Wednesday, November 29, 2006

Ildasm: Viewing attributes

[This was originally posted at]

Yesterday I mentioned ILdasm to view the meta data on an assembly. One of the benefits of .Net is that it requires meta data for all assemblies. You can even create your own strongly-typed meta data via attributes. For example, both NUnit and MSTest use attributes to indicate that a certain method is a unit test. Because all the meta data is public, these external programs can use reflection to loop through all the methods in an assembly, and check which methods have which attributes.

These attributes show up in the IL. If you use Ildasm to view a method, you'll see which attributes it has. The IL snippet below has both the [TestMethod] attribute used by MSTest, and my own custom [Duration] attribute.

.method public hidebysig instance void  Duration_String_1() cil managed
  .custom instance void [Microsoft.VisualStudio.QualityTools.UnitTestFramework]Microsoft.VisualStudio.TestTools.UnitTesting.TestMethodAttribute::.ctor() = ( 01 00 00 00 )
  .custom instance void TestDemo.Duration.DurationAttribute::.ctor(float64) = ( 01 00 7B 14 AE 47 E1 7A 84 3F 00 00 )             // ..{..G.z.?..
  // Code size       18 (0x12)
  .maxstack  2
  .locals init ([0] string s)
  IL_0011:  ret
} // end of method Strings::Duration_String_1

Of course, you'd probably never use Ildasm to read the attributes, but it's good to know that everything is consistent - i.e. meta data is public for any .Net assembly - it's not cryptic information only for debug more or something (like a *.pdb). So it makes sense that external programs like MSTest and NUnit can read these attributes.

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