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Mark boolean P/Invoke arguments with MarshalAs







Breaking Change


A platform invoke method declaration includes a System.Boolean parameter or return value but the System.Runtime.InteropServices.MarshalAsAttribute attribute is not applied to the parameter or return value.

A platform invoke method accesses unmanaged code and is defined by using the Declare keyword in Visual Basic or the System.Runtime.InteropServices.DllImportAttribute. MarshalAsAttribute specifies the marshaling behavior used to convert data types between managed and unmanaged code. Many simple data types, such as System.Byte and System.Int32, have a single representation in unmanaged code and do not need their marshaling behavior specified; the common language runtime automatically supplies the correct behavior.

The Boolean data type has multiple representations in unmanaged code. When the MarshalAsAttribute is not specified, the default marshaling behavior for the Boolean data type is UnmanagedType.Bool. This is a 32-bit integer, which is not appropriate in all circumstances. The Boolean representation required by the unmanaged method should be determined and matched to the appropriate System.Runtime.InteropServices.UnmanagedType. UnmanagedType.Bool is the Win32 BOOL type, which is always 4 bytes. UnmanagedType.U1 should be used for C++ bool or other 1-byte types. For more information, see Default Marshaling for Boolean Types.

To fix a violation of this rule, apply MarshalAsAttribute to the Boolean parameter or return value. Set the value of the attribute to the appropriate UnmanagedType.

Do not suppress a warning from this rule. Even if the default marshaling behavior is appropriate, the code is more easily maintained when the behavior is explicitly specified.

The following example shows two platform invoke methods marked with the appropriate MarshalAsAttribute attributes.

using System;
using System.Runtime.InteropServices;

[assembly: ComVisible(false)]
namespace InteroperabilityLibrary
   internal class NativeMethods
      private NativeMethods() {}

      [DllImport("user32.dll", SetLastError = true)]
      [return: MarshalAs(UnmanagedType.Bool)]
      internal static extern Boolean MessageBeep(UInt32 uType);

                 CharSet = CharSet.Unicode, 
                 SetLastError = true)]
      [return: MarshalAs(UnmanagedType.U1)]
      internal static extern bool StrongNameSignatureVerificationEx(
         [MarshalAs(UnmanagedType.LPWStr)] string wszFilePath,
         [MarshalAs(UnmanagedType.U1)] bool fForceVerification,
         [MarshalAs(UnmanagedType.U1)] out bool pfWasVerified);

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