What is the difference between String and string i

Question

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?

Answer 1

String is not a keyword and it can be used as Identifier whereas string is a keyword and cannot be used as Identifier. And in function point of view both are same.

Answer 2

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..

Answer 3

It's a matter of convention, really. string just looks more like C/C++ style. The general convention is to use whatever shortcuts your chosen language has provided (int/Int for Int32). This goes for "object" and decimal as well.

Theoretically this could help to port code into some future 64-bit standard in which "int" might mean Int64, but that's not the point, and I would expect any upgrade wizard to change any int references to Int32 anyway just to be safe.

Answer 4

New answer after 6 years and 5 months (procrastination).

While string is a reserved C# keyword that always has a fixed meaning, String is just an ordinary identifier which could refer to anything. Depending on members of the current type, the current namespace and the applied using directives and their placement, String could be a value or a type distinct from global::System.String.

I shall provide two examples where using directives will not help.

---

First, when String is a value of the current type (or a local variable):

class MySequence<TElement>
{
  public IEnumerable<TElement> String { get; set; }

  void Example()
  {
    var test = String.Format("Hello {0}.", DateTime.Today.DayOfWeek);
  }
}

The above will not compile because IEnumerable<> does not have a non-static member called Format, and no extension methods apply. In the above case, it may still be possible to use String in other contexts where a type is the only possibility syntactically. For example String local = "Hi mum!"; could be OK (depending on namespace and using directives).

Worse: Saying String.Concat(someSequence) will likely (depending on usings) go to the Linq extension method Enumerable.Concat. It will not go to the static method string.Concat.

---

Secondly, when String is another type, nested inside the current type:

class MyPiano
{
  protected class String
  {
  }

  void Example()
  {
    var test1 = String.Format("Hello {0}.", DateTime.Today.DayOfWeek);
    String test2 = "Goodbye";
  }
}

Neither statement in the Example method compiles. Here String is always a piano string, MyPiano.String. No member (static or not) Format exists on it (or is inherited from its base class). And the value "Goodbye" cannot be converted into it.

Answer 5

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.

e.g.

string place = "world";

Likewise, I think it's generally recommended to use String if you need to refer specifically to the class.

e.g.

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.

Answer 6

string is a reserved word, but String is just a class name. This means that string cannot be used as a variable name by itself.

If for some reason you wanted a variable called string, you'd see only the first of these compiles:

StringBuilder String = new StringBuilder();  // compiles
StringBuilder string = new StringBuilder();  // doesn't compile 

If you really want a variable name called string you can use @ as a prefix:

StringBuilder @string = new StringBuilder();

Another critical difference: Stack Overflow highlights them differently.