What is the difference between the using statement and directive in C#?

this is basically a tutorial question to ask since am a beginner I would like to what is a difference between the using statement we use at start of our C# code to include assembly and namespaces

like this:

using System.Web.Services;

and when we write inside the code within the method or code. like this:

using (SqlDataAdapter adapter = new SqlDataAdapter(cmd))

is there any difference or they both are same, any guidance would be helpful and appreciated.开发者_如何学编程

The first (Using Directive) is to bring a namespace into scope.

This is for example, so you can write

StringBuilder MyStringBuilder = new StringBuilder();

rather than

System.Text.StringBuilder MyStringBuilder = new System.Text.StringBuilder();

The second (Using Statement) one is for correctly using (creating and disposing) an object implementing the IDisposable interface.

For example:

using (Font font1 = new Font("Arial", 10.0f)) 
{
    byte charset = font1.GdiCharSet;
}

Here, the Font type implements IDisposable because it uses unmanaged resources that need to be correctly disposed of when we are no-longer using the Font instance (font1).

using (SqlDataAdapter adapter = new SqlDataAdapter(cmd))

This using disposes the adapter object automatically once the control leaves the using block.

This is equivalent to the call

SqlDataAdapter adapter = new SqlDataAdapter(cmd)
adapter.dispose();

See official documentation on this: http://msdn.microsoft.com/en-us/library/yh598w02(v=vs.71).aspx

They are about as different as you can get.

The first shows intent to use things within a namespace.

The second takes a reference to a disposable object and ensures it is disposed, no matter what happens (like implementing try/finally)

The first allows you to use types that are not defined in your code (tells the compiler where to find the code it needs to reference. REF: http://msdn.microsoft.com/en-us/library/sf0df423(v=VS.100).aspx

The second using makes sure that the memory is released upon the end of the code block, or in the case of an exception. REF: http://msdn.microsoft.com/en-us/library/yh598w02.aspx

Please see the links above for detailed documentation on each.

I'm sure someone will spend a great deal of time answering what amounts to a Google search but here are a couple of links to get you started.

The using Statement (C# Reference) ensures that Dispose is called even if an exception occurs while you are calling methods on the object.

To allow the use of types in a namespace so that you do not have to qualify the use of a type in that namespace use using Directive (C# Reference).

You may find that MSDN is a great resource to spend some time browsing.