There is actually a (subtle) difference between the two. Imagine you have the following code in File1.cs:
// File1.cs
using System;
namespace Outer.Inner
{
class Foo
{
static void Bar()
{
double d = Math.PI;
}
}
}
Now imagine that someone adds another file (File2.cs) to the project that looks like this:
// File2.cs
namespace Outer
{
class Math
{
}
}
The compiler searches Outer
before looking at those using
directives outside the namespace, so it finds Outer.Math
instead of System.Math
. Unfortunately (or perhaps fortunately?), Outer.Math
has no PI
member, so File1 is now broken.
This changes if you put the using
inside your namespace declaration, as follows:
// File1b.cs
namespace Outer.Inner
{
using System;
class Foo
{
static void Bar()
{
double d = Math.PI;
}
}
}
Now the compiler searches System
before searching Outer
, finds System.Math
, and all is well.
Some would argue that Math
might be a bad name for a user-defined class, since there's already one in System
; the point here is just that there is a difference, and it affects the maintainability of your code.
It's also interesting to note what happens if Foo
is in namespace Outer
, rather than Outer.Inner
. In that case, adding Outer.Math
in File2 breaks File1 regardless of where the using
goes. This implies that the compiler searches the innermost enclosing namespace before it looks at any using
directive.
C# language version history:
These are the versions of C# known about at the time of this writing:
- C# 1.0 released with .NET 1.0 and VS2002 (January 2002)
- C# 1.2 (bizarrely enough); released with .NET 1.1 and VS2003 (April 2003). First version to call
Dispose
on IEnumerator
s which implemented IDisposable
. A few other small features.
- C# 2.0 released with .NET 2.0 and VS2005 (November 2005). Major new features: generics, anonymous methods, nullable types, and iterator blocks
- C# 3.0 released with .NET 3.5 and VS2008 (November 2007). Major new features: lambda expressions, extension methods, expression trees, anonymous types, implicit typing (
var
), and query expressions
- C# 4.0 released with .NET 4 and VS2010 (April 2010). Major new features: late binding (
dynamic
), delegate and interface generic variance, more COM support, named arguments, tuple data type and optional parameters
- C# 5.0 released with .NET 4.5 and VS2012 (August 2012). Major features: async programming, and caller info attributes. Breaking change: loop variable closure.
- C# 6.0 released with .NET 4.6 and VS2015 (July 2015). Implemented by Roslyn. Features: initializers for automatically implemented properties, using directives to import static members, exception filters, element initializers,
await
in catch
and finally
, extension Add
methods in collection initializers.
- C# 7.0 released with .NET 4.7 and VS2017 (March 2017). Major new features: tuples, ref locals and ref return, pattern matching (including pattern-based switch statements), inline
out
parameter declarations, local functions, binary literals, digit separators, and arbitrary async returns.
- C# 7.1 released with VS2017 v15.3 (August 2017). New features: async main, tuple member name inference, default expression, and pattern matching with generics.
- C# 7.2 released with VS2017 v15.5 (November 2017). New features: private protected access modifier, Span<T>, aka interior pointer, aka stackonly struct, and everything else.
- C# 7.3 released with VS2017 v15.7 (May 2018). New features: enum, delegate and
unmanaged
generic type constraints. ref
reassignment. Unsafe improvements: stackalloc
initialization, unpinned indexed fixed
buffers, custom fixed
statements. Improved overloading resolution. Expression variables in initializers and queries. ==
and !=
defined for tuples. Auto-properties' backing fields can now be targeted by attributes.
- C# 8.0 released with .NET Core 3.0 and VS2019 v16.3 (September 2019). Major new features: nullable reference-types, asynchronous streams, indices and ranges, readonly members, using declarations, default interface methods, static local functions, and enhancement of interpolated verbatim strings.
- C# 9.0 released with .NET 5.0 and VS2019 v16.8 (November 2020). Major new features: init-only properties, records, with-expressions, data classes, positional records, top-level programs, improved pattern matching (simple type patterns, relational patterns, logical patterns), improved target typing (target-type
new
expressions, target typed ??
and ?
), and covariant returns. Minor features: relax ordering of ref
and partial
modifiers, parameter null checking, lambda discard parameters, native int
s, attributes on local functions, function pointers, static lambdas, extension GetEnumerator
, module initializers, and extending partial.
In response to the OP's question:
What are the correct version numbers for C#? What came out when? Why can't I find any answers about C# 3.5?
There is no such thing as C# 3.5 - the cause of confusion here is that the C# 3.0 is present in .NET 3.5. The language and framework are versioned independently, however - as is the CLR, which is at version 2.0 for .NET 2.0 through 3.5, .NET 4 introducing CLR 4.0, service packs notwithstanding. The CLR in .NET 4.5 has various improvements, but the versioning is unclear: in some places it may be referred to as CLR 4.5 (this MSDN page used to refer to it that way, for example), but the Environment.Version
property still reports 4.0.xxx.
As of May 3, 2017, the C# Language Team created a history of C# versions and features on their GitHub repository: Features Added in C# Language Versions. There is also a page that tracks upcoming and recently implemented language features.
Best Answer
The AppPool's .NET CLR Version is different from the .NET Framework Version.
The .NET CLR Version 4.0 is the CLR base for the following .NET Framework Versions:
So having a .NET CLR Version of 4.0 just means you support the above Frameworks, which will run under their deployed framework version.
Source: https://docs.microsoft.com/en-us/dotnet/standard/clr#clr-versions
Update Feb 2020:
Here is a full mapping of CLR to .NET Framework versions, based off this official documentation. https://docs.microsoft.com/en-us/dotnet/framework/migration-guide/versions-and-dependencies#version-information