The reason for the using
statement is to ensure that the object is disposed as soon as it goes out of scope, and it doesn't require explicit code to ensure that this happens.
As in Understanding the 'using' statement in C# (codeproject) and Using objects that implement IDisposable (microsoft), the C# compiler converts
using (MyResource myRes = new MyResource())
{
myRes.DoSomething();
}
to
{ // Limits scope of myRes
MyResource myRes= new MyResource();
try
{
myRes.DoSomething();
}
finally
{
// Check for a null resource.
if (myRes != null)
// Call the object's Dispose method.
((IDisposable)myRes).Dispose();
}
}
C# 8 introduces a new syntax, named "using declarations":
A using declaration is a variable declaration preceded by the using keyword. It tells the compiler that the variable being declared should be disposed at the end of the enclosing scope.
So the equivalent code of above would be:
using var myRes = new MyResource();
myRes.DoSomething();
And when control leaves the containing scope (usually a method, but it can also be a code block), myRes
will be disposed.
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.
Best Answer
The preferred way to do this is to only put an opening brace
{
after the lastusing
statement, like this: