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Misconceptions regarding Pass By Value and Pass By Reference in C#

Monday, September 21, 2015 0 Comments A+ a-


Introduction:

It is a common question which is asked by interviewers in interviews that "What is Pass By Value and Pass By Reference" or "What is the difference between Pass By Value and Pass By Reference". Most of the beginner level developers and also many of intermediate level developers have misconception on it and they answer it wrong during interview.

When i didn't knew that real difference, i used to answer it like, when we pass primitive types they are passed by value but when we pass reference type, they are passed by reference which i was not aware that it is wrong.

So, today i decided to write on this topic so that readers reading my blog could be aware of the real difference and they can correct their misconception regarding it.

Pass By Value in Value Types:

In .Net Framework all objects are by default passed by value not passed by reference either it is a Value Type (so called Primitive types like int,char,double etc) or Reference Type (class,interface,delegate,string etc).

I will not go in to the details of Value Type and Reference Type definition and concept, you can read about them here.


Consider the following Examples:


First i will show a example using Value Type.

int num1 = 5;
int num2 = num1;
num2 = 10;

Console.WriteLine(num1);



So what will be printed on Console?

If your answer is 5,then you are right, because int is a value type, it is passed by value, which means for the above code num1 has 5 stored in it, when we create num2 and assign it num1 value of num1 is copied to num2 and after that if we change num2 it will not affect num1, ofcourse because we have copied value of num1 to num2, why num1 is to be change.

The same happens when we pass value types to methods. For Example:

We have created a method with sets the value of a int variable to 10.

private void ChangeValue(int i)
{
   i = 10;
}



Now we call it using our previous example:

int num1 = 5;

ChangeValue(num1);
           
Console.WriteLine(num1);



What would be the output on Console now?

Yes it will still output 5 as i already said that value is copied, so when ChangeValue method is called, num1 variable value is copied to so changing i does not changes num1.

Diagrammatic/Pictorial Representation of Pass By Value in Value Types: 

 

Here is diagrammatic representation of Pass By Value.




Pass By Value in Reference Types:

 I have following class of User which is a reference type as classes are reference types :

public class User
{
    public int UserID {get;set;}
    public string Name {get;set;}
}



we create an instance and set its properties then we assign it to another instance and change Name property and then we print Name on Console to check what is printed:

User objUser = new User() 
{ 
     UserID = 1, 
     Name = "Ehsan Sajjad" 
};

User objUser2 = objUser;
objUser2.Name = "Jon Doe";



When we create Instance of class User, an object is create in memory(Heap) and memory is allocated to it and we are storing reference to that memory location in objUser reference memory (mostly Stack) so that we can use it further, otherwise it will get lost in the memory so we need to keep a reference to it for doing different operation in memory.

When we assign objUser to objUser2 reference of object memory location which objUser is holding copied to objUser2 , now we have two seperate copies of reference but they are both are pointing to same memory location which means they both are referencing to same memory location,so changing the value  of Name property will change the value in the object in memory of which we have  reference in objUser and objUser, hence "Jon Doe" will be printed on console and it will be reflected in both references.

Diagrammatic/Pictorial Representation of Pass By Value in Reference Types:




We can see the same behavior using a method,see the following method to change Name property of User object:

public static void ChangeName(User user)
{
    user.Name = "Jon Doe";
}


We call it to change the object state, it will give the same behavior what we saw in assignment case:

User objUser = new User() { UserID = 1, Name = "Ehsan Sajjad" };

ChangeName(objUser);

Console.WriteLine(objUser.Name);



When we are passing reference objUser of User object to method ChangeName, reference of memory location is copied to the local object user of method but they are both are pointing to same memory location which means they both are having reference to same memory location,so changing the value the value of Name property will change the value in the object in memory of which we have  reference in objUser and user, hence "Jon Doe" will be printed on console.


Here is diagramatic representation of it:





When the ChangeName(objUser) is called, as it is referring to same memory location, it will modify the Name property of User object to "Jon Doe".





But think about what would happen if i set the user to null inside ChangeName method like:

public static void ChangeName(User user)
{
    user = null;
}


and now we call it for objUser :

User objUser = new User() 
{ 
     UserID = 1, 
     Name = "Ehsan Sajjad" 
};

ChangeName(objUser);
Console.WriteLine(objUser.Name);


If you are thinking that it will throw Null Reference Exception, then you are wrong, and if you are thinking that it will output Ehsan Sajjad, then you are right and you have understood that reference are passed by value in C# not by reference.

See the pictorial representation to understand better:

 

 

Pass By Reference:

If we want to make objUser null, we will have to pass it to the method via reference which is done in C# using ref Keyword. We will use the above examples again but this time we will pass them by reference and will see what happens so that we can inderstand the difference between these two.

Pass By Reference in Value Types:

We will use the same above example but this time we will be passing by reference. For that first of all we have to change the method signatures of method ChangeValue(int i) to ChangeValue(ref int i), we have added ref keyword with the input parameter which means that when calling this method the argument should be passed by reference to it:
private void ChangeValue(ref int i)
{
   i = 10;
}

 Now we will use the same code as above but we have to use ref keyword at calling side for the parameters that method expect to be passed by reference otherwise you will get compile time error, and your code will not build:

int num1 = 5;

ChangeValue(ref num1);
           
Console.WriteLine(num1);
This will output 10 on the screen, because we are using ref keyword, in the above code when ChangeValue is called the incoming parameter of it has the same memory address of num1 which is passed as argument that's why now modifying the value of i would reflect the change in num1 as well, in pass by reference new memory location is not used for the method parameter so changing the value of it will reflect the variable that is passed from calling side.

Pass By Reference in Reference Types:

We will now check the same thing with reference types, and the behaviour would be same for reference types case as well,first modify the signature of method so that it takes parameter as reference:
public static void ChangeName(ref User user)
{
    user = null;
}


and we will call it simply this way:

User objUser = new User() 
{ 
     UserID = 1, 
     Name = "Ehsan Sajjad" 
};

ChangeName(objUser);
Console.WriteLine(objUser.Name);



Now when we will call it on objUser setting user to null inside ChangeName will also make objUser null because instead of passing the reference by value (in that a new reference memory location is create which points the same object) it is passed by reference, so in this case new copy of reference is not created but the reference of objUser is passed to method which results setting the calling side reference to also change the memory location where it is pointing.
Udemy

How to Convert DataTable to List in C#

Monday, September 14, 2015 0 Comments A+ a-

When writing Data Access Layer, when we get data from the database using ADO.net, most of the times we use DataTable or DataSet to hold the data in memory, then we have to map the DataTable  to List of Model type.


One way is to iterate on the rows of DataTable and create objects of Model and add to List one. Let me show you with example.


I have table in database which has information about Doctors, my table name is Doctor which contains different columns which are for capturing information of Doctor.


I have following Model class on front end which represents Doctor :

public class Doctor
{
    public int ID { get; set; }
    public string Name { get; set; }
    public string ImagePath { get; set; }
    public string Type { get; set; }
}



When we will get Doctors from database in datatable we can create a list of Doctors with following approach in our Data Access:

List<doctor> doctors = new List<doctor>();
for (int i = 0; i < dtDoctors.Rows.Count; i++)
{

    Doctor objDoctor = new Doctor();

    objDoctor.ID = dtDoctors.Rows[i]["DoctorID"].ToString();
    objDoctor.Name = dtDoctors.Rows[i]["Name"].ToString();
    objDoctor.ImagePath = dtDoctors.Rows[i]["Image"].ToString();
    objDoctor.Type = dtDoctors.Rows[i]["Type"].ToString();

    doctorList.Add(objDoctor);
}

It will surely do the job for you, but think that you have many tables in your database and whenever you will fetch data you will have to do a loop the way i showed above and will have to map properties from datatable, which does not look cool as we are rewriting same thing again and again which is against DRY principle.


We can achieve what we have did above using generics,relection and extension methods feature provided by .Net Framework.We can add the following  extension method in our project and just call it to convert DataTable to List of Type which we want to.

Following is the extension method for it:

public static class ExtensionMethods
{
    public static List<T> ToList<T>(this DataTable dt)
    {
        List<T> data = new List<T>();
        foreach (DataRow row in dt.Rows)
        {
            T item = GetItem<T>(row);
            data.Add(item);
        }
        return data;
    }

    private static T GetItem<T>(DataRow dr)
    {
        Type temp = typeof(T);
        T obj = Activator.CreateInstance<T>();

        foreach (DataColumn column in dr.Table.Columns)
        {
            foreach (PropertyInfo pro in temp.GetProperties())
            {
                if (pro.Name == column.ColumnName && dr[column.ColumnName] != DBNull.Value)
                    pro.SetValue(obj, dr[column.ColumnName], null);
                else
                    continue;
            }
        }
        return obj;
    }
}

Now we can call it on instance of DataTable and can specify that convert it to which type of List.

For Example:

List<Doctor> doctors = dtDoctors.ToList<Doctor>();


Now that looks cool, we have just written two extension method for DataTable and now we can reuse it anywhere which enables us to reduce code redundancy.


One thing to consider is that the column names you are returning from your sql query or Stored Procedure should be same as of the properties in the Model class and the datatype should also be matched so that i can be converted.
Udemy

Implementing Repository pattern and Dependency Injection in ADO.net using Generics in C#

Friday, September 04, 2015 1 Comments A+ a-

Nowadays i am trying to learn different design patterns in object oriented paradigm that are pretty useful to implement generic solutions for different scenarios, few weeks back for a job hunt i got an assignment to do which was a web application which would interact with database, so i took it as a challenge and decided to make it loosely coupled using design patterns which were applicable in that scenario.

One of them which implemented in my assignment is repository pattern using generics and with that Dependency Injection using which i injected dependencies of Repository class via constructor.
 


I made generic class which would be inherited by other types against the different tables in the application. in this class i have used different framework features like Reflection and Generics.


My generic class is an abstract class, so it needs to be inherited for making use of it, you will see next how we will use it.


Here is the Repository class:

public abstract class Repository<TEntity> where TEntity : new()
{
    DbContext _context;

    public Repository(DbContext context)
    {
        _context = context;
    }

    protected DbContext Context 
    { 
        get
        {
          return this._context;
        } 
    }

    protected IEnumerable<TEntity> ToList(IDbCommand command)
    {
        using (var record = command.ExecuteReader())
        {
            List<TEntity> items = new List<TEntity>();
            while (record.Read())
            {
                    
                items.Add(Map<TEntity>(record));
            }
            return items;
        }
    }

        
    protected TEntity Map<TEntity>(IDataRecord record)
    {
        var objT = Activator.CreateInstance<TEntity>();
        foreach (var property in typeof(TEntity).GetProperties())
        {
            if (record.HasColumn(property.Name) && !record.IsDBNull(record.GetOrdinal(property.Name)))
                property.SetValue(objT, record[property.Name]);


        }
        return objT;
    }


}

Now i have table in database User whose schema is :

CREATE TABLE [dbo].[tblUser] (
    [UserID]    INT           IDENTITY (1, 1) NOT NULL,
    [FirstName] NVARCHAR (25) NULL,
    [LastName]  NVARCHAR (25) NULL,
    [UserName]  NVARCHAR (25) NULL,
    [Password]  NVARCHAR (25) NULL,
    [IsActive]  BIT           NULL,
    [IsDeleted] BIT           NULL,
    [CreatedBy] INT           NULL,
    [CreatedAt] DATETIME      NULL,
    [UpdatedBy] INT           NULL,
    [UpdatedAt] DATETIME      NULL,
    [Email]     NVARCHAR (50) NULL,
    PRIMARY KEY CLUSTERED ([UserID] ASC)
);



Against this table i have Model class for mapping from table to that type which looks :

public class User
{
    public int UserID { get; set; }

    public string FirstName { get; set; }

    public string LastName { get; set; }

    public string UserName { get; set; }

    public string Password { get; set; }

    public bool IsActive { get; set; }

    public bool IsDeleted { get; set; }

    public DateTime CreatedAt { get; set; }

    public int CreatedBy { get; set; }

    public DateTime UpdatedAt { get; set; }

    public int UpdatedBy { get; set; }

    public string Email { get; set; }
}

We want to fetch data from User table for which we will create a Repository class for User type and the we will write implementation to fetch records from User table from database. Our all methods that need to get data, insert data,update data or delete data from User table will reside in the UserRepository class.

Here is the implementation of User Repository class:

public class UserRepository : Repository<User>
{
    private DbContext _context;
    public UserRepository(DbContext context)
        : base(context)
    {
        _context = context;
    }


    public IList<User> GetUsers()
    {
        using (var command = _context.CreateCommand())
        {
            command.CommandText = "exec [dbo].[uspGetUsers]";

            return this.ToList(command).ToList();
        }
    }

    public User CreateUser(User user)
    {
        using (var command = _context.CreateCommand())
        {
            command.CommandType = CommandType.StoredProcedure;
            command.CommandText = "uspSignUp";

            command.Parameters.Add(command.CreateParameter("@pFirstName", user.FirstName));
            command.Parameters.Add(command.CreateParameter("@pLastName", user.LastName));
            command.Parameters.Add(command.CreateParameter("@pUserName", user.UserName));
            command.Parameters.Add(command.CreateParameter("@pPassword", user.Password));
            command.Parameters.Add(command.CreateParameter("@pEmail", user.Email));

            return this.ToList(command).FirstOrDefault();


        }

    }


    public User LoginUser(string id, string password)
    {
        using (var command = _context.CreateCommand())
        {
            command.CommandType = CommandType.StoredProcedure;
            command.CommandText = "uspSignIn";

            command.Parameters.Add(command.CreateParameter("@pId", id));
            command.Parameters.Add(command.CreateParameter("@pPassword", password));

            return this.ToList(command).FirstOrDefault();
        }
    }


    public User GetUserByUsernameOrEmail(string username, string email)
    {
        using (var command = _context.CreateCommand())
        {
            command.CommandType = CommandType.StoredProcedure;
            command.CommandText = "uspGetUserByUsernameOrEmail";

            command.Parameters.Add(command.CreateParameter("@pUsername", username));
            command.Parameters.Add(command.CreateParameter("@pEmail", email));

            return this.ToList(command).FirstOrDefault();
        }
    }


}


We are done for the UserRepository for new, i have added methods are wrote Stored Procedure to complete the assignment and now i will tell how to make use of it in the Service Layer or in Business Rule to do operations.




First create an interface namely IUserService:

[ServiceContract]
public interface IUserService
{

    [OperationContract]
    IList<User> GetUsers();

    [OperationContract]
    User RegisterUser(User user);

    [OperationContract]
    User Login(string id, string password);

    [OperationContract]
    bool UserNameExists(string username, string email);


}


Here is my WCF Service for User that calls the UserRepository for doing operations:

public class UserService : IUserService
{
    private IConnectionFactory connectionFactory;

    public IList<User> GetUsers()
    {
        connectionFactory = ConnectionHelper.GetConnection();

        var context = new DbContext(connectionFactory);

        var userRep = new UserRepository(context);

        return userRep.GetUsers();
    }

    public User RegisterUser(User user)
    {
        connectionFactory = ConnectionHelper.GetConnection();

        var context = new DbContext(connectionFactory);

        var userRep = new UserRepository(context);

        return userRep.CreateUser(user);
    }


    public User Login(string id, string password)
    {
        connectionFactory = ConnectionHelper.GetConnection();

        var context = new DbContext(connectionFactory);

        var userRep = new UserRepository(context);

        return userRep.LoginUser(id, password);
    }

    public bool UserNameExists(string username, string email)
    {
        connectionFactory = ConnectionHelper.GetConnection();

        var context = new DbContext(connectionFactory);

        var userRep = new UserRepository(context);

        var user = userRep.GetUserByUsernameOrEmail(username, email);
        return !(user != null && user.UserID > 0);

    }
}

You can see that when creating instance of UserRepository i am injecting database context via constructor and then i am calling different methods from userReposiory according to need.

Now in future when i add another table in database, i would create another Repository type and implement its Data Access logic and will call it same way, so applying Dependency Injection and Repository pattern we are following DRY principle to some extent, but i am sure we can make it more better than this.
Udemy

Check If username/email already registered using Remote Validation attribute in asp.net mvc

Tuesday, September 01, 2015 0 Comments A+ a-

When we create Registration form on a web application, we have to check that the email address and username that user is entering is unique and is not already. In asp.net mvc we have Validation Attribute feature in under System.ComponentModel.DataAnnotations and System.Web.Mvc which we can use for different type of Validations before sending data to be saved in persistent location.

In System.Web.Mvc we have Remote attribute which is available in asp.net mvc 4 in which i am implementing it for  this post, i am not sure if it is available in the earlier version of asp.net mvc.


So i have this ViewModel:

    public class SignUpViewModel
    {
        
        public int UserID { get; set; }

        [Required(ErrorMessage = "First Name is required")]
        public string FirstName { get; set; }

        [Required(ErrorMessage = "Last Name is Required")]
        public string LastName { get; set; }

        [Required(ErrorMessage = "Username is Required")]
        [RegularExpression(@"^[a-zA-Z0-9]+$", ErrorMessage = "user name must be combination of letters and numbers only.")]
        public string UserName { get; set; }

        [Required(ErrorMessage = "Password is Required")]
        public string Password { get; set; }

        [Required(ErrorMessage = "Password is Required")]
        [System.Web.Mvc.Compare("Password",ErrorMessage="Both Password fields must match.")]
        public string ConfirmPassword { get; set; }

        [Required(ErrorMessage = "Email Address is required")]
        [EmailAddress(ErrorMessage = "Invalid Email Address")]
        public string Email { get; set; }
    }

As you can see there are laready attributes on the properties of ViewModel like Required which are mandatory fields, RegularExpression attribute for Email Address format validation and Compare attribute for comparing two properties values which is useful here for Password and Repeat Password textbox for making sure that user has verified what password he is setting.

Now what will happen if user enters a username or Email Address which is already registered, one way is to check in post action for username and Email Address  and show user error message that username or Email Address is already taken which does not looks though, as validation should be done before posting data to action using unobtrusive validtion.

Now here we will use Remote attribute which will check for both UserName and Email Address field, here is how it will be done:

 [Required(ErrorMessage = "Username is Required")]
 [RegularExpression(@"^[a-zA-Z0-9]+$", ErrorMessage = "user name must be combination of letters and numbers only.")]
 [Remote("UsernameExists","Account",HttpMethod="POST",ErrorMessage="User name already registered.")]
 public string UserName { get; set; }


[Required(ErrorMessage = "Email Address is required")]
[EmailAddress(ErrorMessage = "Invalid Email Address")]
[Remote("EmailExists", "Account", HttpMethod = "POST", ErrorMessage = "Email address already registered.")]
 public string Email { get; set; }

Remote Attribute has different parameters to be specified:

  1. Action Name which will be called which will have input parameter for this property value
  2. Controller Name of which action will be executed
  3. HttpMethod (Get or Post)
  4. ErrorMessage (Error Message that will be displayed if validation fails)  
Let's come to the action part we need to write action method in the Account controller for Validation, for this post just for clearing to readers i am just checking for specific username and email (my name and dummy email address), but in your project you will need to check in your database for that particular criteria and return true or false respectively.


Here is the action code:

public JsonResult UsernameExists(string username)
{    
     return Json(!String.Equals(username,"ehsansajjad",StringComparison.OrdinalIgnoreCase));
}

public JsonResult EmailExists(string email)
{
     return Json(!String.Equals(email, "ehsansajjad@yahoo.com", StringComparison.OrdinalIgnoreCase));
}

Here is the complete View code:

@model CustomValidationMessageHelper.ViewModels.SignUpViewModel
@{
    Layout = null;
}

<!DOCTYPE html>

<html>
<head>
    <meta name="viewport" content="width=device-width" />
    <title>SignUp</title>
    <link href="@Url.Content("~/Content/jquery.qtip.css")" rel="stylesheet" />

    <script src="@Url.Content("~/Scripts/jquery-1.9.1.js")"></script>
    <script src="@Url.Content("~/Scripts/jquery.unobtrusive-ajax.js")"></script>
    <script src="@Url.Content("~/Scripts/jquery.validate.js")" type="text/javascript"></script>
    <script src="@Url.Content("~/Scripts/jquery.validate.unobtrusive.js")" type="text/javascript"></script>
    <script src="@Url.Content("~/Scripts/jquery.qtip.min.js")" type="text/javascript"></script>

</head>
<body>
    <div>
        @using (Html.BeginForm("SignUp", "Account", FormMethod.Post, new { @class = "form-inline", role = "form" }))
        {
          @Html.AntiForgeryToken()
    
    <div class="row">
        <div class="span8 offset5 aui-page-panel">

            <div>
                    <h2>Sign Up</h2>
                </div>
            <fieldset style="margin-bottom: 40px;">
                

                                <legend style="border-bottom: 1px solid gray; font-size: 16px;">Basic Information</legend>

                <table width="100%" border="0" cellspacing="0" cellpadding="0">
                    <tr id="tr_basic">
                        <td style="vertical-align: top;" width>
                            <div id="basicinfo" style="width: 100%">
                                <div style="height: auto !important; overflow-y: visible;">
                                    <table cellpadding="3">

                                        <tbody>
                                            <tr>
                                                <td width="150">
                                                    @Html.LabelFor(model => model.FirstName, new { @class = "sr-only" })
                                                </td>
                                                <td>
                                                    @Html.TextBoxFor(model => model.FirstName, new { @class = "form-control input-sm" })
                                                    @Html.ValidationMessageFor(model => model.FirstName)
                                                </td>

                                            </tr>
                                            <tr>
                                                <td>
                                                    @Html.LabelFor(model => model.LastName)
                                                </td>
                                                <td>
                                                    @Html.TextBoxFor(model => model.LastName, new { @class = "input-xsmall" })
                                                    @Html.ValidationMessageFor(model => model.LastName)
                                                </td>
                                            </tr>

                                            
                                            <tr>
                                            </tr>

                                        </tbody>
                                    </table>

                                </div>
                        </td>
                    </tr>

                </table>


                <legend style="border-bottom: 1px solid gray; font-size: 16px;">Account Information</legend>
                <table cellpadding="5">
                    <tr>
                        <td width="150">
                            @Html.LabelFor(model => model.UserName)
                        </td>
                        <td>
                            @Html.TextBoxFor(model => model.UserName)
                            @Html.ValidationMessageFor(model => model.UserName)
                        </td>
                        <td id="tdValidate">
                            <img id="imgValidating" src="@Url.Content("~/Images/validating.gif")" style="display:none;" /></td>

                    </tr>
                    <tr>
                        <td>
                            @Html.LabelFor(model => model.Password)
                        </td>
                        <td>
                            @Html.PasswordFor(model => model.Password)
                            @Html.ValidationMessageFor(model => model.Password)


                        </td>

                    </tr>


                    <tr>
                        <td>
                            @Html.LabelFor(m => m.ConfirmPassword, new { @class = "control-label" })
                        </td>

                        <td>
                            @Html.PasswordFor(model => model.ConfirmPassword)
                            @Html.ValidationMessageFor(model => model.ConfirmPassword)

                        </td>
                    </tr>
                    <tr>
                        <td>
                            @Html.LabelFor(m => m.Email, new { @class = "control-label" })
                        </td>

                        <td>
                            @Html.TextBoxFor(model => model.Email)
                            @Html.ValidationMessageFor(model => model.Email)


                        </td>

                    </tr>
                    <tr>
                    <td>
                        <p>
                            <div class="control-group">
                                <div class="controls">
                                    <input id="btnRegister" type="submit" class="btn btn-primary" value="Register" />
                                </div>
                            </div>

                        </p>
                    </td>
                    <td></td>
                </tr>
                </table>
    </div>
     </div>
            }
            </div>
        
</body>
</html>

Now when i will enter username "ehsansajjad" or Email Address "ehsansajjad@yahoo.com" validation will get fired that user already exists :





You can download the sample project from here.