C# command to get struct offset?

Question

Suppose I have a C# struct like this:

[StructLayout(LayoutKind.Explicit)]
struct IMAGE_DOS_HEADER {
    [FieldOffset(60)] public int e_lfanew;
}

<

Answer 1

Yes you can do this using reflection.

FieldOffsetAttribute fieldOffset = 
    (FieldOffsetAttribute)typeof(IMAGE_DOS_HEADER)
        .GetField("e_lfanew")
        .GetCustomAttributes(
            typeof(FieldOffsetAttribute),
            true
        )[0];
Console.WriteLine(fieldOffset.Value);

You can even turn this into a nice method:

static int FieldOffset<T>(string fieldName) {
    if (typeof(T).IsValueType == false) {
        throw new ArgumentOutOfRangeException("T");
    }
    FieldInfo field = typeof(T).GetField(fieldName);
    if (field == null) {
        throw new ArgumentOutOfRangeException("fieldName");
    }
    object[] attributes = field.GetCustomAttributes(
        typeof(FieldOffsetAttribute),
        true
    );
    if (attributes.Length == 0) {
        throw new ArgumentException();
    }
    FieldOffsetAttribute fieldOffset = (FieldOffsetAttribute)attributes[0];
    return fieldOffset.Value;
}

Usage:

Console.WriteLine(FieldOffset<IMAGE_DOS_HEADER>("e_lfanew"));

Answer 2

Use Marshal.OffsetOf:

Marshal.OffsetOf(typeof(IMAGE_DOS_HEADER), "e_lfanew")