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When and why to use aliased name or its class name in c#?

Why and when should we use int, int16, int32, int64, double, string instead of Int, Int16, Int32, Int64, Double, String respectively? I have ready many articles about this but still not getting a proper solution.

This is purely a matter of preference/tradition.
C# language specification states:
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 favoured over use of the complete system type name.

It doesn't matter at all in implementation. I personally prefer using the aliases, and from what I've seen of other code that's the more common choice. There are some who suggest using the BCL names (Int32 etc) everywhere though - including Jeffrey Richter, author of CLR via C# (where he gives that advice).
However, when you're naming methods and types, you should use the BCL name rather than the C#-specific name. That way your code is equally idiomatic to people using your code from other languages. This is the convention adopted within the BCL itself. For example, we have Convert.ToInt32 and Convert.ToSingle instead of Convert.ToInt and Convert.ToFloat.
While it doesn't matter in theory if those are internal or private members, I'd suggest getting in the habit of giving your members the names you'd want to expose publicly - it means you can be consistent there regardless of access, and you're less likely to accidentally let one slip through.

It truly does not matter, just be consistent. I personally use the original, lowercase names.

When to use String and string? [duplicate]

What are the differences between these two and which one should I use?
string s = "Hello world!";
String s = "Hello world!";

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

This YouTube video demonstrates practically how they differ.
But now for a long textual answer.
When we talk about .NET there are two different things one there is .NET framework and the other there are languages (C#, VB.NET etc) which use that framework.
"System.String" a.k.a "String" (capital "S") is a .NET framework data type while "string" is a C# data type.
In short "String" is an alias (the same thing called with different names) of "string". So technically both the below code statements will give the same output.
String s = "I am String";
or
string s = "I am String";
In the same way, there are aliases for other C# data types as shown below:
object: System.Object, string: System.String, bool: System.Boolean, byte: System.Byte, sbyte: System.SByte, short: System.Int16 and so on.
Now the million-dollar question from programmer's point of view: So when to use "String" and "string"?
The first thing to avoid confusion use one of them consistently. But from best practices perspective when you do variable declaration it's good to use "string" (small "s") and when you are using it as a class name then "String" (capital "S") is preferred.
In the below code the left-hand side is a variable declaration and it is declared using "string". On the right-hand side, we are calling a method so "String" is more sensible.
string s = String.ToUpper() ;

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...

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.

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

Against what seems to be common practice among other programmers, I prefer String over string, just to highlight the fact that String is a reference type, as Jon Skeet mentioned.

string is an alias (or shorthand) of System.String. That means, by typing string we meant System.String. You can read more in think link: 'string' is an alias/shorthand of System.String.

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.

String (System.String) is a class in the base class library. string (lower case) is a reserved work in C# that is an alias for System.String. Int32 vs int is a similar situation as is Boolean vs. bool. These C# language specific keywords enable you to declare primitives in a style similar to C.

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.

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.

Coming late to the party: I use the CLR types 100% of the time (well, except if forced to use the C# type, but I don't remember when the last time that was).
I originally started doing this years ago, as per the CLR books by Ritchie. It made sense to me that all CLR languages ultimately have to be able to support the set of CLR types, so using the CLR types yourself provided clearer, and possibly more "reusable" code.
Now that I've been doing it for years, it's a habit and I like the coloration that VS shows for the CLR types.
The only real downer is that auto-complete uses the C# type, so I end up re-typing automatically generated types to specify the CLR type instead.
Also, now, when I see "int" or "string", it just looks really wrong to me, like I'm looking at 1970's C code.

There is no difference.
The C# keyword string maps to the .NET type System.String - it is an alias that keeps to the naming conventions of the language.
Similarly, int maps to System.Int32.

There's a quote on this issue from Daniel Solis' book.
All the predefined types are mapped directly to
underlying .NET types. The C# type names (string) are simply aliases for the
.NET types (String or System.String), so using the .NET names works fine syntactically, although
this is discouraged. Within a C# program, you should use the C# names
rather than the .NET names.

Yes, that's no difference between them, just like the bool and Boolean.

string is a keyword, and you can't use string as an identifier.
String is not a keyword, and you can use it as an identifier:
Example
string String = "I am a string";
The keyword string is an alias for
System.String aside from the keyword issue, the two are exactly
equivalent.
typeof(string) == typeof(String) == typeof(System.String)

Int32.Parse vs int.Parse

It is a common practice to use C# type aliases instead of CTS System.* types (int instead of Int32 and string instead of String). However it's not clear to me what to use to call a static method of a type in this case: an alias or a system type.
Microsoft does not seem to define any guidelies to use aliases instead of System types. But in MSDN aliases are used for variables and CTS equivalents are used for static calls. For example MSDN: Parsing Numeric Strings
int number;
Int32.TryParse(value, out number);
StyleCop defines the contrary in SA1121 - to always use aliases. So int.Parse is ok while Int32.Parse is not.
This question is a matter of style (in my opinion). But I don't understand reasons to use CTS type for static calls.

I personally always use full class names for static method calls. This underlines the fact that they are in fact classes that contain pieces of code instead of simplest possible (primitive) data which the aliases imply.
I always use aliases for variables.

There is absolutely no technical difference, just coding style. Personally I advocate int.Parse, but there are many well-known experts that prefer the opposite.

I strongly suggest to use int.Parse(...) instead of int32.If you immagine someone
read your code running it on 64 bit machine, and it's probbale that he is not aware of that int32 and int are just aliases, it will make him to do a wrong assumptions.
In other words, even if from functional point of view there is no any difference, to avoid ambiguity in the code (referencies to presumably 32 bit code) I would suggest to use int.Parse(..)

What is the difference between String.Format and string.Format (and other static members of primitive data types)?

As far as I can tell, any static member of a class like String or Int32 can also be accessed from the related primitive data type. So, String.Format is the same as string.Format, and Int32.MaxValue is the same as int.MaxValue.
Is there a difference between these two forms? Is one preferred to the other? Even if they are identical, is one generally considered more readable?
Edit: Since they are identical, is one preferred from a human perspective? Would you rather see String.Format or string.Format when reading someone else's code?

There's no difference, these are type aliases in C# for .Net framework types, you're calling the same method underneath.
For example:
int is an alias for System.Int32
string is an alias for System.String
You can find a complete list of these aliases on MSDN here.

Those are not related primitive data types. They are simply shorthands available in C#. string aliases System.String and int aliases System.Int32. Calls to int.MaxValue are calls to Int32.MaxValue. C# just allows you to type it in shorthand similar to what you would type if you were in another C-like language.

Most of the answers have it in general. HOWEVER, there are some cases in which the alias is required. Off the top of my head:
public enum MyEnum:Byte {...} //will not compile
public enum MyEnum:byte {...} //correct
There are a couple other places where you must use the alias; besides that, it's mostly style. The following two rules are generally accepted, perhaps the first more than the second:
Use the alias (lowercase keyword) for all usages of that define a variable or member type (declaration, parameters, casting, generic type closure)
Use the type name (PascalCased class identifier) for all usages of the type in static context (Calling static methods such as parsers or string manipulation methods, or static properties like MinValue/MaxValue).

They're identical. string and int are syntactic aliases for String and Int32 respectively.

string is not a primative type but simply an alias for String. They are the same thing.

Notice that if you go into your IDE and hover your mouse over string, you will see that it is defined by System.String
This is an "alias"... one thing is shorthand for another thing. int is an alias for Int32, byte is an alias for Byte, char is an alias for Char, and so on and so forth.

A funny proof for their interchangeability is the IDE itself.
Start typing List<String> list = new then look at IntelliSense's suggestion: List<string>. It won't even give you List<String> as one of the choices.
To answer the second part of your question, which is an opinion matter, I prefer int and string over upper-case for many reasons:
The bright blue color is more definitive and if you're fast-reading code you cannot mistake the class name for something else
The lesser I need to hit SHIFT+key, the faster I type
IntelliSense will auto-complete the lower-case names, even if you start with uppercase, so to me it's aesthetically unpleasing to see upper- and lower-case versions of the same class in the same line, or even in the same project
The type int is very simple in nature, and I would hate to force my eyes to associate that with having an upper-case letter and two digits in its name (i.e. Int32); it makes it look too specific for no obvious reason
A colleague of mine said specifically using Int32 and String ensures compatibility with future .NET updates; I disagree with whoever says that completely. Prove me wrong, please!

String.Format() requires "using Systems;" but string.Format() doesn't.

Best practice of use member of Primitive types

How I should write: long.MaxValue or Int64.MaxValue?
Are there standard from Microsoft?

long.MaxValue
because you are coding in c#.
Int64.MaxValue is a .NET type.

Default StyleCop settings will tell you to use long.MaxValue.

This is subjective, but I find Int64.MaxValue easier to read, because the capital letter makes it stand out clearly as a static member of a type called Int64, rather than an instance member of a local variable called long (not that there could ever be a local variable called long of course). But I don't think anyone reading your code is going to be confused whichever way you do it!

I would match it to the type of the variable it is being assigned to. If you are using long in the variable declaration, use long.MaxValue.

Jeffrey Richter writes:
I prefer to use the FCL type names and completely avoid the primitive type names.
...
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.

I prefer the Int64.MaxValue.
Also think about this case:
float val = Single.Parse(..);
Single val = Single.Parse(..);
I do believe that the 2nd (FCL type name) is more clear than the 1st (built-in type name).