public struct Angle : IComparable<Angle>, IEquatable<Angle>, IFormattable, IXmlSerializable
{
    public readonly double Radians;
    ...
}

internal static void SetReadonlyField<TItem, TValue>(
    ref TItem self,
    Expression<Func<TItem, TValue>> func,
    TValue value) where TItem : struct
{
    var fieldInfo = self.GetType()
                        .GetField(((MemberExpression)func.Body).Member.Name);
    object boxed = self;
    fieldInfo.SetValue(boxed, value);
    self = (TItem)boxed;
}