为什么我不能在Release版本中将C++中的char*字符串返回给C#？

Question

我试图从C#调用以下简单的C函数:

SIMPLEDLL_API const char* ReturnString()
{
    return "Returning a static string!";
}

使用以下P/Invoke声明(使用或不使用return属性,它没有区别):

[DllImport("SimpleDll")]
[return: MarshalAs(UnmanagedType.LPStr)]
public static extern string ReturnString();

如果DLL是Debug构建但在Release构建(AccessViolationException)中崩溃,则它可以工作.

我正在调用十几个其他简单的函数,这是唯一失败的函数(这里是其他函数:)

[DllImport("SimpleDll")] public static extern int NextInt();
[DllImport("SimpleDll")] public static extern void SetNextInt(int x);
[DllImport("SimpleDll")] public static extern int AddInts(int a, int b);
[DllImport("SimpleDll")] public static extern int AddFourInts(int a, int b, int c, int d);
[DllImport("SimpleDll")] public static extern double AddDoubles(double x, double y);
[DllImport("SimpleDll")] public static extern IntPtr AddDoublesIndirect(ref double x, ref double y);
[DllImport("SimpleDll")] [return: MarshalAs(UnmanagedType.U1)]
public static extern char CharStringArgument([MarshalAs(UnmanagedType.LPStr)]string s);
[DllImport("SimpleDll")] [return: MarshalAs(UnmanagedType.U2)]
public static extern char WCharStringArgument([MarshalAs(UnmanagedType.LPWStr)]string s);
[DllImport("SimpleDll")] [return: MarshalAs(UnmanagedType.LPWStr)]
public static extern string ReturnWString();
[DllImport("SimpleDll")] [return: MarshalAs(UnmanagedType.BStr)]
public static extern string ReturnBSTR();
[DllImport("SimpleDll")] public static extern System.Drawing.Point MakePoint(int x, int y);
[DllImport("SimpleDll")] public static extern IntPtr MakePointIndirect(int x, int y);
[DllImport("SimpleDll")] public static extern int GetPointY(System.Drawing.Point p);
[DllImport("SimpleDll")] public static extern int GetPointYIndirect(ref System.Drawing.Point pp);
[DllImport("SimpleDll")] public static extern int SumIntegers(ref int firstElem, int size);

Answer 1

或者也许尝试使用

[DllImport("SimpleDll")]
public static extern IntPtr ReturnString();

并在您的调用代码中,使用Marshal类

string ret = System.Runtime.InteropServices.Marshal.PtrToStringAnsi(PInvoke.ReturnString());

Answer 2

使用自定义Marshaler也可以这样做:

class ConstCharPtrMarshaler : ICustomMarshaler
{
    public object MarshalNativeToManaged(IntPtr pNativeData)
    {
        return Marshal.PtrToStringAnsi(pNativeData);
    }

    public IntPtr MarshalManagedToNative(object ManagedObj)
    {
        return IntPtr.Zero;
    }

    public void CleanUpNativeData(IntPtr pNativeData)
    {
    }

    public void CleanUpManagedData(object ManagedObj)
    {
    }

    public int GetNativeDataSize()
    {
        return IntPtr.Size;
    }

    static readonly ConstCharPtrMarshaler instance = new ConstCharPtrMarshaler();

    public static ICustomMarshaler GetInstance(string cookie)
    {
        return instance;
    }
}

并像这样使用它:

[DllImport("SimpleDll")]
[return: MarshalAs(UnmanagedType.CustomMarshaler, MarshalTypeRef = typeof(ConstCharPtrMarshaler))]
public static extern string ReturnString();