I am calling the FindMimeFromData in urlmon and I want to make sure I don't
have any memory leaks on the unmanaged side. I have two versions of the
call, one using a ref IntPtr and one using a [MarshalAs
(UnmanagedType.LPWStr)] ref string to represent the LPWSTR*. My hunch is
the latter "MarshalAs" is the appropriate mechanism.
In the case of using the "ref IntPtr", how would I appropriately delete the
memory allocated for the string on the unmanaged side (even if this is not
the best way)? I'm not sure the Marshal.Free functions are doing the trick.
In the case of using "[MarshalAs(UnmanagedType.LPWStr)] ref string", a copy
of the LPWSTR* is made and returned through the managed string. It is my
understanding that the marshalling process would free the memory allocated
for the LPWSTR* parameter on the unmanaged side. Is this correct?
Below is some sample code that uses both calls for illustration:
[DllImport("urlmon.dll", EntryPoint = "FindMimeFromData")]
public static extern int FindMimeFromData1(IntPtr pBC, IntPtr pwzUrl,
IntPtr pBuffer, int cbSize, IntPtr pwzMimeProposed, int
dwMimeFlags,
ref IntPtr ppwzMimeOut, int dwReserved);
[DllImport("urlmon.dll", EntryPoint = "FindMimeFromData")]
public static extern int FindMimeFromData2(IntPtr pBC, IntPtr pwzUrl,
IntPtr pBuffer, int cbSize, IntPtr pwzMimeProposed, int
dwMimeFlags,
[MarshalAs(UnmanagedType.LPWStr)] ref string ppwzMimeOut, int
dwReserved);
public static string GetMimeType(byte[] data) {
IntPtr pBC = IntPtr.Zero;
IntPtr pwzUrl = IntPtr.Zero;
GCHandle hBuffer = GCHandle.Alloc(data, GCHandleType.Pinned);
IntPtr pBuffer = hBuffer.AddrOfPinnedObject();
IntPtr pwzMimeProposed = IntPtr.Zero;
IntPtr pMimeType = IntPtr.Zero;
string mimeType1 = null;
string mimeType2 = null;
try {
int rc1 = FindMimeFromData1(pBC, pwzUrl, pBuffer,
data.Length,
pwzMimeProposed, 0, ref pMimeType, 0);
int rc2 = FindMimeFromData2(pBC, pwzUrl, pBuffer,
data.Length,
pwzMimeProposed, 0, ref mimeType2, 0);
if (rc1 == 0) {
if (pMimeType != IntPtr.Zero) {
mimeType1 = Marshal.PtrToStringAuto
(pMimeType);
// free resources allocated for pMimeType
// ???
}
}
}
finally {
if (hBuffer.IsAllocated)
hBuffer.Free();
}
return mimeType2;
}
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