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.

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. Assuming your code using String compiles to System.String (i.e. you haven't got a using directive for some other namespace with a different String type), 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 that 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..

The best answer I have ever heard about using the provided type aliases in C# comes from Jeffrey Richter in his book CLR Via C#. Here are his 3 reasons:

• I've seen a number of developers confused, not knowing whether to use string or String in their code. Because in C# the string (a keyword) maps exactly to System.String (an FCL type), there is no difference and either can be used.
• In C#, long maps to System.Int64, but in a different programming language, long could map to an Int16 or Int32. In fact, C++/CLI does in fact treat long as an Int32. Someone reading source code in one language could easily misinterpret the code's intention if he or she were used to programming in a different programming language. In fact, most languages won't even treat long as a keyword and won't compile code that uses it.
• The FCL has many methods that have type names as part of their method names. For example, the BinaryReader type offers methods such as ReadBoolean, ReadInt32, ReadSingle, and so on, and the System.Convert type offers methods such as ToBoolean, ToInt32, ToSingle, and so on. Although it's legal to write the following code, the line with float feels very unnatural to me, and it's not obvious that the line is correct:

BinaryReader br = new BinaryReader(...);
float val  = br.ReadSingle(); // OK, but feels unnatural
Single val = br.ReadSingle(); // OK and feels good

So there you have it. I think these are all really good points. I however, don't find myself using Jeffrey's advice in my own code. Maybe I am too stuck in my C# world but I end up trying to make my code look like the framework code.

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.

There is one difference - you can't use String without using System; beforehand.

It's been covered above; however, you can't use string in reflection; you must use String.

System.String is the .NET string class - in C# string is an alias for System.String - so in use they are the same.

As for guidelines I wouldn't get too bogged down and just use whichever you feel like - there are more important things in life and the code is going to be the same anyway.

If you find yourselves building systems where it is necessary to specify the size of the integers you are using and so tend to use Int16, Int32, UInt16, UInt32 etc. then it might look more natural to use String - and when moving around between different .net languages it might make things more understandable - otherwise I would use string and int.

I prefer the capitalized .NET types (rather than the aliases) for formatting reasons. The .NET types are colored the same as other object types (the value types are proper objects, after all).

Conditional and control keywords (like if, switch, and return) are lowercase and colored dark blue (by default). And I would rather not have the disagreement in use and format.

Consider:

String someString; 
string anotherString; 

string and String are identical in all ways (except the uppercase "S"). There are no performance implications either way.

Lowercase string is preferred in most projects due to the syntax highlighting

C# is a language which is used together with the CLR.

string is a type in C#.

System.String is a type in the CLR.

When you use C# together with the CLR string will be mapped to System.String.

Theoretically, you could implement a C#-compiler that generated Java bytecode. A sensible implementation of this compiler would probably map string to java.lang.String in order to interoperate with the Java runtime library.

Lower case string is an alias for System.String. They are the same in C#.

There's a debate over whether you should use the System types (System.Int32, System.String, etc.) types or the C# aliases (int, string, etc). I personally believe you should use the C# aliases, but that's just my personal preference.

string is just an alias for System.String. The compiler will treat them identically.

The only practical difference is the syntax highlighting as you mention, and that you have to write using System if you use String.

Both are same. But from coding guidelines perspective it's better to use string instead of String. This is what generally developers use. e.g. instead of using Int32 we use int as int is alias to Int32

FYI “The keyword string is simply an alias for the predefined class System.String.” - C# Language Specification 4.2.3 http://msdn2.microsoft.com/En-US/library/aa691153.aspx

As the others are saying, they're the same. StyleCop rules, by default, will enforce you to use string as a C# code style best practice, except when referencing System.String static functions, such as String.Format, String.Join, String.Concat, etc...

Using System types makes it easier to port between C# and VB.Net, if you are into that sort of thing.

I'd just like to add this to lfousts answer, from Ritchers book:

The C# language specification states, “As a matter of style, use of the keyword is favored over use of the complete system type name.” I disagree with the language specification; I prefer to use the FCL type names and completely avoid the primitive type names. In fact, I wish that compilers didn’t even offer the primitive type names and forced developers to use the FCL type names instead. Here are my reasons:

• I’ve seen a number of developers confused, not knowing whether to use string or String in their code. Because in C# string (a keyword) maps exactly to System.String (an FCL type), there is no difference and either can be used. Similarly, I’ve heard some developers say that int represents a 32-bit integer when the application is running on a 32-bit OS and that it represents a 64-bit integer when the application is running on a 64-bit OS. This statement is absolutely false: in C#, an int always maps to System.Int32, and therefore it represents a 32-bit integer regardless of the OS the code is running on. If programmers would use Int32 in their code, then this potential confusion is also eliminated.

• In C#, long maps to System.Int64, but in a different programming language, long could map to an Int16 or Int32. In fact, C++/CLI does treat long as an Int32. Someone reading source code in one language could easily misinterpret the code’s intention if he or she were used to programming in a different programming language. In fact, most languages won’t even treat long as a keyword and won’t compile code that uses it.

• The FCL has many methods that have type names as part of their method names. For example, the BinaryReader type offers methods such as ReadBoolean, ReadInt32, ReadSingle, and so on, and the System.Convert type offers methods such as ToBoolean, ToInt32, ToSingle, and so on. Although it’s legal to write the following code, the line with float feels very unnatural to me, and it’s not obvious that the line is correct:

  BinaryReader br = new BinaryReader(...);
  float val = br.ReadSingle(); // OK, but feels unnatural
  Single val = br.ReadSingle(); // OK and feels good
  

• Many programmers that use C# exclusively tend to forget that other programming languages can be used against the CLR, and because of this, C#-isms creep into the class library code. For example, Microsoft’s FCL is almost exclusively written in C# and developers on the FCL team have now introduced methods into the library such as Array’s GetLongLength, which returns an Int64 value that is a long in C# but not in other languages (like C++/CLI). Another example is System.Linq.Enumerable’s LongCount method.

I didn't get his opinion before I read the complete paragraph.

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.

@JaredPar (a developer on the C# compiler and prolific SO user!) wrote a great blog post on this issue. I think it is worth sharing here. It is a nice perspective on our subject.

string vs. String is not a style debate

[...]

The keyword string has concrete meaning in C#. It is the type System.String which exists in the core runtime assembly. The runtime intrinsically understands this type and provides the capabilities developers expect for strings in .NET. Its presence is so critical to C# that if that type doesn’t exist the compiler will exit before attempting to even parse a line of code. Hence string has a precise, unambiguous meaning in C# code.

The identifier String though has no concrete meaning in C#. It is an identifier that goes through all the name lookup rules as Widget, Student, etc … It could bind to string or it could bind to a type in another assembly entirely whose purposes may be entirely different than string. Worse it could be defined in a way such that code like String s = "hello"; continued to compile.

class TricksterString { 
  void Example() {
    String s = "Hello World"; // Okay but probably not what you expect.
  }
}

class String {
  public static implicit operator String(string s) => null;
}

The actual meaning of String will always depend on name resolution. That means it depends on all the source files in the project and all the types defined in all the referenced assemblies. In short it requires quite a bit of context to know what it means.

True that in the vast majority of cases String and string will bind to the same type. But using String still means developers are leaving their program up to interpretation in places where there is only one correct answer. When String does bind to the wrong type it can leave developers debugging for hours, filing bugs on the compiler team, and generally wasting time that could’ve been saved by using string.

Another way to visualize the difference is with this sample:

string s1 = 42; // Errors 100% of the time  
String s2 = 42; // Might error, might not, depends on the code

Many will argue that while this is information technically accurate using String is still fine because it’s exceedingly rare that a codebase would define a type of this name. Or that when String is defined it’s a sign of a bad codebase.

[...]

You’ll see that String is defined for a number of completely valid purposes: reflection helpers, serialization libraries, lexers, protocols, etc … For any of these libraries String vs. string has real consequences depending on where the code is used.

So remember when you see the String vs. string debate this is about semantics, not style. Choosing string gives crisp meaning to your codebase. Choosing String isn’t wrong but it’s leaving the door open for surprises in the future.

Note: I copy/pasted most of the blog posts for archive reasons. I ignore some parts, so I recommend skipping and reading the blog post if you can.

There is one practical difference between string and String.

nameof(String); // compiles
nameof(string); // doesn't compile

This is because string is a keyword (an alias in this case) whereas String is a type.

The same is true for the other aliases as well.

| Alias     | Type             |
|-----------|------------------|
|  bool     |  System.Boolean  |
|  byte     |  System.Byte     |
|  sbyte    |  System.SByte    |
|  char     |  System.Char     |
|  decimal  |  System.Decimal  |
|  double   |  System.Double   |
|  float    |  System.Single   |
|  int      |  System.Int32    |
|  uint     |  System.UInt32   |
|  long     |  System.Int64    |
|  ulong    |  System.UInt64   |
|  object   |  System.Object   |
|  short    |  System.Int16    |
|  ushort   |  System.UInt16   |
|  string   |  System.String   |

First of All, both string & String are not same. There is a difference: String is not a keyword and it can be used as an identifier whereas string is a keyword and cannot be used as identifier.

I am trying to explain with different example : First, when I put string s; into Visual Studio and hover over it I get (without the colour):

That says that string is System.String, right? The documentation is at https://msdn.microsoft.com/en-us/library/362314fe.aspx. The second sentence says "string is an alias for String in the .NET Framework.".

Jeffrey Richter written:

Another way to think of this is that the C# compiler automatically assumes that you have the following using directives in all of your source code files:

using int = System.Int32;
using uint = System.UInt32;
using string = System.String;
...

I’ve seen a number of developers confused, not knowing whether to use string or String in their code. Because in C# string (a keyword) maps exactly to System.String (an FCL type), there is no difference and either can be used.

I prefer to use string because this type is used so much that I don't want the syntax highlighter blending it in with all the other classes. Although it is a class it is used more like a primitive therefore I think the different highlight colour is appropriate.

If you right click on the string keyword and select Go to definition from the context menu it'll take you to the String class - it's just syntactic sugar but it improves readability imo.

A string is a sequential collection of characters that is used to represent text.

A String object is a sequential collection of System.Char objects that represent a string; a System.Char object corresponds to a UTF-16 code unit.

The value of the String object is the content of the sequential collection of System.Char objects, and that value is immutable (that is, it is read-only).

For more information about the immutability of strings, see the Immutability and the StringBuilder class section in msdn.

The maximum size of a String object in memory is 2GB, or about 1 billion characters.

Note : answer is extracted from msdn help section. You can see the full content here in msdn String Class topic under Remarks section

string is short name of System.String. String or System.String is name of string in CTS(Common Type System).

As you already know string is just alias for System.String. But what should I use? it just personal preference.

In my case, I love to use string rather than use System.String because String requires a namespace using System; or a full name System.String.

So I believe the alias string was created for simplicity and I love it!

string is a shortcut for System.String. The only difference is that you don´t need to reference to System.String namespace. So would be better using string than String.

it is common practice to declare a variable using C# keywords. In fact, every C# type has an equivalent in .NET. As another example, short and int in C# map to Int16 and Int32 in .NET. So, technically there is no difference between string and String, but In C#, string is an alias for the String class in .NET framework.

String is the class of string. If you remove System namespace from using statements, you can see that String has gone but string is still here. string is keyword for String. Like
int and Int32
short and Int16
long and Int64
So the keywords are just some words that uses a class. These keywords are specified by C#(so Microsoft, because C# is Microsoft's). Briefly, there's no difference. Using string or String. That doesn't matter. They are same.

declare a string variable with string but use the String class when accessing one of its static members:

String.Format()

Variable

string name = "";

All the above is basically correct. One can check it. Just write a short method

public static void Main()
{
    var s = "a string";
}

compile it and open .exe with ildasm to see

.method private hidebysig static void  Main(string[] args) cil managed
{
  .entrypoint
  // Code size       8 (0x8)
  .maxstack  1
  .locals init ([0] string s)
  IL_0000:  nop
  IL_0001:  ldstr      "a string"
  IL_0006:  stloc.0
  IL_0007:  ret
} // end of method Program::Main

then change var to string and String, compile, open with ildasm and see IL does not change. It also shows the creators of the language prefer just string when difining variables (spoiler: when calling members they prefer String).

There is no major difference between string and String in C#.

String is a class in the .NET framework in the System namespace. The fully qualified name is System.String. The lower case string is an alias of System.String.

But its recommended to use string while declaring variables like:

string str = "Hello";

And we can use String while using any built-in method for strings like String.IsNullOrEmpty().

Also one difference between these two is like before using String we have to import system namespace in cs file and string can be used directly.

Image result for string vs String www.javatpoint.com In C#, string is an alias for the String class in .NET framework. In fact, every C# type has an equivalent in .NET.
Another little difference is that if you use the String class, you need to import the System namespace, whereas you don't have to import namespace when using the string keyword

There are many (e.g. Jeffrey Richter in his book CLR Via C#) who are saying that there is no difference between System.String and string, and also System.Int32 and int, but we must discriminate a little deeper to really squeeze the juice out of this question so we can get all the nutritional value out of it (write better code).

A. They are the Same...

1. to the compiler.
2. to the developer. (We know #1 and eventually achieve autopilot.)

B. They are Different in Famework and in Non-C# Contexts. Different...

1. to OTHER languages that are NOT C#
2. in an optimized CIL (was MSIL) context (the .NET VM assembly language)
3. in a platform-targeted context -- the .NET Framework or Mono or any CIL-type area
4. in a book targeting multiple .NET Languages (such as VB.NET, F#, etc.)

So, the true answer is that it is only because C# has to co-own the .NET space with other languages that this question even exists.

C. To Summarize...

You use string and int and the other C# types in a C#-only targeted audience (ask the question, who is going to read this code, or use this library). For your internal company, if you only use C#, then stick to the C# types.

...and you use System.String and System.Int32 in a multilingual or framework targeted audience (when C# is not the only audience). For your internal organization, if you also use VB.NET or F# or any other .NET language, or develop libraries for consumption by customers who may, then you should use the "Frameworky" types in those contexts so that everyone can understand your interface, no matter what universe they are from. (What is Klingon for System.String, anyway?)

HTH.

Essentially, there is no difference between string and String in C#.

String is a class in the .NET framework in the System namespace under System.String, Whereas, lower case string is an alias of System.String.

Logging the full name of both types can prove this

string s1= "hello there 1";
String s2 = "hello there 2";
        
Console.WriteLine(s1.GetType().FullName); // System.String
Console.WriteLine(s2.GetType().FullName); // System.String

It is recommended to use string over String but it's really a matter of choice. Most developers use string to declare variables in C# and use System.String class to use any built-in string methods like for an example , the String.IsNullOrEmpty() method.

All differences were discussed at length.

But if you're after a simple answer for your code, then just know that since Visual Studio 2015 if you try to use String the IDE suggests you to turn it to string.

In the cases both are valid, always prioritize string.

There are at least 4 differences:

1- 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.

2- you can't use String without "using System".so you write less code by using "string".

3- 'String' is better naming convention than 'string', as it is a type not variable.

4- "string" is a C# keyword and syntax highlighted in most coding editors, but not "String".