ILAsm: The Mono Implementation

This time, I’ll write about some issues I faced during development of Mono’s ILAsm and how I got around them. I’ve had to make several changes that make Mono’s ILAsm stricter than MS.NET’s or outright incompatible.

First of all, we have generic parameters. In MS.NET’s ILAsm, !0 and !!0 are simply emitted as VAR 0 and MVAR 0 in the metadata, meaning that you could do something like:

.method public static !!0 Foo(!!1 bar)
{
    ldarg bar
    ret
}

Even though !!1 refers to a generic parameter that does not exist, MS.NET’s ILAsm will gladly emit it, and not even warn you. Due to an API “limitation” in Cecil, Mono’s ILAsm cannot do this. All GenericParameter objects must have an owner (i.e. the type or method definition/reference) and must be contained in that owner’s GenericParameters collection at the correct index. Therefore, Mono’s ILAsm would throw an error for the above code, since !!0 is out of bounds. This is a slightly annoying incompatibility, but there’s not much I can do about it other than patching Cecil. That’s only going to happen, though, if it won’t cause API breaks for version 1.0.

Second, properties like .file alignment, .imagebase, and .stackreserve aren’t fully supported. Cecil currently has no way of setting these on a module. ILAsm will check the values and error if they’re invalid, though.

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