Verify that a property is Init Only

CheckInitOnly.cs

public static class PropertyChecker
{
    // usage: PropertyChecker.CheckInitOnly(() => foo.Bar);
    public static void CheckInitOnly(Expression<Func<object>> expression, bool allowEmptyStrings = false)
    {
        if (expression.Body is not UnaryExpression { Operand: MemberExpression memberExpression })
        {
            if (expression.Body is not MemberExpression tmp)
            {
                throw new InvalidOperationException($"Expected a {nameof(MemberExpression)}.");
            }

            memberExpression = tmp;
        }

        var property = (PropertyInfo)memberExpression.Member;

        var setMethod = property.GetSetMethod();

        setMethod.Should().NotBeNull();

        var isExternalInitType = typeof(System.Runtime.CompilerServices.IsExternalInit);

        setMethod!.ReturnParameter.GetRequiredCustomModifiers().Should().Contain(isExternalInitType);
    }
}