c# - 如何在运行时知道属于结构体的 char 数组的长度

Question

在下面的代码中抛出异常。

'type could not be marshaled because the length of an embedded array instance does not match the declared length in the layout'

我知道这是因为字符串和字符数组的长度不匹配，所以我尝试用空白字符修复字符串的长度。

要添加空白字符，我应该知道运行时目标字符数组的长度，但我不能，因为字符数组成员还为空。

如何获取 char 数组的长度？还有比我更好的方法吗？

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct TestStruct
{
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 5)] public char[] CharArray;
}

private void Button1_Click(object sender, RoutedEventArgs e)
{
    var _TestStruct = new TestStruct();
    var _IntPtr = Marshal.AllocHGlobal(Marshal.SizeOf(_TestStruct));
    var _String = "a";

    _TestStruct.CharArray = _String.ToCharArray();
    Marshal.StructureToPtr(_TestStruct, _IntPtr, false); // ERROR

    // void TestFunction(IntPtr _Intptr);
    Marshal.FreeHGlobal(_IntPtr);
}

Answer 1

如果将 5 定义为常量，则可以用零填充 char 数组以使其填充所需的大小:

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct TestStruct
{
    public const int CharArraySize = 5;

    [MarshalAs(UnmanagedType.ByValArray, SizeConst = CharArraySize)] public char[] CharArray;
}

private void Button1_Click(object sender, RoutedEventArgs e)
{
    TestStruct _TestStruct = new TestStruct();

    IntPtr _IntPtr = Marshal.AllocHGlobal(Marshal.SizeOf(_TestStruct));

    string _String = "a";

    _TestStruct.CharArray = _String.ToCharArray();
    if (_TestStruct.CharArray.Length < TestStruct.CharArraySize)
        _TestStruct.CharArray = _TestStruct.CharArray.Concat(Enumerable.Repeat((char)0, TestStruct.CharArraySize - _TestStruct.CharArray.Length)).ToArray();

    Marshal.StructureToPtr(_TestStruct, _IntPtr, false); // This works now

    // void TestFunction(IntPtr _Intptr);

    Marshal.FreeHGlobal(_IntPtr);
}

您可以使用反射自动执行此过程:

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct TestStruct
{
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 5)] public char[] CharArray;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 10)] public char[] CharArray2;
}

private void Button1_Click(object sender, RoutedEventArgs e)
{
    TestStruct _TestStruct = new TestStruct();

    IntPtr _IntPtr = Marshal.AllocHGlobal(Marshal.SizeOf(_TestStruct));

    _TestStruct.CharArray = "abcd".ToCharArray();
    _TestStruct.CharArray2 = "ab".ToCharArray();
    _TestStruct = (TestStruct)FixArraySizes(_TestStruct); // Due to TestStruct being a struct, we must unbox the result.

    Marshal.StructureToPtr(_TestStruct, _IntPtr, false);

    // void TestFunction(IntPtr _Intptr);

    Marshal.FreeHGlobal(_IntPtr);
}

private object FixArraySizes(object obj) {
    var objType = obj.GetType();
    var fieldInfos = objType.FindMembers(MemberTypes.Field, BindingFlags.Public | BindingFlags.Instance, (info, _) => ((FieldInfo)info).FieldType == typeof(char[]), null);
    foreach (FieldInfo info in fieldInfos) {
        var fieldValue = (char[])info.GetValue(obj);
        var sizeOfArray = (int?)info.GetCustomAttributesData().FirstOrDefault(attr => attr.AttributeType == typeof(MarshalAsAttribute))?.NamedArguments.FirstOrDefault(arg => arg.MemberName == "SizeConst").TypedValue.Value;
        if (sizeOfArray.HasValue)
        {
            fieldValue = FixArraySize(fieldValue, sizeOfArray.Value);
            info.SetValue(obj, fieldValue);
        }
    }

    return obj;
}

private char[] FixArraySize(char[] array, int expectedSize)
{
    if (array.Length < expectedSize)
        array = array.Concat(Enumerable.Repeat((char)0, expectedSize - array.Length)).ToArray();
    return array;
}