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    <p>Win32 functions almost never return a <code>HRESULT</code>. Instead they return a <code>BOOL</code> or use special values to indicate error (e.g. <code>CreateFile</code> returns <code>INVALID_HANDLE_VALUE</code>). They store the error code in a per-thread variable, which you can read with <code>GetLastError()</code>. <code>SetLastError=true</code> instructs the marshaler to read this variable after the native function returns, and stash the error code where you can later read it with <code>Marshal.GetLastWin32Error()</code>. The idea is that the .NET runtime may call other Win32 functions behind the scenes which mess up the error code from your p/invoke call before you get a chance to inspect it.</p> <p>Functions which return a <code>HRESULT</code> (or equivalent, e.g. <code>NTSTATUS</code>) belong to a different level of abstraction than Win32 functions. Generally these functions are COM-related (above Win32) or from <code>ntdll</code> (below Win32), so they don't use the Win32 last-error code (they might call Win32 functions internally, though). </p> <p><code>PreserveSig=false</code> instructs the marshaler to check the return <code>HRESULT</code> and if it's not a success code, to create and throw an exception containing the <code>HRESULT</code>. The managed declaration of your <code>DllImport</code>ed function then has <code>void</code> as its return type.</p> <p>Remember, the C# or VB compiler cannot check the <code>DllImport</code>ed function's unmanaged signature, so it has to trust whatever you tell it. If you put <code>PreserveSig=false</code> on a function which returns something other than a <code>HRESULT</code>, you will get strange results (e.g. random exceptions). If you put <code>SetLastError=true</code> on a function which does not set the last Win32 error code, you will get garbage instead of a useful error code.</p>
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    1. COI dont have experience with COM objects so let me ask one more question regarding creation of method signature. The question is: when I see that COM function returns HRESULT I can mark my method as returning void and set PreserveSig=false (as you said), or set PreserveSig=true and mark my method as returning IntPtr to manually examine the returned code?
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    2. COYes, that is correct, except that HRESULTs are UInt32s, not IntPtrs.
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    3. COThank you - you have been very helpful.
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