Predefined types

C# provides a set of predefined types, most of which will be familiar to C and C++ developers.
The predefined reference types are object and string. The type object is the ultimate base type of all other types. The type string is used to represent Unicode string values. Values of type string are immutable.
The predefined value types include signed and unsigned integral types, floating-point types, and the types bool, char, and decimal. The signed integral types are sbyte, short, int, and long; the unsigned integral types are byte, ushort, uint, and ulong; and the floating-point types are float and double.
The bool type is used to represent boolean values: values that are either true or false. The inclusion of bool makes it easier to write self-documenting code, and also helps eliminate the all-too-common C++ coding error in which a developer mistakenly uses "=" when "==" should have been used. In C#, the example int i = ...; F(i); if (i = 0) // Bug: the test should be (i == 0) G(); results in a compile-time error because the expression i = 0 is of type int, and if statements require an expression of type bool.
The char type is used to represent Unicode characters. A variable of type char represents a single 16-bit Unicode character.
The decimal type is appropriate for calculations in which rounding errors caused by floating point representations are unacceptable. Common examples include financial calculations such as tax computations and currency conversions. The decimal type provides 28 significant digits.
The table below lists the predefined types, and shows how to write literal values for each of them. Type Description Example object The ultimate base type of all other types object o = null; string String type; a string is a sequence of Unicode characters string s = "hello"; sbyte 8-bit signed integral type sbyte val = 12; short 16-bit signed integral type short val = 12; int 32-bit signed integral type int val = 12; long 64-bit signed integral type long val1 = 12; long val2 = 34L; byte 8-bit unsigned integral type byte val1 = 12; ushort 16-bit unsigned integral type ushort val1 = 12; uint 32-bit unsigned integral type uint val1 = 12; uint val2 = 34U; ulong 64-bit unsigned integral type ulong val1 = 12; ulong val2 = 34U; ulong val3 = 56L; ulong val4 = 78UL; float Single-precision floating point type float val = 1.23F; double Double-precision floating point type double val1 = 1.23; double val2 = 4.56D; bool Boolean type; a bool value is either true or false bool val1 = true; bool val2 = false; char Character type; a char value is a Unicode character char val = 'h'; decimal Precise decimal type with 28 significant digits decimal val = 1.23M;
Each of the predefined types is shorthand for a system-provided type. For example, the keyword int refers to the struct System.Int32. As a matter of style, use of the keyword is favored over use of the complete system type name.
Predefined value types such as int are treated specially in a few ways but are for the most part treated exactly like other structs. Operator overloading enables developers to define new struct types that behave much like the predefined value types. For instance, a Digit struct can support the same mathematical operations as the predefined integral types, and can define conversions between Digit and predefined types.
The predefined types employ operator overloading themselves. For example, the comparison operators == and != have different semantics for different predefined types: Two expressions of type int are considered equal if they represent the same integer value. Two expressions of type object are considered equal if both refer to the same object, or if both are null. Two expressions of type string are considered equal if the string instances have identical lengths and identical characters in each character position, or if both are null.
The example using System; class Test { static void Main() { string s = "Test"; string t = string.Copy(s); Console.WriteLine(s == t); Console.WriteLine((object)s == (object)t); } } produces the output True False because the first comparison compares two expressions of type string, and the second comparison compares two expressions of type object.