c sharp Faqs

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C# Frequently Asked Questions

The C# team posts answers to common questions

  • How do I use an alias for a namespace or class?

    Use the using directive to create an alias for a long namespace or class name.
    You can then use it anywhere you normally would have used that class or
    namespace. The using alias has a scope within the namespace you declare it in. Sample code:

    // Namespace: 
    using act = System.Runtime.Remoting.Activation; 
    // Class: 
    using list = System.Collections.ArrayList; 
    ... 
    list l = new list(); // Creates an ArrayList 
    act.UrlAttribute foo; // Equivalent to System.Runtime.Remoting.Activation.UrlAttribute foo

    [Author: Jon Skeet]

  • What’s the difference between override and new?

    This is all to do with polymorphism. When a virtual method is called on a reference, the actual type of
    the object that the reference refers to is used to decide which method implementation to use. When a method
    of a base class is overridden in a derived class, the version in the derived class is used, even if the calling
    code didn’t “know” that the object was an instance of the derived class. For instance:

    public class Base 
    { 
        public virtual void SomeMethod() 
        { 
        } 
    } 
    
    public class Derived : Base 
    { 
        public override void SomeMethod() 
        { 
        } 
    } 
    
    ... 
    
    Base b = new Derived(); 
    b.SomeMethod();

    will end up calling Derived.SomeMethod if that overrides Base.SomeMethod. Now,
    if you use the new keyword instead of override, the method in the derived class
    doesn’t override the method in the base class, it merely hides it. In that case, code like this:

    public class Base 
    { 
        public virtual void SomeOtherMethod() 
        { 
        } 
    } 
    
    public class Derived : Base 
    { 
        public new void SomeOtherMethod() 
        { 
        } 
    } 
    
    ... 
    
    Base b = new Derived(); 
    Derived d = new Derived(); 
    b.SomeOtherMethod(); 
    d.SomeOtherMethod();

    Will first call Base.SomeOtherMethod (line 3), then Derived.SomeOtherMethod (line 4).
    They’re effectively two entirely separate methods which happen to have the same name, rather than the derived method
    overriding the base method.

    If you don’t specify either new or overrides, the resulting output is the same
    as if you specified new, but you’ll also get a compiler warning (as you may not be aware that you’re
    hiding a method in the base class method, or indeed you may have wanted to override it, and merely forgot to
    include the keyword).

    That provides the basics of overriding and the difference between new and override,
    but you should really see a book or tutorial for a more in-depth look at polymorphism.

    [Author: Jon Skeet]

  • Why doesn’t C# have checked exceptions?

    Checked exceptions are a very hotly debated topic in some circles, particularly for experienced Java developers moving to, or additionally learning, C#. Here are some resources that discuss the issue in depth:

    Note that without the CLR itself supporting checked exceptions, it would be effectively impossible for C# to do so alone.
    [Author: Jon Skeet]

  • What’s the difference between cast syntax and using the as operator?

    Using the as operator differs from a cast in C# in three important ways:

    1. It returns null when the variable you are trying to convert is not of
      the requested type or in it’s inheritance chain, instead of throwing an exception.
    2. It can only be applied to reference type variables converting to reference types.
    3. Using as will not perform user-defined conversions, such as implicit or
      explicit conversion operators, which casting syntax will do.

    There are in fact two completely different operations defined in IL that
    handle these two keywords (the castclass and
    isinst instructions) – it’s not
    just “syntactic sugar” written by C# to get this different behavior. The
    as operator appears to be slightly faster in v1.0 and v1.1 of Microsoft’s
    CLR compared to casting (even in cases where there are no invalid casts
    which would severely lower casting’s performance due to exceptions).

    [Author: Jon Skeet]

  • What’s the difference between string and System.String?

    C# defines a number of aliases for CLR types. They may be used interchangably, and even mixed together, e.g.
    string x = new System.String(' ', 5);.
    These are the aliases defined:

    Alias CLR type
    string System.String
    sbyte System.SByte
    byte System.Byte
    short System.Int16
    ushort System.UInt16
    int System.Int32
    uint System.UInt32
    long System.Int64
    ulong System.UInt64
    char System.Char
    float System.Single
    double System.Double
    bool System.Boolean
    decimal System.Decimal

    [Author: Jon Skeet]

  • What’s the difference between an event and a delegate?

    Put simply, an event gives more limited access than a delegate. If an event is made public, code in
    other classes can only add or remove handlers for that event; they can’t necessarily fire it, find out
    all the handlers for it, or remove handlers they don’t know about. Events also allow more flexibility in
    terms of how the handlers are stored. For more details on this, see
    Eric
    Gunnerson’s article on the topic
    .

    [Author: Jon Skeet]

  • Why can’t I use static and const together?

    All constants declarations are implicitly static, and the C# specification states that the
    (redundant) inclusion of the static modifier is prohibited. I believe this is to avoid the
    confusion which could occur if a reader were to see two constants, one declared static and one not – they
    could easily assume that the difference in specification implied a difference in semantics. Having said
    that, there is no prohibition on redundantly specifying an access modifier which is also the default one,
    where there is a choice. For instance, a (concrete) method can be explicitly marked as private despite that being the
    default. The rule appears to be that where there is no choice (e.g. a method declaration in an interface)
    the redundant modifier is prohibited. Where there is a choice, it’s allowed.

    [Author: Jon Skeet]

  • What character escape sequences are available?

    C# defines the following character escape sequences:

    • \' – single quote, needed for character literals
    • \" – double quote, needed for string literals
    • \\ – backslash
    • – Unicode character 0
    • \a – Alert (character 7)
    • \b – Backspace (character 8)
    • \f – Form feed (character 12)
    • \n – New line (character 10)
    • \r – Carriage return (character 13)
    • \t – Horizontal tab (character 9)
    • \v – Vertical quote (character 11)
    • \uxxxx – Unicode escape sequence for character with hex value xxxx
    • \xn[n][n][n] – Unicode escape sequence for character with hex value nnnn (variable length version of \uxxxx)
    • \Uxxxxxxxx – Unicode escape sequence for character with hex value xxxxxxxx (for generating surrogates)

    Of these, \a, \f, \v, \x and \U are rarely used in my experience.

    [Author: Jon Skeet]

  • What does an @ before the start of a string literal mean?

    A string literal such as @"c:\Foo" is called a verbatim string literal. It basically means,
    “don’t apply any interpretations to characters until the next quote character is reached”. So, a verbatim
    string literal can contain backslashes (without them being doubled-up) and even line separators. To get a
    double-quote (") within a verbatim literal, you need to just double it, e.g.
    @"My name is ""Jon""" represents the string My name is "Jon". Verbatim string literals
    which contain line separators will also contain the white-space at the start of the line, so I tend
    not to use them in cases where the white-space matters. They’re very handy for including XML or SQL in your source
    code though, and another typical use (which doesn’t need line separators) is for specifying a file system path.

    It’s worth noting that it doesn’t affect the string itself in any way: a string specified as a verbatim string literal
    is exactly the same as a string specified as a normal string literal with appropriate escaping. The debugger will
    sometimes choose to display a string as a verbatim string literal – this is solely for ease of viewing the string’s
    contents without worrying about escaping.

    [Author: Jon Skeet]

  • How are parameters passed in C#? Are they passed by reference or by value?

    By default, all parameters are passed by value in C#. Parameters are only passed by reference if you explicitly include
    an out or ref modifier. However, you need to be aware that when the type of the
    parameter is a reference type, you’re passing a reference rather than an actual object. For more information, see my
    article on parameter passing and
    the C#
    Programmer’s Reference
    .

    [Author: Jon Skeet]

  • Does C# have templates like C++?

    Although C# doesn’t have templates, and isn’t likely to get them, it is
    getting a feature called generics which will be available in the next version
    of .NET and Visual Studio. Generics will be a feature in the CLR itself,
    and most languages targetting the CLR will change to support it. Generics provide a
    lot of the functionality of C++ templates (mostly in terms of type safety) but in a more
    restricted (and therefore potentially less confusing) way.

    For more information, see:

    [Author: Jon Skeet]

  • Why doesn’t C# have VB.NET’s ‘with’ operator?

    Many people, including the C# language designers, believe that ‘with’ often harms readability, and is more of a curse than a blessing. It is clearer to declare a local variable with a meaningful name, and use that variable to perform multiple operations on a single object, than it is to have a block with a sort of implicit context.
    For more information, see the Ask a C# Language Designer page.
    [Author: Jon Skeet]
  • What are the advantages of C# over VB.NET and vice versa?

    The choice between C# and VB.NET is largely one of subjective
    preference. Some people like C#’s terse syntax, others like VB.NET’s
    natural language, case-insensitive approach. Both have access to the same
    framework libraries. Both will perform largely equivalently (with a few
    small differences which are unlikely to affect most people, assuming
    VB.NET is used with Option Strict on). Learning the .NET framework itself is
    a much bigger issue than learning either of the languages, and it’s perfectly possible
    to become fluent in both – so don’t worry too much about which to plump for. There are,
    however, a few actual differences which may affect your decision:

    VB.NET Advantages
    • Support for optional parameters – very handy for some COM interoperability
    • Support for late binding with Option Strict off – type safety at compile time
      goes out of the window, but legacy libraries which don’t have strongly typed interfaces
      become easier to use.
    • Support for named indexers (aka properties with parameters).
    • Various legacy VB functions (provided in the Microsoft.VisualBasic namespace, and can be
      used by other languages with a reference to the Microsoft.VisualBasic.dll). Many of
      these can be harmful to performance if used unwisely, however, and many people believe they should
      be avoided for the most part.
    • The with construct: it’s a matter of debate as to whether this is an advantage or not,
      but it’s certainly a difference.
    • Simpler (in expression – perhaps more complicated in understanding) event handling, where
      a method can declare that it handles an event, rather than the handler having to be set up in code.
    • The ability to implement interfaces with methods of different names. (Arguably this makes it harder
      to find the implementation of an interface, however.)
    • Catch ... When ... clauses, which allow exceptions to be filtered based on runtime expressions
      rather than just by type.
    • The VB.NET part of Visual Studio .NET compiles your code in the background. While this is considered an
      advantage for small projects, people creating very large projects have found that the IDE slows down considerably
      as the project gets larger.
    C# Advantages
    • XML documentation generated from source code comments. (This is coming in VB.NET with
      Whidbey (the code name for the next version of Visual Studio and .NET), and there are tools which will do it with existing VB.NET code
      already.)
    • Operator overloading – again, coming to VB.NET in Whidbey.
    • Language support for unsigned types (you can use them from VB.NET, but they aren’t in the language
      itself). Again, support for these is coming to VB.NET in Whidbey.
    • The using statement, which makes unmanaged resource disposal simple.
    • Explicit interface implementation, where an interface which is already implemented in a base class
      can be reimplemented separately in a derived class. Arguably this makes the class harder to understand,
      in the same way that member hiding normally does.
    • Unsafe code. This allows pointer arithmetic etc, and can improve performance in some situations.
      However, it is not to be used lightly, as a lot of the normal safety of C# is lost (as the name implies).
      Note that unsafe code is still managed code, i.e. it is compiled to IL, JITted, and run within the CLR.

    Despite the fact that the above list appears to favour VB.NET (if you don’t mind waiting for Whidbey),
    many people prefer C#’s terse syntax enough to make them use C# instead.

    [Author: Jon Skeet]

  • Where is the C# specification?

    There are two versions of the C# specification – one from Microsoft, and one from ECMA. They are the same in all important respects (a few pieces of explanatory wording are different, but nothing that affects the specification itself) but the numbering of sections is different.

    The ECMA specification is available as a PDF, or you can email ECMA and they will send you a hard copy for free. Details are on the ECMA website. Alternatively, Jon Jagger’s HTML version is available and is a useful download. It is from a draft version, but it’s very close to the final version. I don’t know whether or not Jon Jagger is planning to make an HTML copy of version 2 of the specification when it’s released.

    The Microsoft version is available online or within the offline version of the MSDN – look for “specifications, Visual C#” in the index to find it quickly.

  • Why don’t relational operators return bool as a type?

    Section 10.9.2 of the language specification says:

    A binary operator must take two parameters, at least one of which must have the class or struct type in which the operator is declared. Parameters of the shift operators (§7.8) are further constrained. A binary operator can return any type.

    Why isn’t it limited to just returning bool?

    There is some weirdness here because of the C# ability to support nullable types, such as the SQL types (ie SQLInt32, SQLSingle, etc.). These type can either have a value or be null.

    One of the strange features of the SQL expression syntax is if either operand in a binary operation is null, the result is null. So, when we want to write the comparison operator for SQLInt32, we have a problem if the result type is constrained to bool, since null is neither true or false.

    C# therefore allows the comparison operators to return another type as long as the resulting type has operator true and operator false defined. That gives us enough support to be able to write:

    SQLInt32 x = …;

    SQLInt32 y = …;

    if (x == y)

    {

        // x and y or equal

    }

    else if (x != y)

    {

        // x and y are not equal

    }

    else

    {

        // the result of the comparison is null

    }

     

    Which allows the full richness of SQL types to be expressed.

    Author: Eric Gunnerson]

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