Export (0) Print
Expand All

DllImportAttribute.PreserveSig Field

Indicates whether unmanaged methods that have HRESULT or retval return values are directly translated or whether HRESULT or retval return values are automatically converted to exceptions.

Namespace:  System.Runtime.InteropServices
Assemblies:   mscorlib (in mscorlib.dll)
  System.Runtime.InteropServices (in System.Runtime.InteropServices.dll)

'Declaration
Public PreserveSig As Boolean

Set the PreserveSig field to true to directly translate unmanaged signatures with HRESULT or retval values; set it to false to automatically convert HRESULT or retval values to exceptions. By default, the PreserveSig field is true.

When true, the resulting method signature returns an integer value that contains the HRESULT value. In this case, you must manually inspect the return value and respond accordingly in your application.

When you set the PreserveSig field to false, the resulting method signature contains a void return type instead of an integer (HRESULT) return type. When the unmanaged method produces an HRESULT, the runtime automatically ignores a return value of S_OK (or 0) and does not throw an exception. For HRESULTs other than S_OK, the runtime automatically throws an exception that corresponds to the HRESULT. Note that the DllImportAttribute attribute only performs this conversion to methods that return an HRESULT.

You might decide to change the default error reporting behavior from HRESULTs to exceptions in cases where exceptions better fit the error reporting structure of your application.

This field is similar to the PreserveSigAttribute; however, in contrast to the PreserveSig field, the default value for the attribute is false.

In some cases, Visual Basic developers use the DllImportAttribute, instead of using the Declare statement, to define a DLL function in managed code. Setting the PreserveSig field is one of those cases.

The following code example uses the DllImportAttribute to import the unmanaged SHAutoComplete function once with the PreserveSig field set to true and again with the PreserveSig field set to false. This code example causes the SHAutoComplete function to generate any errors with an exception one time and an HRESULT the next.

Imports System
Imports System.Collections.Generic
Imports System.ComponentModel
Imports System.Runtime.InteropServices



Module Win32
    ' The SHAutoComplete function allows you  
    ' to add auto-compete functionality to your 
    ' Windows Forms text boxes. In .NET Framework  
    ' 1.1 and earlier, you can use SHAutoComplete. 
    ' Later versions have this ability built in without 
    ' requiring platform invoke. 
    ' See the MSDN documentation of the  
    ' SHAutoComplete function for the  
    ' complete set of flags. 

    Public Enum SHAutoCompleteFlags
        SHACF_DEFAULT = &H1
        SHACF_FILESYSTEM = &H1
    End Enum  


    ' Use the DllImportAttribute to import the SHAutoComplete function.  
    ' Set the PreserveSig to false to specify exception errors.
    <DllImportAttribute("shlwapi.dll", EntryPoint:="SHAutoComplete", ExactSpelling:=True, PreserveSig:=False)> _
    Public Sub SHAutoComplete(ByVal hwndEdit As IntPtr, ByVal dwFlags As SHAutoCompleteFlags)
    End Sub 


    ' Use the DllImportAttribute to import the SHAutoComplete function.  
    ' Use the default value of the PreserveSig field to specify HRESULT errors.
    <DllImportAttribute("shlwapi.dll", EntryPoint:="SHAutoComplete", ExactSpelling:=True)> _
    Public Function SHAutoCompleteHRESULT(ByVal hwndEdit As IntPtr, ByVal dwFlags As SHAutoCompleteFlags) As Integer 
    End Function 
End Module 



Module Program

    Sub Main()
        Run()

    End Sub 


    Sub Run()
        ' Create a null (nothing in Visual Basic) IntPtr 
        ' to pass to the SHAutoComplete method.  Doing so 
        ' creates a failure and demonstrates the two ways   
        ' that the PreserveSig property allows you to handle  
        ' failures.   
        ' Normally, you would pass a handle to a managed 
        ' Windows Forms text box. 
        Dim iPtr As New IntPtr(0)

        ' Call the SHAutoComplete function using exceptions. 
        Try
            Console.WriteLine("Calling the SHAutoComplete method with the PreserveSig field set to false.")

            Win32.SHAutoComplete(iPtr,Win32.SHAutoCompleteFlags.SHACF_DEFAULT)
        Catch e As Exception
            Console.WriteLine("Exception handled: " + e.Message)
        End Try

        Console.WriteLine("Calling the SHAutoComplete method with the PreserveSig field set to true.")

        ' Call the SHAutoComplete function using HRESULTS. 
        Dim HRESULT As Integer = Win32.SHAutoCompleteHRESULT(iPtr,Win32.SHAutoCompleteFlags.SHACF_DEFAULT)

        Console.WriteLine("HRESULT handled: " + HRESULT.ToString())

    End Sub 
End Module

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0, 2.0, 1.0

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8
Show:
© 2015 Microsoft