Open();



            string s = cassandraClient.describe_cluster_name();

            List<ksdef> keyspaces = cassandraClient.describe_keyspaces();

Hope this will help another poor soul.

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SQL GROUP BY Example

The GROUP BY clause causes data in a table (or any data source) to be divided into groups based on the expressions (or conditions) given in the GROUP BY clause. Seems confusing? Probably.. Let's see what it means,

Say you have Column1, Column 2..etc on your table.

Now you say GROUP BY Column1 , what this does is it divides your table's data based on Column1. So all data which has  Column1='Apple' will be in one group, then all data which has  Column1='Orange' will be in another group.

Again if you had GROUP BY Column1, Column2 then your data will be divided into groups of Column1 + Column2 combinations . If Column2 had values of Green and Red in different rows then one group would be based on Apple+Red another group Apple+Green. So you get the point.

So in a nutshell,

GROUP BY creates groups for output rows, according to unique combination of values specified in the GROUP BY clause

But why would you need to divide them into groups? So we can operate on those groups.

I'll use AdventureWorks database as an example.

Download it here,
https://msftdbprodsamples.codeplex.com/releases/view/93587

SELECT SalesPersonID, YEAR(OrderDate) AS OrderYear,

COUNT(CustomerID) AS All_Customers

FROM Sales.SalesOrderHeader

GROUP BY SalesPersonID, YEAR(OrderDate);

The above query will return all unique combinations of SalesPersonID  + YEAR(OrderDate) and display all customers each of those combinations.

In other words this means you are seeing all customers for a particular Sales Person in a given year.

Point to note:

SELECT DISTINCT SalesPersonID ,YEAR(OrderDate) AS OrderYear

FROM Sales.SalesOrderHeader

and

SELECT  SalesPersonID ,YEAR(OrderDate) AS OrderYear

FROM Sales.SalesOrderHeader

GROUP BY SalesPersonID, YEAR(OrderDate);

Will give you the same results.

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ASP.Net Webforms Inline tags and code blocks

There are many elements inside a webforms (.aspx) page. To somebody beginning asp.net webforms these may seem quite similar and be confusing at first. (Well it was for me when I started learning asp.net webforms)
So hopefully this might help another soul.

Elements of a asp.net page (Inline tags and code blocks) can be listed as follows,

1. Directives

These come in many flavors like Page directives,
 ex : <%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs"  Trace="true" %>

or  Import directives,

ex : <%@ namespace="System.Drawing" %>

There are many more directives other than these two.

2. ASP.Net expressions

This is expression builder syntax, typically used to get values from web.config

ex: <%$ ConnectionStrings:AdventureWorks2008 %>  

3. User Controls

These are references to user created controls which are similar to web controls that enhances reusability 

ex: <PH:MainMenu Title="Menu Control" id="mainmenu" runat="server" />

4. Render blocks

Used for outputting elements like variables

ex:  If you have a variable named age then,

<%= age %>

will output the value of age

5. Code blocks

These are similar to javascript tags just that these run on the server,

<script runat="Server" language="c#" > ....some server side code here .... </script>  

6. Server Controls

These are the controls we drag and drop from the designer (Labels, Buttons etc)

ex: <asp:Label id="myLabel" runat="Server" />

7. Data binding controls

Used for biniding expressions such as Eval or Bind, these are typically used with data controls like Gridview
ex : <%# Eval("DBFieldName") %>

   

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DataContract vs Serializable WCF

When applied to a class Serializable attribute serializes everything inside of it including private fields thus the developer has no control as what to expose.

When DataContract attribute is applied to a class it will serialize only the elements decorated with DataMember attribute thus the developer will have more control.

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ServiceContract Name property WCF

This property can give your contract a name which is different from the actual interface name. So this way if you decide to change the interface name of a already running WCF service your client wont break.

Ex:

 [ServiceContract(Name="IGoodbyeService")]

    public interface IHelloService

    {

        [OperationContract]

        string GetMessage(string name);

    }

Now the contract name would be IGoodbyeService rather than  IHelloService.

To verify in the WSDL generated check the porttype property it should be IGoodbyeService

Note: Same can be done for OperationContract as well 

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Basic Elements of WCF Appconfig

All config items should be inside ,

<system.serviceModel>

This is the WCF namespace provided by Microsoft.

Inside the <system.serviceModel> element you have the <services> element. This is where you configure all your services.
For each service you have a <service> element inside <services> element.
A particular service can be exposed using more than one endpoint. (Ex: http , net.tcp etc)
So for each endpoint in a service you configure a <endpoint> element.
An endpoint element will have three essential things,

1. address
2. binding
3. contract  

Here is a  sample config (not complete) to get an idea.

The service name is HelloService (class and namespace both are HelloService) and has two endpoints (http and net.tcp) for the service.

<?xml version="1.0" encoding="utf-8" ?>


<configuration>

  <system.serviceModel>

    <services>

      <service name="HelloService.HelloService">

        <endpoint address="HelloService" binding="basicHttpBinding" contract="HelloService.IHelloService">

        </endpoint>

        <endpoint address="HelloService" binding="netTcpBinding" contract="HelloService.IHelloService">

        </endpoint>

      </service>

    </services>

  </system.serviceModel>

</configuration>

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system.web vs system.webserver

I have not worked in IIS6 and mostly I have worked on IIS 7+ and this came up when I was following a very old article (2003) ASP.Net article.

Anyway system.web is IIS6 and classic mode.
system.webserver is IIS7+  and integrated mode.

One scenario you might come across is when adding httpmodules to your ASP.Net project. If you want backward compatibility then you should add the module section to both system.web section (which is there by default for all ASP.Net projects ) and to system.webserver.

<system.web>
    <httpModules>
    </httpModules>
</system.web>

And

<system.webServer>
    <modules>
    </modules>
</system.webServer>

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SQL JOIN Example

Joins can be quite perplexing initially hopefully this simple example will help. 

Joins are used to combine data from multiple tables (usually two tables)

There are three types of joins,

1. INNER

2. OUTER - FULL, LEFT, RIGHT

3. CROSS

For all examples I'll be using the below given tables and use the Name field as the condition.

                                                                         Table1

Id1	Name1
1	Padmika
2	Dissanayake
3	Sukitha
4	Pahan

                                                                        Table2

Id2	Name2
1	Fernando
2	Malaka
3	Padmika
4	Sukitha

1. INNER JOIN

Returns only the set of records that match the Name field in both Table 1 and Table2

SELECT * FROM Table1

INNER JOIN Table2

ON Table1.Name1 = Table2.Name2

Result,

Id1	Name1	Id2	Name2
1	Padmika	3	Padmika
3	Sukitha	4	Sukitha

* This gets executed in the following manner, first get the first record from Table 1 get its name field compare it to Table 2 first record name field if it’s a match return it, now compare with Table2 second record name field…  etc.

You will see that only the records that have a common name is returned

This is similar to the intersection of two sets (Table1 and Table2) based on Name.  

2. OUTER JOIN

2.1 FULL OUTER JOIN

Returns all records in Table 1 and Table 2, with matching records from both sides where available. If there is no match, the missing side will contain null.

SELECT * FROM Table1

FULL OUTER JOIN Table2

ON Table1.Name1 = Table2.Name2

Result,

Id1	Name1	Id2	Name2
1	Padmika	3	Padmika
2	Dissanayake	null	Null
3	Sukitha	4	Sukitha
4	Pahan	null	Null
null	Null	1	Fernando
null	Null	2	Malaka

This is similar to the union of two sets (Table1 and Table2) based on Name.

2.2 LEFT OUTER JOIN

 Returns the complete set of records from Table 1 (Left table), with the matching records for Name field (where available) in Table 2. If there is no match, the right side will contain null.

SELECT * FROM Table1

LEFT OUTER JOIN Table2

ON Table1.Name1 = Table2.Name2

Results,

Id1	Name1	Id2	Name2
1	Padmika	3	Padmika
2	Dissanayake	null	null
3	Sukitha	4	Sukitha
4	Pahan	null	null

Special case of the above result would be to select records that do not have any matching records from the right table (Table2)

To do this just add a ‘WHERE Table2.Id2 IS NULL’ clause to the above query.

Ex: You have a Customer table and an Order table (Has a CustomerID column) and want to find out which customers have not placed any orders  

2.3 RIGHT OUTER JOIN

Similar to the above but all the right table (Table2) records will return rather than the left table ones.

SELECT * FROM Table1

RIGHT OUTER JOIN Table2

ON Table1.Name1 = Table2.Name2

Results,

Id1	Name1	Id2	Name2
null	null	1	Fernando
null	null	2	Malaka
1	Padmika	3	Padmika
3	Sukitha	4	Sukitha

Note : When inspected carefully you will see that in this example,

OUTER JOIN result = LEFT JOIN result + RIGHT JOIN result – INNER JOIN result

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SQL COUNT Example

COUNT returns the number of rows in a select statement (Also note that it doesn't consider null values),

Note : I'll be using the AdventureWorks2008R2 sample database which can be downloaded from here,

http://msftdbprodsamples.codeplex.com/releases/view/59211


Example 1,

/* By using the * we return the number of records in the Sales.Customer table */

select

count(*)

from Sales.Customer;

Example 2,

/* By using StoreID inside the COUNT() function we return the number of store id's in the Sales.Customer table */

select

count(StoreID)

from Sales.Customer;

Example 3,
  /* By using "DISTINCT StoreID" inside the COUNT() function we return the unique number of store id's in the Sales.Customer table */

select

count(distinct StoreID)

from Sales.Customer;

Note: From a performance point of view it is better to use 1 instead of * inside the COUNT function.

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DDL vs DML in SQL Server

DDL
Data Definition Language (DDL) statements are used to define the database structure or schema. Examples:

• CREATE - to create objects in the database
• ALTER - alters the structure of the database
• DROP - delete objects from the database

DML
Data Manipulation Language (DML) statements are used for managing data within schema objects. Examples:

• SELECT - retrieve data
• INSERT - add data
• UPDATE - modify data
• DELETE - remove data

Note : This is not limited to SQL Server

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SQL Server Data Types

Here are the SQL server data types for quick reference,

1. Exact numerics – (bigint, bit, decimal, int, money, numeric, smallint)
2. Approximate numerics (float, real)
3. Date and time (date, datetime2, datetime, datetimeoffset, time)
4. Character strings (char, varchar, text)
5. Unicode character strings (nchar, ntext, nvarchar)
6. Binary strings (binary, varbinary, image)
7. Other data types (cursor, timestamp, uniqueidentifier, table)
8. Large valued data types (varchar(max), nvarchar(max))
9. Large object data types (text, ntext, image, xml)

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ASP.NET ViewState example

ViewState is a client side state management mechanism. It is not the only state management mechanism. Some other examples would be,

For client side state management,

·         Hidden Field

·         Cookies

·         Control State

For server side,

·         Session

·         Application Object

·         Caching

·         Database

ViewState should be used to persist only small amounts of data as having lots of data in ViewState will degrade performance.

Also by default ViewState does not encrypt data but only encodes them to Base64. So generally sensitive data should not be kept there.

However you can encrypt ViewState by setting  ViewStateEncryptionMode="Always" as a page level directive,

<%@Page ViewStateEncryptionMode="Always" %>

 or for all pages by setting in web.config,

<configuration>
   <system.web>
      <pages ViewStateEncryptionMode="Always" />
   </system.web>
</configuration>

There is a lot of confusion about ViewState and Form input preservation on postbacks.

An example would be, how a textbox persists data even when View State is set to off.

This is because many ASP.Net controls (TextBox, ListBox, RadioButtonList etc) have implemented the IPostBackDataHandler interface and calls the LoadPostData(), which repopulates them.

Here is a small example of using ViewState persist state,

1. Create a ASP.Net web application 
2. Add a Web Form named ViewStateExample
3. Paste the below given code to the apsx and apsx.cs files respectively
4. Make sure to rename of the namespace LearnASPWebForms to your namespace

  

ViewStateExample.aspx

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="ViewStateExample.aspx.cs" Inherits="LearnASPWebForms.ViewStateExample" %>



<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" >

<head id="Head1" runat="server">

    <title>ViewStateExample</title>

</head>

<body>

    <form id="testform" runat="server">

        <asp:Label runat="server" Text="Enter a Value" />

        <asp:TextBox runat="server" id="FieldToBePreserved" /><br />

        <asp:Button runat="server" id="SubmitForm" onclick="SubmitForm_Click" text="Submit and add to ViewState" />

        <asp:Button runat="server" id="SubmitDontSave" text="Submit but dont save to ViewState" OnClick="SubmitDontSave_Click"  />

        <br /><br />

        Value from ViewState: <asp:Label runat="server" id="ViewStateValue" BorderStyle="Dotted" />

    </form> 

</body>

</html>

CodeBehind ViewStateExample.aspx.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace LearnASPWebForms
{
    public partial class ViewStateExample : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (ViewState["Field"] != null)
                ViewStateValue.Text = ViewState["Field"].ToString();
            else
                ViewStateValue.Text = "Nothing in ViewState";
        }

        protected void SubmitForm_Click(object sender, EventArgs e)
        {
            ViewState["NameOfUser"] = FieldToBePreserved.Text;
            ViewStateValue.Text = FieldToBePreserved.Text;
        }

        protected void SubmitDontSave_Click(object sender, EventArgs e)
        {
            //This does nothing
        }

    }
}

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