Explain Dependency Injection with an example

One of the very common interview questions, asked these days. This is the most common approach used today to solve dependencies between objects. In many of the enterprise class ASP.NET application, Dependency Injection is a common standard to follow. Let us understand Dependency Injection with an example.
In the example above, Employee class depends on EmployeeDAL class to get the data from the database. In GetAllEmployees() method of the Employee class, we create an instance of the EmployeeDAL (Employee Data Access Layer) class and then invoke SelectAllEmployees() method. This is tight coupling, EmployeeDAL is tightly copuled with the Employee class. Everytime the EmployeeDAL class changes, the Employee class also needs to change. EmployeeDAL cannot be mocked and hence unit testing becomes cumbersome and time consuming.

The same example can be re-written using dependency injection as shown below. First thing to notice is that, we are using interface types instead of concrete types. Using interfaces help us to plugin any implemenation of the interface, with less or no code modification at all. We are not creating the instance of the EmployeeDAL in the Employee class, instead we are passing it as a parameter to the constructor of the Employee class. As, we are injecting an instance of a class into a class that depends on it, we can call this process as Dependency Injection.
Dependency Injection is of 2 types.
1. Constructor Injection
2. Setter Injection.

We have already seen how to use Constructor Injection in the example above. An, example for Setter Injection is shown below. We are injecting an object instance through the Setter property, instead of a constructor. Hence, we call Setter Injection. It is very important to use the property EmployeeDataObject to access the instance of IEmployeeDAL, rather than the private variable employeeDAL. The property checks to see if employeeDAL is null, and throws the exception accordingly.

Difference between EnableViewState and ViewStateMode properties

1. Using EnableViewState property we only have 2 options
     We can turn off view state altogether,
                              or
     Enable viewstate for the entire page and then turn it off on a control-by-control basis.

2. If you want to turn of ViewState for the entire page and only enable it for specific controls on the page, then we have to use ViewStateMode property in conjunction with EnableViewState.

3. EnableViewState property only accepts true or false values and the default value is true, where as ViewStateMode property can have a value of - Enabled, Disabled and inherit. Inherit is the default value for ViewStateMode property.

4. ViewStateMode property is introduced in ASP.NET 4, where as EnableViewState exists from a long time.

5. If EnableViewState is to True, only then the ViewStateMode settings are applied, where as, if EnableViewState is set to False then the control will not save its view state, regardless of the ViewStateMode setting. In short if EnableViewState is set to False, ViewStateMode setting is not respected.

6. To disable view state for a page and to enable it for a specific control on the page, set the EnableViewState property of the page and the control to true, set the ViewStateMode property of the page to Disabled, and set the ViewStateMode property of the control to Enabled.

ASP.NET Page is very slow. What will you do to make it fast

This is a very common asp.net interview question asked in many interviews. There are several reasons for the page being slow. We need to identify the cause.

1. Find out which is slow, is it the application or the database : If the page is executing SQL queries or stored procedures, run those on the database and check how long do they take to run. If the queries are taking most of the time, then you know you have to tune the queries for better performance. To tune the queries, there are several ways and I have listed some of them below.
   a) Check if there are indexes to help the query
   b) Select only the required columns, avoid Select *.
   c) Check if there is a possiblity to reduce the number of joins
   d) If possible use NO LOCK on your select statements
   e) Check if there are cursors and if you can replace them with joins

2. If the queries are running fast, then we know it is the application code that is causing the slowness. Isolate the page event that is causing the issue by turning tracing on. To turn tracing on, set Trace="true" in the page directive. Once you have tracing turned on you should see trace information at the bottom of the page as shown in the image below. In this case Page Load event is taking the maximum time. So we know, the code in Page_Load event is causing the issue. Once you look at the code, you should be able to nail down the issue.

When I create a new ASP.NET 4 web application, the web.config file is almost empty. What happened to all the configuration elements that were there prior to ASP.NET 4?

All the major configuration settings are moved into machine.config file, and all the applications will inherit the setting from this file. If an application needs to override the default settings, we can do so using the application specific configuration (web.config) file. ASP.NET 4 applications have clean web.config files.


If you create a new asp.net 4 empty web application, the only entry that you will find is shown below.



Note: If you create a new ASP.NET Web Application, instead of a new ASP.NET Empty Web Application, you will find a lot more confiuration entries in the web.config file. This is because, the template for ASP.NET web application overrides some of the default settings inherited from machine.config.

MVC Interview Questions on razor views

What are the 2 popular asp.net mvc view engines?
1. Razor
2. .aspx

What symbol would you use to denote, the start of a code block in razor views?
@

What symbol would you use to denote, the start of a code block in aspx views?
<%= %>

In razor syntax, what is the escape sequence character for @ symbol?
The escape sequence character for @ symbol, is another @ symbol

When using razor views, do you have to take any special steps to proctect your asp.net mvc application from cross site scripting (XSS) attacks?
No, by default content emitted using a @ block is automatically HTML encoded to protect from cross site scripting (XSS) attacks.

When using aspx view engine, to have a consistent look and feel, across all pages of the application, we can make use of asp.net master pages. What is asp.net master pages equivalent, when using razor views?
To have a consistent look and feel when using razor views, we can make use of layout pages. Layout pages, reside in the shared folder, and are named as _Layout.cshtml

What are sections?
Layout pages, can define sections, which can then be overriden by specific views making use of the layout. Defining and overriding sections is optional.

What are the file extensions for razor views?
1. .cshtml - If the programming lanugaue is C#
2. .vbhtml - If the programming lanugaue is VB

How do you specify comments using razor syntax?
Razor syntax makes use of @* to indicate the begining of a comment and *@ to indicate the end. An example is shown below.
@* This is a Comment *@

MVC Interview Questions on Filters

What is the use of action filters in an MVC application?
Action Filters allow us to add pre-action and post-action behavior to controller action methods.

If I have multiple filters impleted, what is the order in which these filters get executed?
1. Authorization filters
2. Action filters
3. Response filters
4. Exception filters

What are the different types of filters, in an asp.net mvc application?
1. Authorization filters
2. Action filters
3. Result filters
4. Exception filters

Give an example for Authorization filters in an asp.net mvc application?
1. RequireHttpsAttribute
2. AuthorizeAttribute

Which filter executes first in an asp.net mvc application?
Authorization filter

What are the levels at which filters can be applied in an asp.net mvc application?
1. Action Method
2. Controller
3. Application

Is it possible to create a custom filter?
Yes

What filters are executed in the end?
Exception Filters

Is it possible to cancel filter execution?
Yes

What type of filter does OutputCacheAttribute class represents?
Result Filter

MVC Interview Questions on Routing

What is the significance of ASP.NET routing?
ASP.NET MVC uses ASP.NET routing, to map incoming browser requests to controller action methods. ASP.NET Routing makes use of route table. Route table is created when your web application first starts. The route table is present in the Global.asax file.

What are the 3 segments of the default route, that is present in an ASP.NET MVC application?
1st Segment - Controller Name
2nd Segment - Action Method Name
3rd Segment - Parameter that is passed to the action method

Example: http://pragimtech.com/Customer/Details/5
Controller Name = Customer
Action Method Name = Details
Parameter Id = 5

ASP.NET MVC application, makes use of settings at 2 places for routing to work correctly. What are these 2 places?
1. Web.Config File : ASP.NET routing has to be enabled here.
2. Global.asax File : The Route table is created in the application Start event handler, of the Global.asax file.

What is the adavantage of using ASP.NET routing?
In an ASP.NET web application that does not make use of routing, an incoming browser request should map to a physical file. If the file does not exist, we get page not found error.

An ASP.NET web application that does make use of routing, makes use of URLs that do not have to map to specific files in a Web site. Because the URL does not have to map to a file, you can use URLs that are descriptive of the user's action and therefore are more easily understood by users.

What are the 3 things that are needed to specify a route?
1. URL Pattern - You can include placeholders in a URL pattern so that variable data can be passed to the request handler without requiring a query string.
2. Handler - The handler can be a physical file such as an .aspx file or a controller class.
3. Name for the Route - Name is optional.

Is the following route definition a valid route definition?
{controller}{action}/{id}
No, the above definition is not a valid route definition, because there is no literal value or delimiter between the placeholders. Therefore, routing cannot determine where to separate the value for the controller placeholder from the value for the action placeholder.

What is the use of the following default route?
{resource}.axd/{*pathInfo}
This route definition, prevent requests for the Web resource files such as WebResource.axd or ScriptResource.axd from being passed to a controller.
 
What is the difference between adding routes, to a webforms application and to an mvc application?
To add routes to a webforms application, we use MapPageRoute() method of the RouteCollection class, where as to add routes to an MVC application we use MapRoute() method.

How do you handle variable number of segments in a route definition?
Use a route with a catch-all parameter. An example is shown below. * is referred to as catch-all parameter.
controller/{action}/{*parametervalues}

What are the 2 ways of adding constraints to a route?
1. Use regular expressions
2. Use an object that implements IRouteConstraint interface

Give 2 examples for scenarios when routing is not applied?
1. A Physical File is Found that Matches the URL Pattern - This default behaviour can be overriden by setting the RouteExistingFiles property of the RouteCollection object to true.
2. Routing Is Explicitly Disabled for a URL Pattern - Use the RouteCollection.Ignore() method to prevent routing from handling certain requests.