How to marshal a struct with a nested array of structs when using P/Invoke C#

Your declarations are not quite right:

• c is declared as a pointer, so you would have needed UnmanagedType.LPArray
• e is declared as a pointer, so it should also use have used LPArray.
  • If it's actually a single object and not an array then InnerStruct needs to be a class and the declaration should be public InnerStruct e;
• But you can't embed arrays in structs. You'd need to marshal them manually. Make sure to use Marshall.FreeHGlobal in a finally or using.
• Exactly where the size of the array is coming from is not clear.
• CallingConvention.StdCall is probably wrong: most C code uses CallingConvention.Cdecl.

[StructLayout(LayoutKind.Sequential)]
public struct OuterStruct
{
    public uint d;
    public IntPtr e;
}

[StructLayout(LayoutKind.Sequential)]
public struct InnerStruct
{
    public uint a;
    public uint b;
    public IntPtr c;
}

Marshalling manually looks like this

var o = new OuterStruct();
var array = new InnerStruct[5];
// set values etc
var arrayPtr = IntPtr.Zero;
var bytePtr = IntPtr.Zero;
try
{
    for (var i = 0; i < array.Length; i++)
    {
        array[i].c = Marshal.AllocHGlobal(SomeByteArray.Length);
        Marshal.Copy(SomeByteArray, 0, array[i].c, SomeByteArray.Length);
    }
    arrayPtr = Marshal.AllocHGlobal(array.Length * Marshal.SizeOf<InnerStruct>());
    Marshal.Copy(array, 0, arrayPtr, array.Length * Marshal.SizeOf<InnerStruct>());

    // make call
}
finally
{
    Marshal.FreeHGlobal(arrayPtr);
    foreach (var x in array)
        Marshal.FreeHGlobal(x.c);
}