What are the uses of “using” in C#?

Question

User kokos answered the wonderful Hidden Features of C# question by mentioning the using keyword. Can you elaborate on that? What are the uses of

Answer 1

I've used it a lot in the past to work with input and output streams. You can nest them nicely and it takes away a lot of the potential problems you usually run into (by automatically calling dispose). For example:

        using (FileStream fs = new FileStream("c:\file.txt", FileMode.Open))
        {
            using (BufferedStream bs = new BufferedStream(fs))
            {
                using (System.IO.StreamReader sr = new StreamReader(bs))
                {
                    string output = sr.ReadToEnd();
                }
            }
        }

Answer 2

You can make use of the alias namespace by way of the following example:

using LegacyEntities = CompanyFoo.CoreLib.x86.VBComponents.CompanyObjects;

This is called a using alias directive as as you can see, it can be used to hide long-winded references should you want to make it obvious in your code what you are referring to e.g.

LegacyEntities.Account

instead of

CompanyFoo.CoreLib.x86.VBComponents.CompanyObjects.Account

or simply

Account   // It is not obvious this is a legacy entity

Answer 3

using is used when you have a resource that you want disposed after it's been used.

For instance if you allocate a File resource and only need to use it in one section of code for a little reading or writing, using is helpful for disposing of the File resource as soon as your done.

The resource being used needs to implement IDisposable to work properly.

Example:

using (File file = new File (parameters))
{
    *code to do stuff with the file*
}

Answer 4

Another great use of using is when instantiating a modal dialog.

Using frm as new Form1

Form1.ShowDialog

' do stuff here

End Using

Answer 5

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.

Answer 6

Since a lot of people still do:

using (System.IO.StreamReader r = new System.IO.StreamReader(""))
using (System.IO.StreamReader r2 = new System.IO.StreamReader("")) {
   //code
}

I guess a lot of people still don't know that you can do:

using (System.IO.StreamReader r = new System.IO.StreamReader(""), r2 = new System.IO.StreamReader("")) {
   //code
}