HijackProperty

HijackProperty.cs

// Example Usage:
// class FakeDateTime {
// 	public DateTime UtcNow => DateTime.Parse("1970-01-01");
// }
// 
// using (var hijack = new HijackProperty(typeof(DateTime), "UtcNow", typeof(FakeDateTime))) {
// 		Console.WriteLine($"It's currently: {DateTime.Now}");
// }

public class HijackProperty : IDisposable
{
    private readonly IntPtr originalData;
    private readonly IntPtr originalAddress;
    // Hijacks any property and replaces it with the same property on another type
    public HijackProperty(Type sourceType, string property, Type targetType)
    {
        var source = sourceType.GetProperty(property)?.GetMethod;
        var target = targetType.GetProperty(property)?.GetMethod;

        if (source == null || target == null)
        {
            return;
        }

        originalAddress = GetMethodAddress(source);
        IntPtr tar = GetMethodAddress(target);
        originalData = Marshal.ReadIntPtr(originalAddress);

        Marshal.Copy(new[] { Marshal.ReadIntPtr(tar) }, 0, originalAddress, 1);
    }

    // Gets the memory pointer to the requested method by looking it up on the MethodTable
    private static IntPtr GetMethodAddress(MethodInfo mi)
    {
        const ushort slotNumberMask = 0xffff;
        const int mtOffset32Bit = 0x28;
        const int mtOffset64Bit = 0x40;

        IntPtr address;

        RuntimeHelpers.PrepareMethod(mi.MethodHandle);

        IntPtr md = mi.MethodHandle.Value;
        IntPtr mt = mi.DeclaringType!.TypeHandle.Value;

        if (mi.IsVirtual)
        {
            int offset = IntPtr.Size == 4 ? mtOffset32Bit : mtOffset64Bit;
            IntPtr ms = Marshal.ReadIntPtr(mt + offset);
            long shift = Marshal.ReadInt64(md) >> 32;
            int slot = (int)(shift & slotNumberMask);
            address = ms + slot * IntPtr.Size;
        }
        else
        {
            address = md + 8;
        }

        return address;
    }

    public void Dispose()
    {
        Marshal.Copy(new[] { originalData }, 0, originalAddress, 1);
    }
}