Bug 53 - Obtaining FieldInfo from expression in LINQ ConstantExpression returns null.
Summary: Obtaining FieldInfo from expression in LINQ ConstantExpression returns null.
Alias: None
Product: Runtime
Classification: Mono
Component: General ()
Version: unspecified
Hardware: All All
: Lowest trivial
Target Milestone: ---
Assignee: Bugzilla
Depends on:
Reported: 2011-07-22 18:10 UTC by Andrew Finnell
Modified: 2014-01-12 01:02 UTC (History)
3 users (show)

Tags: linq
Is this bug a regression?: ---
Last known good build:

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Description Andrew Finnell 2011-07-22 18:10:39 UTC
Unit Test (Passes on .NET fails on Mono 2.10.1):

using System;
using System.Collections.Generic;
using System.Linq.Expressions;
using NUnit.Framework;

namespace MonoLinq
    public class TestLinqConstantExpression
        void UseNonConstMemberVariable<T>(IEnumerable<T> data, Expression<Func<T, Boolean>> input)
            var expression = (BinaryExpression)input.Body;
            var memberExpression = (MemberExpression) expression.Right;
            var constantExpression = (ConstantExpression) memberExpression.Expression;
            var memberInfo = constantExpression.Type;
            var fieldInfo = memberInfo.GetField(memberExpression.Member.Name);

        public void MonoLinqSupportsNonConstMemberFieldInExpression()
            var thisHasToBeNonConst = "a";
            var items = new List<String>();
            UseNonConstMemberVariable(items, item => item == thisHasToBeNonConst);

Mono Version:

Mono JIT compiler version 2.10.1 (tarball)
Copyright (C) 2002-2011 Novell, Inc and Contributors. www.mono-project.com
        TLS:           normal
        SIGSEGV:       normal
        Notification:  Thread + polling
        Architecture:  x86
        Disabled:      none
        Misc:          softdebug
        LLVM:          supported, not enabled.
        GC:            Included Boehm (with typed GC and Parallel Mark)
Comment 1 Andrew Finnell 2011-07-23 09:46:39 UTC
This appears to be an abuse of LINQ on my part. In .NET constantExpression.Type just happens to be the same MethodInfo of memberExpression.Member. This does not appear to be the case in Mono.

The correct way to use LINQ in this scenario is:

var constantExpression = memberExpression.Expression as ConstantExpression;
var memberInfo = memberExpression.Member;
if (memberInfo is FieldInfo)
    var value = ((FieldInfo)memberInfo).GetValue(constantExpression.Value);

Downgrading this to trivial.
Comment 2 Rodrigo Kumpera 2014-01-12 01:02:52 UTC
The above works on mono 3.2.5 in the same as on .NET 4.5