从IntPtr整理结构数组-仅第一个值正确整理

Question

I'm passing an address to an array of structures into a C# DLL from an external program.

I thought that I would first make a simple test, in order to see if the approach would work, by trying to marshal a pointer into an array of structs on the C# side.

Given the following struct:

[StructLayout(LayoutKind.Sequential)]
struct TestStruct
{
    public int id;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 32)]
    public string someString;
}

and the following code which attempts to read the struct array (the section up until the for loop is just to simulate the pointer being passed from the other program):

TestStruct[] testStructs = new TestStruct[3];
testStructs[0].id = 1;
testStructs[0].someString = "Value 1";
testStructs[1].id = 2;
testStructs[1].someString = "Value 2";
testStructs[2].id = 3;
testStructs[2].someString = "Value 3";

int size = Marshal.SizeOf(testStructs[0]);
IntPtr ptrFirst = Marshal.AllocHGlobal(size);
Marshal.StructureToPtr(testStructs[0], ptrFirst, true);

long ptrAddrLong = ptrFirst.ToInt64();

for (int i = 0; i < testStructs.Length; i++) {
    IntPtr thisPtr = new IntPtr(ptrAddrLong);
    TestStruct testStruct = Marshal.PtrToStructure<TestStruct>(thisPtr);
    ptrAddrLong += size;
}

can anyone shed any light why, when I debug through the for loop, only the first testStruct item is marshalled correctly from the pointer? All subsequent items contain garbage in the fields, as it appears the pointer address is incorrect after the first iteration.

First iteration:

Second iteration:

Size is reported as 36, which seems to be correct.

I've tried using explicit layout, but this didn't make any difference.

Thanks

Answer 1

Why do you think that Marshal.StructureToPtr marshals anything more than you have told it to marshal?

Marshal.StructureToPtr(testStructs[0], ptrFirst, true); marshals a single TestStruct to the memory at ptrFirst .

It is basically just a smart memory copy that takes into account all those marshaling attributes.

PS: And take into account that memory at ptrFirst can hold at most Marshal.SizeOf(testStructs[0]) . So you can't (at least without risking some nasty memory access problem ) read a memory behind ptrFirst + size .