How does one pass nullptr through P/Invoke declared with a StringBuilder?

Question

Consider a typical P/Invoke declaration like this:

[DllImport("advapi32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
[return: MarshalAs(UnmanagedType.Bool)]
internal static extern bool LookupAccountName(
string SystemName,
string accountName,
IntPtr pSid,
ref uint cbSid,
[MarshalAs(UnmanagedType.LPWStr)] StringBuilder ReferencedDomainName,
ref uint ReferencedDomainNameCount,
out SID_NAME_USE SIDUse);

MSDN documentation for LookupAccountName says that pSid and ReferencedDomainName may be nullptr if the customer wishes. Passing nullptr for pSid is easy; just pass IntPtr.Zero. But what should one pass for a StringBuilder?

I don't want to pass an empty StringBuilder, because I don't want this call to fail with ERROR_INSUFFICIENT_BUFFER.

@pm100: That fails because `IntPtr` is not convertible to `StringBuilder`. — Billy ONeal, Mar 05 '13 at 23:48
@pm100: Can't do that. (Other pieces of code currently use this call and I don't want to fix that up everywhere) — Billy ONeal, Mar 05 '13 at 23:50
other answer is good. But FYI you can create a second dllimport for the same function but with a different name so that you can fiddle with the param definitons (EntryPoint=xxx) — pm100, Mar 05 '13 at 23:57

score 8 · Accepted Answer · answered Mar 05 '13 at 23:50

8

You can just pass null for this parameter; it will be marshalled as a null pointer.

answered Mar 05 '13 at 23:50

Thomas Levesque

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How does one pass nullptr through P/Invoke declared with a StringBuilder?

1 Answers1