Useful Delegates in .NET

Func<T, TResult> Delegate
Encapsulates a method that has one parameter and return a value of the type specified by the TResult parameter.

Use this method to represent a method that can be passed as a parameter without explicitly declaring a custom delegate.


Action<T>  Delegate
Encapsulate a method that has one parameter and does not return a value.

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Lambda Expressions in C#

Lambda expression is a anonymous function that you can use to create delegates or expression tree types. These are useful for writing LINQ query expressions.

Expression Lambdas

A lambda expression with a statement on right side of => operator. These are used to construct expression trees. 

Statement Lambdas

Resembles an expression lambda except that statements are enclosed in braces. 

Async Lambdas

Incorporate asynchronous processing by using async and await keywords.

Generic Delegates ##

a delegate can define its own type parameters. A code that uses generic delegate can specify type argument to create a closed constructed type, just like when instantiating or creating a generic. 


What is a method group in C# (SO)

Lambdas with Standard Query Operators

Many standard query operators have an input parameter whose type is one of Func<T,TResult> family of generic delegates. 

Type inference in Lambdas

Usually you don't have to specify the type of input parameters because compiler can infer the type based on lambda body,  the parameters delegate type etc. 

Resources

Lambda Expressions (MSDN)
Lambda Expressions and Expression Trees Introduction (CodeProject)
Using Variance for Func and Action Generic Delegates 

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Session State Modes in ASP.NET

ASP.NET session state supports several different storage options for session data. Each option is identified by value in the SessionStateMode enum. ASP.NET Session has the type HttpSessionStateBase type. Session has 2 methods, Session.Clear() and Session.Abandon(). See the difference.

InProc (default)

Store session state in web server memory.

StateServer 

• Store session state in a separate process called ASP.NET state service. 
• This ensures session state is preserved if the web app is restarted. 
• Makes session state available to multiple web servers in a server farm. 
• If using stateServer the objects stored in the session should be serializable. 
• If using in a server farm you must have same machine key in all machines. 

SQL Server

• Store in SQL Server DB. Same as state server, ensures preservation of data when web app restarted and on web farms. 
• Objects should be serializable
• You must first install ASP.NET session state database in SQL Server
• You can use fail over cluster 

Custom

Enables you to specify custom storage provider. See how to implement session state store provider. There's one for Redis.

Off

Disable session state
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HttpSessionState 

• Provides access to session-state values as well as session-level settings and lifetime management methods.
• Enables you to store information associated with a unique browser session across multiple requests
• Can access these through Session property of the current HttpContext, or Session property of the Page
• Session data is associated with a specific browser using a unique identifier. By default, this identifier is stored in non-expiring cookie in the browser.
• You can also use Timeout property set a timeout
• When a new session begins session Start event is raised
• When session times out, the Abandon method is called.
• When ASP.NET application shut down the session End event is raised. (The End event is raised only when the session state mode is set to InProc)
• Session state does not persist across application boundaries
• Session values are stored in the memory of the web server.

Session object in ASP.NET

http://stackoverflow.com/questions/2874078/asp-net-session-sessionid-changes-between-requests
http://stackoverflow.com/questions/tagged/sessionid
http://stackoverflow.com/questions/940742/difference-between-session-and-httpcontext-current-session

In ASP.NET For session state management we have,

• View State
• Control state
• Hidden fields
• Cookies
• Query Strings

ASP.NET MVC doesn't have view state or postbacks.

Changing ASP.NET_SessionId cookie name
http://stackoverflow.com/q/1017144/340046

Resources
Session-State Modes - MSDN
http://blogs.msdn.com/b/kenkilty/archive/2014/07/03/asp-net-session-state-using-sql-sever-in-memory.aspx

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Delegates in C#

In simple words delegate is a type that references to a method with a particular parameter list and a return type. You can invoke(call) the method through the delegate instance. Delegates are used to pass methods as arguments to other methods. Event handlers are methods invoked through delegates. Delegate is a similar concept as function pointers in C and C++.

The ability to pass methods as parameters makes delegates ideal for defining callbacks. (When to use delegates, link2). Delegates are ideal to encapsulate codes. For example when you attach an event handler to a button, the handler becomes a delegate. The button doesn't need to know what it does. (Difference between Delegate and an Event).

Delegate types are derived from Delegate class. Delegate types are sealed and cannot be derived from. The ability to pass delegate as a parameter to a method and to call the delegate at a later time is known as asynchronous callback. 

When to use Delegates and Interfaces

Interfaces and delegates allows developer to separate type declaration and implementation. Use the following guidelines to decide.

Use delegates when,

• Event based design pattern is used
• Easy composition is desired

Use interfaces when,

• There are group of related methods that might be called
• Class only needs one implementation of the method

Source : MSDN, StackExchange

Delegates with Named Methods

Delegate with Anonymous Method

C# 2.0 introduced anonymous methods. Anonymous methods enables you to omit parameter list. This means anonymous method can be converted to delegates with variety of signatures. (This is not possible with Lambda expressions). Anonymous methods provide a way to pass a code block as a delegate parameter. They are basically methods without a name.

Lambda Expressions

Lambda expression is a anonymous function that you can use to create delegates or expression tree types. These are useful for writing LINQ query expressions. 

To create lambda expressions you specify any input parameters on the left side of => lambda operator and the expression or statement block on the other side. You can assign this expression to a delegate type. 



There are different types of lambda expressions,

• Expression lambdas
• Statement lambdas
• Async lambdas

Lambda Expressions (MSDN)

Resources

http://msdn.microsoft.com/en-us/library/ms173171.aspx
http://msdn.microsoft.com/en-us/library/98dc08ac.aspx 

Ways to declare, instansitate and use delegates (MSDN)

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HTML Elements

Upto HTML 4.01. block level elements and inline elements. 

inline elements: 

• respect l to r margins and padding. not top and bottom
• cannot have width and height set
• allow other elements to sit to their l and r
• can display things before and after it, on the same line.
• span, a, strong, em, img, input, abbr, acronym

block

• respect all of those
• force a line break after the block element
• demands its own line with whitespace around it. 
• <p>, <div>, h1 to h6, ul, ol, dl, li, dt, dl, table, pre, form

From HTML5 onwards

https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Content_categories

Talking about attributes, you can use data-* attributes to store extra information on standard semantic HTML elements. (link)

HTML5 WAI-ARIA is a spec defining support for accessible web apps. (link, link)

Difference between aria and data attributes

s

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