Some features in .NET Framework 3.0

Implicitly typed variables and arrays

Implicit variables are useful for improve writing and reading code (not in all cases) and they are key for LINQ query expressions:

Before:

Dictionary<string, string> dict = new Dictionary<string, string>();
int[] Numbers = new int[] { 1, 2, 3, 4, 5 };
string[] s = new string[] { "a", "b", "c", "d" };

Now:

var dict = new Dictionary<string, string>();
var Numbers = new[] { 1, 2, 3, 4, 5 };
var s = new[] { "a", "b", "c", "d" };

Extensions Methods

That's an amazing feature for extend interfaces and classes that we don't have the source code or we don't want to modify directly.

Example: Suppose you have an interface that has two properties: Name and Age. Also, it has a method that display its data as 'Name=NameValue Age=AgeValue':

public interface ITestClass
{
    string Name { get; set; }
    int Age { get; set; }

    void Show();
}

But, we need to show this data in a different way in several parts of our code, for example: 'NameValue (AgeValue)'. By extending this interface we can simply invoke the method as 'tc.ShowCustom();' like if ShowCustom is part of this interface:

public static class TestClassExtension
{
    public static void ShowCustom(this ITestClass tc)
    {
        MessageBox.Show(tc.Name + " (" + tc.Age.ToString() + ")");
    }
}

...

ITestClass tc = GetTestClassInteface();
tc.Name = "Oscar";
tc.Age = 36;

tc.Show();
tc.ShowCustom(); // Now it appears like if this method is part of ITestClass (!!! amazing !!!)

NOTE: You can extend standard and base classes/interfaces like String, IEnumerator, etc.

Lambda Expressions

They are key for LINQ expressions and for reduce the use of delegates, here a little example:

var list = new List<string>();

list.Add("one");
list.Add("two");
list.Add("three");

bool bExist = list.Exists(delegate(string s) { return s == "two"; });

And using Lambda Expressions:

bool bExist = list.Exists(s => s == "two");

Anonymous types

Allows the on-the-fly creation of structures:

var person = new { Name = "Oscar", Age = 36 };

MessageBox.Show(person.Name + "(" + person.Age + ")");

Object/Collection Initializers

We can now initialize any property when we are creating some instance for objects or collections:

Timer t = new Timer() { Interval = 30000, Enabled = true };

var list = new List<string> { "one", "two", "three" };

References:

C# 3.0 Tutorial By Jonathan Worthington

C# Version 3.0 Specification MSDN