I claim this task.

go! :)

Part one :)

Part two.

Sorry I'm short on time today (and will be out of town for the next two days). 
I'll
try to review more stuff tonight (in about 6 hours). For the moment...

I think you're missing calls to object creation in the second rule.
E.g.

public void CallPInvoke_CreateObject_GetError ()
{
    MessageBeep(5);
    new CallGetLastErrorAfterPInvokeTest ();
    Marshal.GetLastWin32Error();
}

since the rule has no clue what's being done in the ctor it should warn that 
the call
to GetLastError is too late.

Second note: I forgot to say (but it's no big deal ;-) but the two rules will 
be in a
new assembly (and namespace) named Gendarme.Rules.Interoperability

Yes, object creation was missing. I have added newobj. Initobj is not needed, 
since
it just initializes the memory for the struct. (Parameterized struct 
constructors are
invoked with call. I have added a test for this).

The namespace is now: Gendarme.Rules.Interoperability

Documentation:
Gendarme.Rules.Interoperability

PInvokeShouldNotBeVisibleRule
This rule checks for PInvoke methods that are internal to the analyzed assembly.

Bad exmaple:
[DllImport ("User32.dll")]
public static extern Boolean MessageBeep (UInt32 beepType);

Good example:

[DllImport ("User32.dll")]
internal static extern Boolean MessageBeep (UInt32 beepType);

CallGetLastErrorAfterPInvokeRule
Marshal.GetLastWin32Error () should be called directly after a PInvoke call.
Intermediate calls could overwrite the error.

Bad example:
public void DestroyError () {
    MessageBeep (2);
    Console.WriteLine ("Beep");
    int error = Marshal.GetLastWin32Error ();
}

Good example:
public void GetError () {
    MessageBeep (2);
    int error = Marshal.GetLastWin32Error ();
    Console.WriteLine ("Beep");
}


public void DontUseGetLastError () {
    MessageBeep (2);
    Console.WriteLine ("Beep");
}

got something important missing in #2 ... ;-)

fixed :) ( + unittest )

Ok, everything is fine and complete :)
Closing!

p.s. It's fun to see it doesn't get repetitive ;-)

As discussed in IRC here is an update that supports branching (simple version 
of #82,
does not support try/catch etc, just simple branches and switches). It also 
needs
access to AddIfNew and StackEntryAnalysis.GetNextInstruction (needs to be public
static) from #82.

The rule follows all branches and triggers if one branch calls a (not 
whitelisted)
method and GetError afterwards.

Updated to fix the multiple PInvoke bug.

The two last defects are real one, so no more false positives!
Thanks a lot for continuing the fixes after the task was closed :)
Sebastien