What is the difference between String and string in C#?
Example (note the case):
string s = "Hello world!";
String s = "Hello world!";
What are the guidelines for the use of each? And what are the differences ?
string is an alias in C# for System.String .
So technically, there is no difference. It's like int vs. System.Int32 .
As far as guidelines, it's generally recommended to use string any time you're referring to an object.
eg
string place = "world";
Likewise, I think it's generally recommended to use String if you need to refer specifically to the class.
eg
string greet = String.Format("Hello {0}!", place);
This is the style that Microsoft tends to use in their examples.
It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.
Just for the sake of completeness, here's a brain dump of related information...
As others have noted, string is an alias for System.String . They compile to the same code, so at execution time there is no difference whatsoever. This is just one of the aliases in C#. The complete list is:
object: System.Object
string: System.String
bool: System.Boolean
byte: System.Byte
sbyte: System.SByte
short: System.Int16
ushort: System.UInt16
int: System.Int32
uint: System.UInt32
long: System.Int64
ulong: System.UInt64
float: System.Single
double: System.Double
decimal: System.Decimal
char: System.Char
Apart from string and object , the aliases are all to value types. decimal is a value type, but not a primitive type in the CLR. The only primitive type which doesn't have an alias is System.IntPtr .
In the spec, the value type aliases are known as "simple types". Literals can be used for constant values of every simple type; no other value types have literal forms available. (Compare this with VB, which allows DateTime literals, and has an alias for it too.)
There is one circumstance in which you have to use the aliases: when explicitly specifying an enum's underlying type. For instance:
public enum Foo : UInt32 {} // Invalid
public enum Bar : uint {} // Valid
That's just a matter of the way the spec defines enum declarations - the part after the colon has to be the integral-type production, which is one token of sbyte , byte , short , ushort , int , uint , long , ulong , char ... as opposed to a type production as used by variable declarations for example. It doesn't indicate any other difference.
Finally, when it comes to which to use: personally I use the aliases everywhere for the implementation, but the CLR type for any APIs. It really doesn't matter too much which you use in terms of implementation - consistency among your team is nice, but no-one else is going to care. On the other hand, it's genuinely important that if you refer to a type in an API, you do so in a language neutral way. A method called ReadInt32 is unambiguous, whereas a method called ReadInt requires interpretation. The caller could be using a language which defines an int alias for Int16 , for example. The .NET framework designers have followed this pattern, good examples being in the BitConverter , BinaryReader and Convert classes.
String stands for System.String and it is a .NET Framework type. string is an alias in the C# language for System.String . Both of them are compiled to System.String in IL (Intermediate Language), so there is no difference. Choose what you like and use that. If you code in C#, I'd prefer string as it's a C# type alias and well-known by C# programmers.
I can say the same about ( int , System.Int32 ) etc..
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