How can I use reflection to set this object's property?

Question

I have an external library I am using, namely Aspose.Email.dll (available on NuGet). It has a PageInfo class. Go To Definition shows the following for it in Visual Studio:

using System;

namespace Aspose.Email
{
    public class PageInfo
    {
        protected PageInfo next;

        public int AbsoluteOffset { get; }
        public int ItemsPerPage { get; }
        public bool LastPage { get; }
        public virtual PageInfo NextPage { get; }
        public int PageOffset { get; }
        public int TotalCount { get; }
    }
}

Long story short, I need to create a PageInfo object. How can I create one using Reflection and also set its ItemsPerPage property?

I have tried this:

var result = (PageInfo)FormatterServices.GetUninitializedObject(typeof(PageInfo));
typeof(PageInfo).GetProperty("ItemsPerPage", BindingFlags.Instance | BindingFlags.Public).SetValue(result, 1);

The problem is that SetValue errors out with Property set method not found.

What are you doing that you need to create a PageInfo? Perhaps if you ask a new question about "How can I do X without crateing my own PageInfo?" you may have a solution better than using reflection. — Scott Chamberlain
– Scott Chamberlain, Commented Jul 14, 2016 at 18:22
I was afraid someone might ask. I am programming an abstract interface library to consolidate two different mail libraries I am using together (this and the Exchange library) I am trying to consolidate a way of paging data between the two. Unfortunately a big caveat with this IMAP library is that you can't really build an arbitrary paging query, but must always use a linked paging object after you first make a call to get paged data. Its because of the nature of the library and how its API was implemented that it becomes a bit of a pain. — Alexandru
– Alexandru, Commented Jul 14, 2016 at 19:06

Eli Arbel · Accepted Answer · 2016-07-14 15:50:11Z

2

Getter-only properties do not have setters. They use a readonly backing field that can only be set in the constructor.

You can change the properties to have private setter, e.g.

public int ItemsPerPage { get; private set; }

If you do not have access to the code, you can find the matching field using GetField and set its value.

typeof(PageInfo).GetTypeInfo().DeclaredFields
    .First(f => f.Name.Contains("ItemsPerPage")).SetValue(result, 1);

edited Jul 14, 2016 at 15:50

answered Jul 14, 2016 at 15:48

Eli Arbel

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6 Comments

Alexandru Over a year ago

Can't change the library as I don't have the source, so I can't edit the ItemsPerPage property.

Alexandru Over a year ago

Checking the library using dotPeek, and also enumerating through the DeclaredFields...I am at a bit of an impass, because...their named fields contain names such as #=qOeu6uj6DPtGwJ1qAj8pvJQ== and #=qQACP_gKR9skNS8JCd3aPRw==, leading me to believe they are hashing them or something for each version of the library, which sucks because upgrade-ability goes out the window...

Alexandru Over a year ago

What do you mean when you say to grab the field name from the getter and use it? Oh, you mean just use the named fields like #=qOeu6uj6DPtGwJ1qAj8pvJQ==?

Alexandru Over a year ago

I was hoping to avoid that. I guess there's no other way huh? I will park the issue for now then...

Jon Over a year ago

It's not a sealed class, so can you create your subclass of PageInfo, and override that ItemsPerPage property using the 'new' keyword? public new int ItemsPerPage { get; set; } ??

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