Returning a structure from a call to a C++ DLL's function to C#

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Returning a structure from a call to a DLL to C#

Here is my code in C++

struct abc
{
    int a;
    int b[5]
};

struct abc mystruct()
{
    struct abc a1;
    a1.a=12;
    for(int i=0;i<5;i++)
        a1.b[i]=i*i;
    return a2;
}

I want to call this function in C# Win32 application after having created the DLL in C++. My C# structure is like this:

public struct abc
{
    public int a;
    public int []b;
};

[DllImport("C:\\mydll.dll")]
public static extern abc mystruct();

private void button5_Click(object sender, EventArgs e)
{
    abc a2=new bcd () ;
    a2.b = new int[5];
    a2 = mystruct(10);
}

Whenever I compile it it givse me this error:

"Method's type signature is not PInvoke compatible."

Can anyone please provide me a solution to this problem. If I remove the array from the struct it compiles successfully.

Thanks in advance!