# Built-in reference types (C# reference)

C# has a number of built-in reference types. They have keywords or operators that are synonyms for a type in the .NET library.

## The object type

The object type is an alias for System.Object in .NET. In the unified type system of C#, all types, predefined and user-defined, reference types and value types, inherit directly or indirectly from System.Object. You can assign values of any type to variables of type object. Any object variable can be assigned to its default value using the literal null. When a variable of a value type is converted to object, it is said to be boxed. When a variable of type object is converted to a value type, it is said to be unboxed. For more information, see Boxing and Unboxing.

## The string type

The string type represents a sequence of zero or more Unicode characters. string is an alias for System.String in .NET.

Although string is a reference type, the equality operators == and != are defined to compare the values of string objects, not references. This makes testing for string equality more intuitive. For example:

string a = "hello";
string b = "h";
// Append to contents of 'b'
b += "ello";
Console.WriteLine(a == b);
Console.WriteLine(object.ReferenceEquals(a, b));


This displays "True" and then "False" because the content of the strings are equivalent, but a and b do not refer to the same string instance.

The + operator concatenates strings:

string a = "good " + "morning";


This creates a string object that contains "good morning".

Strings are immutable--the contents of a string object cannot be changed after the object is created, although the syntax makes it appear as if you can do this. For example, when you write this code, the compiler actually creates a new string object to hold the new sequence of characters, and that new object is assigned to b. The memory that had been allocated for b (when it contained the string "h") is then eligible for garbage collection.

string b = "h";
b += "ello";


The [] operator can be used for readonly access to individual characters of a string. Valid index values start at 0 and must be less than the length of the string:

string str = "test";
char x = str[2];  // x = 's';


In similar fashion, the [] operator can also be used for iterating over each character in a string:

string str = "test";

for (int i = 0; i < str.Length; i++)
{
Console.Write(str[i] + " ");
}
// Output: t e s t


String literals are of type string and can be written in two forms, quoted and @-quoted. Quoted string literals are enclosed in double quotation marks ("):

"good morning"  // a string literal


String literals can contain any character literal. Escape sequences are included. The following example uses escape sequence \\ for backslash, \u0066 for the letter f, and \n for newline.

string a = "\\\u0066\n F";
Console.WriteLine(a);
// Output:
// \f
//  F


Note

The escape code \udddd (where dddd is a four-digit number) represents the Unicode character U+dddd. Eight-digit Unicode escape codes are also recognized: \Udddddddd.

Verbatim string literals start with @ and are also enclosed in double quotation marks. For example:

@"good morning"  // a string literal


The advantage of verbatim strings is that escape sequences are not processed, which makes it easy to write, for example, a fully qualified Windows file name:

@"c:\Docs\Source\a.txt"  // rather than "c:\\Docs\\Source\\a.txt"


To include a double quotation mark in an @-quoted string, double it:

@"""Ahoy!"" cried the captain." // "Ahoy!" cried the captain.


## The delegate type

The declaration of a delegate type is similar to a method signature. It has a return value and any number of parameters of any type:

public delegate void MessageDelegate(string message);
public delegate int AnotherDelegate(MyType m, long num);


In .NET, System.Action and System.Func types provide generic definitions for many common delegates. You likely don't need to define new custom delegate types. Instead, you can create instantiations of the provided generic types.

A delegate is a reference type that can be used to encapsulate a named or an anonymous method. Delegates are similar to function pointers in C++; however, delegates are type-safe and secure. For applications of delegates, see Delegates and Generic Delegates. Delegates are the basis for Events. A delegate can be instantiated by associating it either with a named or anonymous method.

The delegate must be instantiated with a method or lambda expression that has a compatible return type and input parameters. For more information on the degree of variance that is allowed in the method signature, see Variance in Delegates. For use with anonymous methods, the delegate and the code to be associated with it are declared together.

## The dynamic type

The dynamic type indicates that use of the variable and references to its members bypass compile-time type checking. Instead, these operations are resolved at run time. The dynamic type simplifies access to COM APIs such as the Office Automation APIs, to dynamic APIs such as IronPython libraries, and to the HTML Document Object Model (DOM).

Type dynamic behaves like type object in most circumstances. In particular, any non-null expression can be converted to the dynamic type. The dynamic type differs from object in that operations that contain expressions of type dynamic are not resolved or type checked by the compiler. The compiler packages together information about the operation, and that information is later used to evaluate the operation at run time. As part of the process, variables of type dynamic are compiled into variables of type object. Therefore, type dynamic exists only at compile time, not at run time.

The following example contrasts a variable of type dynamic to a variable of type object. To verify the type of each variable at compile time, place the mouse pointer over dyn or obj in the WriteLine statements. Copy the following code into an editor where IntelliSense is available. IntelliSense shows dynamic for dyn and object for obj.

class Program
{
static void Main(string[] args)
{
dynamic dyn = 1;
object obj = 1;

// Rest the mouse pointer over dyn and obj to see their
// types at compile time.
System.Console.WriteLine(dyn.GetType());
System.Console.WriteLine(obj.GetType());
}
}


The WriteLine statements display the run-time types of dyn and obj. At that point, both have the same type, integer. The following output is produced:

System.Int32
System.Int32


To see the difference between dyn and obj at compile time, add the following two lines between the declarations and the WriteLine statements in the previous example.

dyn = dyn + 3;
obj = obj + 3;


A compiler error is reported for the attempted addition of an integer and an object in expression obj + 3. However, no error is reported for dyn + 3. The expression that contains dyn is not checked at compile time because the type of dyn is dynamic.

The following example uses dynamic in several declarations. The Main method also contrasts compile-time type checking with run-time type checking.

using System;

namespace DynamicExamples
{
class Program
{
static void Main(string[] args)
{
ExampleClass ec = new ExampleClass();
Console.WriteLine(ec.exampleMethod(10));
Console.WriteLine(ec.exampleMethod("value"));

// The following line causes a compiler error because exampleMethod
// takes only one argument.
//Console.WriteLine(ec.exampleMethod(10, 4));

dynamic dynamic_ec = new ExampleClass();
Console.WriteLine(dynamic_ec.exampleMethod(10));

// Because dynamic_ec is dynamic, the following call to exampleMethod
// with two arguments does not produce an error at compile time.
// However, it does cause a run-time error.
//Console.WriteLine(dynamic_ec.exampleMethod(10, 4));
}
}

class ExampleClass
{
static dynamic field;
dynamic prop { get; set; }

public dynamic exampleMethod(dynamic d)
{
dynamic local = "Local variable";
int two = 2;

if (d is int)
{
return local;
}
else
{
return two;
}
}
}
}
// Results:
// Local variable
// 2
// Local variable