.net under the covers — using/Using/use

Something I had occasion to look at the other day was exactly how the auto-disposal mechanism actually works for the out of the box .net languages: and there are some interesting quirks to be found. Take the equivalent sample code fragments

	using (var r = new StringReader(text))
	{
	Console.WriteLine(r.ReadToEnd());
	}

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	Using r As New StringReader(text)
	Console.WriteLine(r.ReadToEnd())
	End Using

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	use r = new StringReader(text)
	printfn "%A" <| r.ReadToEnd()

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and for completeness, C++/CLI stack based disposal

	{
	StringReader r(text);
	Console::WriteLine(r.ReadToEnd());
	}

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All languages and configurations render this as a try {} finally { Dispose() } construction (apart from stack-based C++/CLI semantics, which generates a try {} catch { Dispose(); throw;} Dispose()), but there are devils in the details. In release mode, C# and VB compile to the same IL -- just dispose if the used value is not null

	finally
	{
	IL_001a: ldloc.1
	IL_001b: brfalse.s IL_0023
	IL_001d: ldloc.1
	IL_001e: callvirt instance void [mscorlib]System.IDisposable::Dispose()
	IL_0023: endfinally
	} // end handler

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whereas F# compiles to

	finally
	{
	IL_002c: ldloc.0
	IL_002d: isinst [mscorlib]System.IDisposable
	IL_0032: stloc.3
	IL_0033: ldloc.3
	IL_0034: brfalse.s IL_003f
	IL_0036: ldloc.3
	IL_0037: callvirt instance void [mscorlib]System.IDisposable::Dispose()
	IL_003c: ldnull
	IL_003d: pop
	IL_003e: endfinally
	IL_003f: ldnull
	IL_0040: pop
	IL_0041: endfinally
	} // end handler
	// which ILSpy decompiled to
	finally
	{
	IDisposable disposable = r as IDisposable;
	if (disposable != null)
	{
	disposable.Dispose();
	}
	}

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which is unexpected, as the compiler enforces the subject of a use initialisation to be an IDisposable anyway -- but, importantly, it does not test for null on the original reference.

In debug mode, all three languages differ, C# flips the test around

	finally
	{
	IL_001e: ldloc.1
	IL_001f: ldnull
	IL_0020: ceq
	IL_0022: stloc.2
	IL_0023: ldloc.2
	IL_0024: brtrue.s IL_002d
	IL_0026: ldloc.1
	IL_0027: callvirt instance void [mscorlib]System.IDisposable::Dispose()
	IL_002c: nop
	IL_002d: endfinally
	} // end handler

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VB does something a bit more like F#

	finally
	{
	IL_001f: ldloc.1
	IL_0020: ldnull
	IL_0021: ceq
	IL_0023: ldc.i4.0
	IL_0024: ceq
	IL_0026: stloc.2
	IL_0027: ldloc.2
	IL_0028: brfalse.s IL_0031
	IL_002a: ldloc.1
	IL_002b: callvirt instance void [mscorlib]System.IDisposable::Dispose()
	IL_0030: nop
	IL_0031: nop
	IL_0032: endfinally
	} // end handler
	// which ILSpy decompiled to (not recognising it as a Using block even when decompiling to VB!)
	finally
	{
	bool flag = r != null;
	if (flag)
	{
	((IDisposable)r).Dispose();
	}
	}

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and F# just adds one of its usual bursts of gratuitous branching in the middle

	finally
	{
	IL_0029: ldloc.0
	IL_002a: isinst [mscorlib]System.IDisposable
	IL_002f: stloc.s 4
	IL_0031: ldloc.s 4
	IL_0033: brfalse.s IL_0037
	IL_0035: br.s IL_0039
	IL_0037: br.s IL_0043
	IL_0039: ldloc.s 4
	IL_003b: callvirt instance void [mscorlib]System.IDisposable::Dispose()
	IL_0040: ldnull
	IL_0041: pop
	IL_0042: endfinally
	IL_0043: ldnull
	IL_0044: pop
	IL_0045: endfinally
	} // end handler

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The C++/CLI code can rely on the stack-based object not being null, so eschews any checks

	fault
	{
	IL_0022: ldloc.0
	IL_0023: callvirt instance void [mscorlib]System.IDisposable::Dispose()
	IL_0028: endfinally
	} // end handler
	IL_0029: ldloc.0
	IL_002a: callvirt instance void [mscorlib]System.IDisposable::Dispose()
	// which ILSpy decompiled to
	catch
	{
	((IDisposable)r).Dispose();
	throw;
	}
	((IDisposable)r).Dispose();

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