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The input file is truncated in some way that prevents it from being read as an IL image.
The input file is not a PE/COFF file. That is, it is not formatted as a Windows executable or dynamic link library, and is also not formatted as a Portable.Net object file (see ilasm(1) for details on the object file format).
The input file is a valid PE/COFF file, but it does not contain an IL binary within it. It is probably an ordinary native Windows executable that cannot be processed by ildasm.
The input file is a valid PE/COFF file that contains a valid IL binary, but the IL data is for an unsupported version.
There was something wrong with the order of sections within the file that caused the loader to need to seek backwards. This will normally only happen for IL binaries that have been incorrectly generated or corrupted in some way.
The IL binary was so large that it could not be loaded into memory prior to disassembly.
The PE/COFF file or IL binary within the file contained a "relative virtual address", or RVA, that was invalid. This will normally only happen for IL binaries that have been incorrectly generated or corrupted in some way.
The metadata section was invalid in some way. Usually this indicates a corrupted IL binary.
The metadata section was mostly valid, but it used features for which we do not have sufficient documentation to parse successfully.
The metadata section was valid, but it referred to types in another file that could not be fully resolved.
Depending on how the Portable.NET development tools were built, errors of this kind may appear in addition to the other errors listed above. They provide additional diagnostics that mainly of interest to the authors. If you come across a file that produces such an error, then contact the authors.