Hidden Features of C#? [closed]

Question

This came to my mind after I learned the following from this question:

where T : struct

We, C# developers, all know the basics of C#. I mean declarations, conditionals, loops, operators, etc.

Some of us even mastered the stuff like Generics, anonymous types, lambdas, LINQ, ...

But what are the most hidden features or tricks of C# that even C# fans, addicts, experts barely know?

Here are the revealed features so far:

Keywords

• yield by Michael Stum
• var by Michael Stum
• using() statement by kokos
• readonly by kokos
• as by Mike Stone
• as / is by Ed Swangren
• as / is (improved) by Rocketpants
• default by deathofrats
• global:: by pzycoman
• using() blocks by AlexCuse
• volatile by Jakub Šturc
• extern alias by Jakub Šturc

Attributes

• DefaultValueAttribute by Michael Stum
• ObsoleteAttribute by DannySmurf
• DebuggerDisplayAttribute by Stu
• DebuggerBrowsable and DebuggerStepThrough by bdukes
• ThreadStaticAttribute by marxidad
• FlagsAttribute by Martin Clarke
• ConditionalAttribute by AndrewBurns

Syntax

• ?? (coalesce nulls) operator by kokos
• Number flaggings by Nick Berardi
• where T:new by Lars Mæhlum
• Implicit generics by Keith
• One-parameter lambdas by Keith
• Auto properties by Keith
• Namespace aliases by Keith
• Verbatim string literals with @ by Patrick
• enum values by lfoust
• @variablenames by marxidad
• event operators by marxidad
• Format string brackets by Portman
• Property accessor accessibility modifiers by xanadont
• Conditional (ternary) operator (?:) by JasonS
• checked and unchecked operators by Binoj Antony
• implicit and explicit operators by Flory

Language Features

• Nullable types by Brad Barker
• Anonymous types by Keith
• __makeref __reftype __refvalue by Judah Himango
• Object initializers by lomaxx
• Format strings by David in Dakota
• Extension Methods by marxidad
• partial methods by Jon Erickson
• Preprocessor directives by John Asbeck
• DEBUG pre-processor directive by Robert Durgin
• Operator overloading by SefBkn
• Type inferrence by chakrit
• Boolean operators taken to next level by Rob Gough
• Pass value-type variable as interface without boxing by Roman Boiko
• Programmatically determine declared variable type by Roman Boiko
• Static Constructors by Chris
• Easier-on-the-eyes / condensed ORM-mapping using LINQ by roosteronacid
• __arglist by Zac Bowling

Visual Studio Features

• Select block of text in editor by Himadri
• Snippets by DannySmurf

Framework

• TransactionScope by KiwiBastard
• DependantTransaction by KiwiBastard
• Nullable<T> by IainMH
• Mutex by Diago
• System.IO.Path by ageektrapped
• WeakReference by Juan Manuel

Methods and Properties

• String.IsNullOrEmpty() method by KiwiBastard
• List.ForEach() method by KiwiBastard
• BeginInvoke(), EndInvoke() methods by Will Dean
• Nullable<T>.HasValue and Nullable<T>.Value properties by Rismo
• GetValueOrDefault method by John Sheehan

Tips & Tricks

• Nice method for event handlers by Andreas H.R. Nilsson
• Uppercase comparisons by John
• Access anonymous types without reflection by dp
• A quick way to lazily instantiate collection properties by Will
• JavaScript-like anonymous inline-functions by roosteronacid

Other

• netmodules by kokos
• LINQBridge by Duncan Smart
• Parallel Extensions by Joel Coehoorn

Answer 1

Another note on event handlers: you can simply create a raise extension method like so:

public static class EventExtensions {
    public static void Raise<T>(this EventHandler<T> @event, 
                                object sender, T args) where T : EventArgs {
        if(@event!= null) {
            @event(sender, args);
        }
    }
}

Then you can use it to raise events:

public class MyImportantThing {
    public event EventHandler<MyImportantEventEventArgs> SomethingHappens;
    ...
    public void Bleh() {
        SomethingHappens.Raise(this, new MyImportantEventEventArgs { X=true });
    }
}

This method has the added advantage of enforcing a coding standard (using EventHandler<>).

There isn't a point in writing the same exact function over and over and over again. Perhaps the next version of C# will finally have an InlineAttribute that can be placed on the extension method and will cause the compiler to inline the method definition (which would make this way nearly standard, and the fastest).

edit: removed temp variable inside extension method based on comments

Answer 2

On the basis that this thread should be entitled "things you didn't know about C# until recently despite thinking you already knew everything", my personal feature is asynchronous delegates.

Until I read Jeff Richter's C#/CLR book (excellent book, everyone doing .NET should read it) I didn't know that you could call any delegate using BeginInvoke / EndInvoke. I tend to do a lot of ThreadPool.QueueUserWorkItem calls (which I guess is much like what the delegate BeginInvoke is doing internally), but the addition of a standardised join/rendezvous pattern may be really useful sometimes.

Answer 3

Near all the cool ones have been mentioned. Not sure if this one's well known or not

C# property/field constructor initialization:

var foo = new Rectangle() 
{ 
    Fill = new SolidColorBrush(c), 
    Width = 20, 
    Height = 20 
};

This creates the rectangle, and sets the listed properties.

I've noticed something funny - you can have a comma at the end of the properties list, without it being a syntax error. So this is also valid:

var foo = new Rectangle() 
{ 
    Fill = new SolidColorBrush(c), 
    Width = 20, 
    Height = 20,
};

Answer 4

Atrribute Targets

Everyone has seen one. Basically, when you see this:

[assembly: ComVisible(false)]

The "assembly:" portion of that attribute is the target. In this case, the attribute is applied to the assembly, but there are others:

[return: SomeAttr]
int Method3() { return 0; } 

In this sample the attribute is applied to the return value.

Answer 5

One great class I like is System.Xml.XmlConvert which can be used to read values from xml tag. Especially, if I am reading a boolean value from xml attribute or element, I use

bool myFlag  = System.Xml.XmlConvert.ToBoolean(myAttribute.Value);

Note: since boolean type in xml accepts 1 and 0 in addition to "true" and "false" as valid values, using string comparison in this case is error-prone.

Answer 6

Apologies for posting so late, I am new to Stack Overflow so missed the earlier opportunity.

I find that EventHandler<T> is a great feature of the framework that is underutilised.

Most C# developers I come across still define a custom event handler delegate when they are defining custom events, which is simply not necessary anymore.

Instead of:

public delegate void MyCustomEventHandler(object sender, MyCustomEventArgs e);

public class MyCustomEventClass 
{
    public event MyCustomEventHandler MyCustomEvent;
}

you can go:

public class MyCustomEventClass 
{
    public event EventHandler<MyCustomEventArgs> MyCustomEvent;
}

which is a lot more concise, plus you don't get into the dilemma of whether to put the delegate in the .cs file for the class that contains the event, or the EventArgs derived class.

Answer 7

What about IObservable?

Pretty much everybody knows IEnumerable but their mathematical dual seems to be unknown IObservable. Maybe because its new in .NET 4.

What it does is instead of pulling the information (like an enumerable) it pushes information to the subscriber(s) of the observerable.

Together with the Rx extensions it will change how we deal with events. Just to illustrate how powerful it is check a very short example here.

Answer 8

OK, it may seem obvious, but I want to mention the Object.Equals method (the static one, with two arguments).

I'm pretty sure many people don't even know about it, or forget it's there, yet it can really help in some cases. For instance, when you want to compare two objects for equality, not knowing if they're null. How many times did you write something like that :

if ((x == y) || ((x != null && y != null) && x.Equals(y)))
{
    ...
}

When you can just write :

if (Object.Equals(x, y))
{
    ...
}

(Object.Equals is actually implemented exactly like in the first code sample)

Answer 9

string.Empty

I know it's not fantastical (ludicrously odd), but I use it all the time instead of "".

And it's pretty well hidden until someone tells you it's there.

Answer 10

I'm late to this party, so my first choices are already taken. But I didn't see anyone mention this gem yet:

Parallel Extensions to the .NET Framework

It has things like replace with Parallel.For or foreach with Parallel.ForEach

---

Parallel Sample :
In your opinion, how many CLR object can be created in one second?
See fallowing example :

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Threading;
using System.Threading.Tasks;

namespace ObjectInitSpeedTest
{
   class Program
   {
       //Note: don't forget to build it in Release mode.
       static void Main()
       {
           normalSpeedTest();           
           parallelSpeedTest();

           Console.ForegroundColor = ConsoleColor.White;
           Console.WriteLine("Press a key ...");
           Console.ReadKey();
       }

       private static void parallelSpeedTest()
       {
           Console.ForegroundColor = ConsoleColor.Yellow;
           Console.WriteLine("parallelSpeedTest");

           long totalObjectsCreated = 0;
           long totalElapsedTime = 0;

           var tasks = new List<Task>();
           var processorCount = Environment.ProcessorCount;

           Console.WriteLine("Running on {0} cores", processorCount);

           for (var t = 0; t < processorCount; t++)
           {
               tasks.Add(Task.Factory.StartNew(
               () =>
               {
                   const int reps = 1000000000;
                   var sp = Stopwatch.StartNew();
                   for (var j = 0; j < reps; ++j)
                   {
                       new object();
                   }
                   sp.Stop();

                   Interlocked.Add(ref totalObjectsCreated, reps);
                   Interlocked.Add(ref totalElapsedTime, sp.ElapsedMilliseconds);
               }
               ));
           }

           // let's complete all the tasks
           Task.WaitAll(tasks.ToArray());

           Console.WriteLine("Created {0:N} objects in 1 sec\n", (totalObjectsCreated / (totalElapsedTime / processorCount)) * 1000);
       }

       private static void normalSpeedTest()
       {
           Console.ForegroundColor = ConsoleColor.Green;
           Console.WriteLine("normalSpeedTest");

           const int reps = 1000000000;
           var sp = Stopwatch.StartNew();
           sp.Start();
           for (var j = 0; j < reps; ++j)
           {
               new object();
           }
           sp.Stop();

           Console.WriteLine("Created {0:N} objects in 1 sec\n", (reps / sp.ElapsedMilliseconds) * 1000);
       }
   }
}

Answer 11

C# 3.0's LINQ query comprehensions are full-blown monadic comprehensions a la Haskell (in fact they were designed by one of Haskell's designers). They will work for any generic type that follows the "LINQ pattern" and allows you to write in a pure monadic functional style, which means that all of your variables are immutable (as if the only variables you used were IDisposables and IEnumerables in using and foreach statements). This is helpful for keeping variable declarations close to where they're used and making sure that all side-effects are explicitly declared, if there are any at all.

 interface IFoo<T>
  { T Bar {get;}
  }

 class MyFoo<T> : IFoo<T> 
  { public MyFoo(T t) {Bar = t;}
    public T Bar {get; private set;} 
  }

 static class Foo 
  { public static IFoo<T> ToFoo<T>(this T t) {return new MyFoo<T>(t);}

    public static void Do<T>(this T t, Action<T> a) { a(t);}

    public static IFoo<U> Select<T,U>(this IFoo<T> foo, Func<T,U> f) 
     { return f(foo.Bar).ToFoo();
     }
  }

 /* ... */

 using (var file = File.OpenRead("objc.h"))
 { var x = from f in file.ToFoo()
           let s = new Scanner(f)
           let p = new Parser {scanner = s}
           select p.Parse();

   x.Do(p => 
    { /* drop into imperative code to handle file 
         in Foo monad if necessary */      
    });

 }

Answer 12

I see a lot of people replicate the functionality of Nullable<T>.GetValueOrDefault(T).

Answer 13

Not really hidden, but useful. When you've got an enum with flags, you can use shift-left to make things clearer. e.g.

[Flags]
public enum ErrorTypes {
    None              = 0,
    MissingPassword   = 1 << 0,
    MissingUsername   = 1 << 1,
    PasswordIncorrect = 1 << 2 
}

Answer 14

My favorite attribute: InternalsVisibleTo

At assembly level you can declare that another assembly can see your internals. For testing purposes this is absolutely wonderful.

Stick this in your AssemblyInfo.cs or equivalent and your test assembly get full access to all the internal stuff that requires testing.

[assembly: InternalsVisibleTo("MyLibrary.Test, PublicKey=0024...5c042cb")]

As you can see, the test assembly must have a strong name to gain the trust of the assembly under test.

Available in .Net Framework 2.0+, Compact Framework 2.0+ and XNA Framework 1.0+.

Answer 15

I love using the @ character for SQL queries. It keeps the sql nice and formatted and without having to surround each line with a string delimiter.

string sql = @"SELECT firstname, lastname, email
               FROM users
               WHERE username = @username AND password = @password";

Answer 16

The extern alias keyword to reference two versions of assemblies that have the same fully-qualified type names.

Answer 17

Need to return an empty IEnumerable?

public IEnumerable<T> GetEnumerator(){
  yield break;
}

Answer 18

You can limit the life and thus scope of variables by using { } brackets.

{
    string test2 = "3";
    Console.Write(test2);
}

Console.Write(test2); //compile error

test2 only lives within the brackets.

Answer 19

Ability to use LINQ Expressions to perform strongly-typed reflection:

static void Main(string[] args)
{
  var domain = "matrix";
  Check(() => domain);
  Console.ReadLine();
}

static void Check<T>(Expression<Func<T>> expr)
{
  var body = ((MemberExpression)expr.Body);
  Console.WriteLine("Name is: {0}", body.Member.Name);
  Console.WriteLine("Value is: {0}", ((FieldInfo)body.Member)
   .GetValue(((ConstantExpression)body.Expression).Value));
}

// output:
// Name is: 'domain'
// Value is: 'matrix'

More details are available at How to Find Out Variable or Parameter Name in C#?

Answer 20

You can use any Unicode character in C# names, for example:

public class MyClass
{
    public string Hårføner()
    {
        return "Yes, it works!";
    }
}

You can even use Unicode escapes. This one is equivalent to the above:

public class MyClass
{
    public string H\u00e5rføner()
    {
        return "Yes, it (still) works!";
    }
}

Answer 21

You can store colors in Enum.

public enum MyEnumColors : uint
{
    Normal          = 0xFF9F9F9F,
    Active          = 0xFF8EA98A,
    Error           = 0xFFFF0000
}

Answer 22

Using "~" operator with FlagAttribute and enum
Sometime we would use Flag attribute with enum to perform bitwise manipulation on the enumeration.

 [Flags]
 public enum Colors
 {
    None  = 0,
    Red   = 1,
    Blue  = 2,
    White = 4,
    Black = 8,
    Green = 16,
    All   = 31 //It seems pretty bad...
 }

Notice that, the value of option "All" which in enum is quite strange.
Instead of that we can use "~" operator with flagged enum.

 [Flags]
 public enum Colors
 {
    None  = 0,
    Red   = 1,
    Blue  = 2,
    White = 4,
    Black = 8,
    Green = 16,
    All   = ~0 //much better now. that mean 0xffffffff in default.
 }

Answer 23

How about the FlagsAttribute on an enumeration? It allows you to perform bitwise operations... took me forever to find out how to do bitwise operations in .NET nicely.

Answer 24

Also useful, but not commonly used : Constrained Execution Regions.

A quote from BCL Team blog :

Constrained execution regions (CER's) exist to help a developer write her code to maintain consistency. The CLR doesn't guarantee that the developer's code is correct, but the CLR does hoist all of the runtime-induced failure points (ie, async exceptions) to either before the code runs, or after it has completed. Combined with constraints on what the developer can put in a CER, these are a useful way of making strong guarantees about whether your code will execute. CER's are eagerly prepared, meaning that when we see one, we will eagerly JIT any code found in its statically-discoverable call graph. If the CLR's host cares about stack overflow, we'll probe for some amount of stack space as well (though perhaps not enough stack space for any arbitrary method*). We also delay thread aborts until the CER has finished running.

It can be useful when making edits to more than one field of a data structure in an atomic fashion. So it helps to have transactions on objects.

Also CriticalFinalizerObject seems to be hidden(at least who are not writing unsafe code). A CriticalFinalizerObject guarantees that garbage collection will execute the finalizer. Upon allocation, the finalizer and its call graph are prepared in advance.

Answer 25

You can add and remove delegates with less typing.

Usual way:

handler += new EventHandler(func);

Less typing way:

handler += func;

Answer 26

fixed statement

This statement prevents the garbage collector from relocating a movable variable. Fixed can also be used to create fixed size buffers.

The fixed statement sets a pointer to a managed variable and "pins" that variable during the execution of statement.

stackalloc

The stackalloc allocates a block of memory on the stack.

Answer 27

I have often come across the need to have a generic parameter-object persisted into the viewstate in a base class.

public abstract class BaseListControl<ListType,KeyType,ParameterType>
                 : UserControl 
                 where ListType : BaseListType
                 && ParameterType : BaseParameterType, new
{

    private const string viewStateFilterKey = "FilterKey";

    protected ParameterType Filters
    {
        get
        {
            if (ViewState[viewStateFilterKey] == null)
                ViewState[viewStateFilterKey]= new ParameterType();

            return ViewState[viewStateFilterKey] as ParameterType;
        }
        set
        {
            ViewState[viewStateFilterKey] = value;
        }
    }

}

Usage:

private void SomeEventHappened(object sender, EventArgs e)
{
    Filters.SomeValue = SomeControl.SelectedValue;
}

private void TimeToFetchSomeData()
{
    GridView.DataSource = Repository.GetList(Filters);
}

This little trick with the "where ParameterType : BaseParameterType, new" is what makes it really work.

With this property in my baseclass, I can automate handling of paging, setting filter values to filter a gridview, make sorting really easy, etc.

I am really just saying that generics can be an enormously powerful beast in the wrong hands.

Answer 28

Labeling my endregions...

#region stuff1
 #region stuff1a
 //...
 #endregion stuff1a
#endregion stuff1

Answer 29

ConditionalAttribute

Allows you to tell the compiler to omit the call to the method marked with the attribute under certain conditions (#define).

The fact that the method call is omitted also means that its parameters are not evaluated. This is very handy and it's what allows you to call expensive validation functions in Debug.Assert() and not worry about them slowing down your release build.

Answer 30

When defining custom attributes you can use them with [MyAttAttribute] or with [MyAtt]. When classes exist for both writings, then a compilation error occures.

The @ special character can be used to distinguish between them:

[AttributeUsage(AttributeTargets.All)]
public class X: Attribute
{}

[AttributeUsage(AttributeTargets.All)]
public class XAttribute: Attribute
{}

[X]      // Error: ambiguity
class Class1 {}

[XAttribute]   // Refers to XAttribute
class Class2 {}

[@X]      // Refers to X
class Class3 {}

[@XAttribute]   // Refers to XAttribute
class Class4 {}