C# 3.0 new features

1. Implicitly typed local variables and arrays:  A new keyword is introduced called var. This allows you to declare a variable and let the compiler figure out the type  

ex:

        // Declare a variable of type int

            var i = 5;

            // Declare an int array

            var a = new[] { 1, 10, 100, 1000 };

            // Usage:

            for (var i = 0; i < 5; i++) { }

            foreach (var item in MyList) { }

            using (var file = new StreamReader(“Filename.txt”)) { } 

2. Auto-implement properties: Use this when no additional logic is required in the property accessors. 

ex:

        // Previously we had to do this,

        private string _name;

        public string Name

        {

            get { return _name; }

            set { _name = value; }

        }

        // Now we can shorthand it, there’s no need to declare the _name variable

        public string Name { get; set; } // The compiler creates a private anonymous backing field

Behind the scene, compiler automatically creates private, anonymous fields that can be accessed from the above auto-implemented properties.

 3. Object and collection initializers: Set properties inline during object creation.    

 ex:

    public class A {

    public string Name { get; set; }

    public string Address { get; set; }

}

// Previously we would have to do,

A a = new A();

a.Name = “…”;

a.Address = “…”;

// Now we can do this,

A a = new A { Name = “…”, Address = “…” };

// Similarly, we can initialize collections,

List<int> MyList = new List<int> { 0, 1, 2, 3, 4 };

 4. Extension methods: Now you can add methods to existing types without creating a new derived type, recompiling or modifying the original type’s code. This is incredibly useful in writing cleaner code,

ex:

public static void ExportToExcel(this DataSet ds)
{
    //do something…
}

Extension methods are declared by specifying the “this” keyword as the modifier in the first parameter. The ExportToExcel method is declared as an extension method for the DataSet class.
Extension methods are invoked as soon the namespace of the containing static class is included.
In the below code snippet, the extension method is declared inside
namespace Orcas.Samples
{
   public static class Extensions
   {
       public static void ExportToExcel(this DataSet ds)
      {
             //do something…
      }
   }
}

Above method is invoked in the client program as,
using Orcas.Samples;
DataSet ds = new DataSet();
ds.ExportToExcel();

5. Anonymous types: If you need a small class with read only properties, you can now use a simple syntax,     

ex:

// a is an anonymous type, the compiler creates the class with

// private string fields _name and _address

// and public string properties Name and Address,

var a = new { Name = “Dev”, Address = “Delhi” };

// a.Name and a.Address are read-only,

string s = a.Name;

 6. Query keywords (LINQ): We are now able to use TSQL like syntax inside C# to query data sources (could be objects, collections, XML, or SQL databases). Introduces a host of new keywords like from, where, select, group, orderby, join, etc.      

ex:

// The data source,

     int[] numbers = { 0, 1, 2, 3, 4, };

     // Create the query,

     var lessthan3 =

         from num in numbers

         where num < 3

         select num;

     // Output: 0 1 2

     foreach (var m in lessthan3) {

         Console.Write(m.ToString() + ” “);

     }

7. Partial method definitions: A partical class (introduced in C# 2.0) can now have partial methods. This simply means the method signature is in one partial class and the implementation in another. There are some constraints, like the method is implicitly private (and therefore cannot be virtual) and must return void.      

 ex:

public partial class A

{

    // Method definition

    partial void Method();

}

public partial class A

{

    // Method implementation

    partial void Method() { /* Do Work */ }

}