The "using" Keyword in C#

Introduction

I was at work today, and someone asked me what happens when you use the using keyword in C#. Most people will tell you that it is something you use that will clean up any unmanaged resources for the specified object, which is not incorrect. But what actually happens at the IL level? How does it "clean up" unmanaged resources? I had my assumptions, but I really didn't know. So, I set out to find out for myself.

Tests

I decided I was going to run through a couple of tests:

Test #1

I wanted to see what the generated MSIL code looks like when I use the using keyword. So, I wrote some very simple sample code in C#, compiled it, then I decompiled it using ildasm to see the MSIL code.

Test #2

I wanted to see if I could write code without using the using keyword that would generate the exact same MSIL code. This process was a little more trial and error, but was fairly easy.

Test #1

Here is my sample code:

[STAThread]
private static void Main(string[] args)
{
      using (Bitmap bitmap1 = new Bitmap(100, 100))
      {
            Console.WriteLine("Width: {0}, Height: {1}", bitmap1.Width, bitmap1.Height);
      }
      Console.ReadLine();
}

As you can see... nothing special in the code. Create a new Bitmap inside a using statement, write some output to the console, wait for user input, then exit.

What does this look like when we build the app, then decompile it into MSIL? Check it out:

.method private hidebysig static void Main(string[] args) cil managed
{
      .custom instance void [mscorlib]System.STAThreadAttribute::.ctor()
      .entrypoint
      .maxstack 4
      .locals init (
            [0] [System.Drawing]System.Drawing.Bitmap bitmap1)
      L_0000: ldc.i4.s 100
      L_0002: ldc.i4.s 100
      L_0004: newobj instance void 
	[System.Drawing]System.Drawing.Bitmap::.ctor(int32, int32)
      L_0009: stloc.0 
      L_000a: ldstr "Width: {0}, Height: {1}"
      L_000f: ldloc.0 
      L_0010: callvirt instance int32 [System.Drawing]System.Drawing.Image::get_Width()
      L_0015: box int32
      L_001a: ldloc.0 
      L_001b: callvirt instance int32 [System.Drawing]System.Drawing.Image::get_Height()
      L_0020: box int32
      L_0025: call void [mscorlib]System.Console::WriteLine(string, object, object)
      L_002a: leave.s L_0036
      L_002c: ldloc.0 
      L_002d: brfalse.s L_0035
      L_002f: ldloc.0 
      L_0030: callvirt instance void [mscorlib]System.IDisposable::Dispose()
      L_0035: endfinally 
      L_0036: call string [mscorlib]System.Console::ReadLine()
      L_003b: pop 
      L_003c: ret 
      .try L_000a to L_002c finally handler L_002c to L_0036
}

Test #1 Results

So the results from test #1 are interesting. The using keyword is basically a try - finally block, without a catch , where IDisposable.Dispose() is called in the finally . One interesting thing to note is that the Bitmap constructor is called before the try block begins. This tells me that if, in the Bitmap constructor, an unmanaged resource is allocated, but not freed, then the constructor throws an exception, the unmanaged resource will not get freed by a call to the IDisposable.Dispose(). This also assumes that the IDisposable is implemented properly. Therefore the using keyword is useless if the IDisposable is not implemented properly. The constructor should clean up resources if it fails, and the Dispose() method should clean up all unmanaged resources. Chances are good that Microsoft has implemented IDisposable correctly in their classes, so watch out for this if you are implementing your own IDisposable.

Test #2

Based on the MSIL code that resulted from Test #1, I decided to write the same code using try - finally blocks. This is what I came up with:

[STAThread]
private static void Main(string[] args)
{
      Bitmap bitmap1 = new Bitmap(100, 100);
      try
      {
            Console.WriteLine("Width: {0}, Height: {1}", bitmap1.Width, bitmap1.Height);
      }
      finally
      {
            if (bitmap1 != null)
            {
                  bitmap1.Dispose();
            }
      }
      Console.ReadLine();
}

And this is what it looked like in MSIL:

.method private hidebysig static void Main(string[] args) cil managed
{
      .custom instance void [mscorlib]System.STAThreadAttribute::.ctor()
      .entrypoint
      .maxstack 4
      .locals init (
            [0] [System.Drawing]System.Drawing.Bitmap bitmap1)
      L_0000: ldc.i4.s 100
      L_0002: ldc.i4.s 100
      L_0004: newobj instance void 
	[System.Drawing]System.Drawing.Bitmap::.ctor(int32, int32)
      L_0009: stloc.0 
      L_000a: ldstr "Width: {0}, Height: {1}"
      L_000f: ldloc.0 
      L_0010: callvirt instance int32 [System.Drawing]System.Drawing.Image::get_Width()
      L_0015: box int32
      L_001a: ldloc.0 
      L_001b: callvirt instance int32 [System.Drawing]System.Drawing.Image::get_Height()
      L_0020: box int32
      L_0025: call void [mscorlib]System.Console::WriteLine(string, object, object)
      L_002a: leave.s L_0036
      L_002c: ldloc.0 
      L_002d: brfalse.s L_0035
      L_002f: ldloc.0 
      L_0030: callvirt instance void [System.Drawing]System.Drawing.Image::Dispose()
      L_0035: endfinally 
      L_0036: call string [mscorlib]System.Console::ReadLine()
      L_003b: pop 
      L_003c: ret 
      .try L_000a to L_002c finally handler L_002c to L_0036
}

Test #2 Results

It is almost exactly like the MSIL generated from the using keyword, except this calls Image.Dispose() rather than IDisposable.Dispose(), but the effect is the same. Just for fun, I regenerated C# code using Lutz Roeder's .NET Reflector, which is an invaluable tool, and here is the result:

[STAThread]
private static void Main(string[] args)
{
      using (Bitmap bitmap1 = new Bitmap(100, 100))
      {
            Console.WriteLine("Width: {0}, Height: {1}", bitmap1.Width, bitmap1.Height);
      }
      Console.ReadLine();
}

This looks exactly like our original code!!!

Conclusion

This was really a fun experiment that revealed a lot about the way .NET works behind the scenes. I hope you guys enjoy this as much as I did.

History

• 10^th January, 2007: Initial post