What is the difference between String and string in C#?


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, 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.


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?

No difference. byte is alias to System.Byte, the same way int is alias to System.Int32, long to System.Int64, string to System.String, ...

Difference between byte vs Byte data types in C#

The byte keyword is an alias for the System.Byte data type.

They represent the same data type, so the resulting code is identical. There are only some differences in usage:

  • You can use byte even if the System namespace is not included. To use Byte you have to have a using System; at the top of the page, or specify the full namespace System.Byte.

  • There are a few situations where C# only allows you to use the keyword, not the framework type, for example:


enum Fruits : byte // this works
  Apple, Orange

enum Fruits : Byte // this doesn't work
  Apple, Orange

They are aliases and are interchangeable. However, stylistically, for declarations, I use the lowercased string, and for the static methods, I use String.

string foo = "bar";

if( foo != String.Empty )
   Console.WriteLine(String.Format("foo.Length = {0}", foo.Length));

String vs string

There is no difference. string (lower case) is just an alias for System.String.

It is conceptually similar to something like this:

using int=System.Int32