en-US/about_BadImageFormat.help.txt
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ABOUT BAD IMAGE FORMAT
SHORT DESCRIPTION Describes some possible causes of a `BadImageFormatException` or `InvalidProgramException` when using the `New-ILDelegate` command. LONG DESCRIPTION The `BadImageFormatException` or `InvalidProgramException` is thrown by the Common Language Runtime (CLR) whenever it detects invalid IL. This topic will go over some of the potential reasons this can occur. This is far from an exhaustive list and mostly goes over the common cases. Invalid evaluation stack state When returning from a method the evaluation stack must either be empty (in the case of a `void` return method) or contain exactly one item (in the case of a return). The following example shows an evaluation stack with too many items. ```powershell il { [int]@() } { # Load `0` as an int32 onto the eval stack. ldc.i4.0 # Load a second `0` as an int32 onto the eval stack. ldc.i4.0 # The CLR expects only a single item on the eval stack, but sees two. ret } ``` The following example shows an evaluation stack with too few items. ```powershell il { [int]@() } { # Load `0` as an int32 onto the eval stack. ldc.i4.0 # Pull an int32 off the eval stack and pass to Console.WriteLine as an # argument. call { [void] [Console]::WriteLine([int]) } # Eval stack is empty, but the method is declared to return an int32. ret } ``` .maxstack is too low Methods must declare to the CLR the maximum amount of items that they will put on the evaluation stack. The default for this module (when not specified with `.maxstack`) is `8`. ```powershell il { [void]@() } { ldc.i4.0 ldc.i4.0 ldc.i4.0 ldc.i4.0 ldc.i4.0 ldc.i4.0 ldc.i4.0 ldc.i4.0 # Since `.maxstack` has not been specified it has defaulted to 8. This # nineth item will result in invalid IL. Adding `.maxstack 9` would make # this valid. ldc.i4.0 pop; pop; pop; pop; pop; pop; pop; pop; pop ret } ``` ```powershell il { [void]@() } { .maxstack 1 ldc.i4.0 # Since `.maxstack` was declared as 1, this second item will result in # invalid IL. Changing `.maxstack` to 2 would make this valid. ldc.i4.0 pop; pop ret } ``` Leaving a structured exception handling region incorrectly Exiting a structured exception handling (SEH) region requires a specific instruction for each type of SEH region. The following table describes which instruction to use when exiting each SEH region type. | SEH Type | Instruction | | --------- | ------------------ | | `catch` | `leave.s`, `leave` | | `finally` | `endfinally` | | `fault` | `endfault` | | `filter` | `endfilter` | Incorrect: ```powershell il { [void]@() } { .try { newobj { [void] [Exception].new() } throw } catch { # Invalid, cannot `ret` from a catch block. ret } } ``` Correct: ```powershell il { [void]@() } { .try { newobj { [void] [Exception].new() } throw } catch { # Pop the exception off the eval stack. pop leave.s exitSEH } exitSEH: ret } ``` Missing `ret` A method must use the `ret` instruction to exit, regardless of if it has a non-void return. Incorrect: ```powershell il { [void]@() } { ldstr 'Does not return, but still must exit with ret' call { [void] [Console]::WriteLine([string]) } } ``` Correct: ```powershell il { [void]@() } { ldstr 'Does not return, but still must exit with ret' call { [void] [Console]::WriteLine([string]) } ret } ``` |