PathRenameExtension - Replaces the extension of a file name with a new extension. If the file name does not contain an extension, the extension will be attached to the end of the string.

[DllImport("shlwapi.dll", EntryPoint = "PathRenameExtensionW", SetLastError = True, CharSet = CharSet.Unicode)]

static extern bool PathRenameExtension([MarshalAs(UnmanagedType.LPTStr)]System.Text.StringBuilder lpszPath,

<DllImport("shlwapi.dll", EntryPoint:="PathRenameExtensionW", SetLastError:=True, CharSet:=CharSet.Unicode)> _

Public Function PathRenameExtension(<MarshalAs(UnmanagedType.LPTStr)>lpszPath As System.Text.StringBuilder, _

Public Declare Function PathRenameExtension Lib "shlwapi" Alias "PathRenameExtensionA" _

Documentation

[PathRenameExtension] on MSDN

Access PInvoke.net directly from VS: