c# - C#和.NET中P/Invoking时的特性检测

Question

我正在尝试找到一种在 P/调用之前检测功能是否存在的好方法。例如调用 native StrCmpLogicalW功能:

[SuppressUnmanagedCodeSecurity]
internal static class SafeNativeMethods
{
   [DllImport("shlwapi.dll", CharSet = CharSet.Unicode)]
   public static extern int StrCmpLogicalW(string psz1, string psz2);
}

在某些没有此功能的系统上会崩溃。

i don't want to perform version checking ，因为这是不好的做法，有时可能是错误的(例如，当功能向后移植时，或者当功能可以卸载时)。

正确的方法是检查 存在 shlwapi.dll 的导出:

private static _StrCmpLogicalW: function(String psz1, String psz2): Integer;
private Boolean _StrCmpLogicalWInitialized;

public int StrCmpLogicalW(String psz1, psz2)
{
    if (!_StrCmpLogialInitialized)
    {
        _StrCmpLogicalW = GetProcedure("shlwapi.dll", "StrCmpLogicalW");
        _StrCmpLogicalWInitialized = true;
    }

    if (_StrCmpLogicalW)
       return _StrCmpLogicalW(psz1, psz2)
    else
       return String.Compare(psz1, psz2, StringComparison.CurrentCultureIgnoreCase);
}

当然，问题在于 C# 不支持函数指针，即:

_StrCmpLogicalW = GetProcedure("shlwapi.dll", "StrCmpLogicalW");

做不到。

所以我正在尝试寻找替代语法来在 .NET 中执行相同的逻辑。到目前为止，我有以下伪代码，但我遇到了困难:

[SuppressUnmanagedCodeSecurity]
internal static class SafeNativeMethods
{
   private Boolean IsSupported = false;
   private Boolean IsInitialized = false;

   [DllImport("shlwapi.dll", CharSet = CharSet.Unicode, Export="StrCmpLogicalW", CaseSensitivie=false, SetsLastError=true, IsNative=false, SupportsPeanutMandMs=true)]
   private static extern int UnsafeStrCmpLogicalW(string psz1, string psz2);

   public int StrCmpLogicalW(string s1, string s2)
   {
       if (!IsInitialized) 
       {
          //todo: figure out how to loadLibrary in .net
          //todo: figure out how to getProcedureAddress in .net
          IsSupported = (result from getProcedureAddress is not null);
          IsInitialized = true;
       }

       if (IsSupported) 
          return UnsafeStrCmpLogicalW(s1, s2);
       else
          return String.Compare(s1, s2, StringComparison.CurrentCultureIgnoreCase);
   }
}

我需要一些帮助。

---

我想检测存在的一些导出的另一个例子是:

• dwmapi.dll::DwmIsCompositionEnabled
• dwmapi.dll::DwmExtendFrameIntoClientArea
• dwmapi.dll::DwmGetColorizationColor
• dwmapi.dll::DwmGetColorizationParameters(未记录¹，尚未按名称导出，序号 127)
• dwmapi.dll::127(未记录¹，DwmGetColorizationParameters)

¹ 从 Windows 7 SP1 开始

.NET 中必须已经有一个设计模式来检查操作系统功能的存在。谁能指出在 .NET 中执行特征检测的首选方式的示例？

Answer 1

您可以 P/Invoke 到 LoadLibraryW 以加载 shlwapi.dll，然后 P/Invoke 到 GetProcAddressW 以找到“StrCmpLogicalW”。如果返回 NULL，则表示它不存在。

您不需要 GetProcAddressW 的实际返回值 - 只要它不是 NULL，您就知道您可以使用您选择的 P/Invoke 声明。

请注意，GetProcAddressW 还支持按序数值导出的函数。

编辑:如果您想遵循某种模式，那么这可能有效:

首先定义一个辅助类 NativeMethodResolver，它会告诉您库中是否存在某个方法:

public static class NativeMethodResolver
{
    public static bool MethodExists(string libraryName, string methodName)
    {
        var libraryPtr = LoadLibrary(libraryName);
        var procPtr = GetProcAddress(libraryPtr, methodName);

        return libraryPtr != UIntPtr.Zero && procPtr != UIntPtr.Zero;
    }

    [DllImport("Kernel32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
    private static extern UIntPtr LoadLibrary(string lpFileName);

    [DllImport("Kernel32.dll", SetLastError = true, CharSet = CharSet.Ansi)]
    private static extern UIntPtr GetProcAddress(UIntPtr hModule, string lpProcName);
}

上面的帮助类可以被 SafeNativeMethod 的派生类使用，这有助于在锅炉镀一些常见的东西:

public abstract class SafeNativeMethod
{
    private readonly string libraryName;
    private readonly string methodName;
    private bool resolved;
    private bool exists;

    protected SafeNativeMethod(string libraryName, string methodName)
    {
        this.libraryName = libraryName;
        this.methodName = methodName;
    }

    protected bool CanInvoke
    {
        get
        {
            if (!this.resolved)
            {
                this.exists = Resolve();
                this.resolved = true;
            }

            return this.exists; 
        }            
    }

    private bool Resolve()
    {
        return NativeMethodResolver.MethodExists(this.libraryName, this.methodName);
    }
}

定义自己的 Invoke 方法的派生类然后可以调用基 CanInvoke 来查看是否应该返回默认值(或默认实现)来代替所寻找的 native 方法的返回值。根据您的问题，我将以 shlwapi.dll/StrCmpLogicalW 和 dwmapi.dll/DwmIsCompositionEnabled 作为 SafeNativeMethod 的示例实现:

public sealed class SafeStrCmpLogical : SafeNativeMethod
{
    public SafeStrCmpLogical()
        : base("shlwapi.dll", "StrCmpLogicalW")
    {           
    }

    public int Invoke(string psz1, string psz2)
    {
        return CanInvoke ? StrCmpLogicalW(psz1, psz2) : 0;
    }

    [DllImport("shlwapi.dll", SetLastError = true, CharSet = CharSet.Unicode)]
    private static extern int StrCmpLogicalW(string psz1, string psz2);
}

public sealed class SafeDwmIsCompositionEnabled : SafeNativeMethod
{
    public SafeDwmIsCompositionEnabled()
        : base("dwmapi.dll", "DwmIsCompositionEnabled")
    {
    }

    public bool Invoke()
    {
        return CanInvoke ? DwmIsCompositionEnabled() : false;
    }

    [DllImport("dwmapi.dll", SetLastError = true, PreserveSig = false)]
    private static extern bool DwmIsCompositionEnabled();
}

然后可以像这样使用这两个:

static void Main()
{
    var StrCmpLogical = new SafeStrCmpLogical();
    var relation = StrCmpLogical.Invoke("first", "second");

    var DwmIsCompositionEnabled = new SafeDwmIsCompositionEnabled();
    var enabled = DwmIsCompositionEnabled.Invoke();
}