Ayende @ Rahien

It's a girl

Mutable Linq Expressions

As you probably know, System.Linq.Expression is considered to be immutable. This is a major pain when it comes to working with them, and many Linq providers end up creating a parallel hierarchy of expressions that are mutable.

What I am going to show you now is probably going to get some people in Microsoft pretty angry with me, but hey, if they used ReSharper, they wouldn’t have this problem. In a true TDD fashion, let us start with a failing test:

var constant = Expression.Constant(5);
var captureReference = constant;

// put something here that may not change the captureReference variable

Debug.Assert(ReferenceEquals(constant, captureReference));

We then move on to look at the actual ConstantExpression code:


Do you see the hook that I am going to plug into?

Coming back to my ReSharper plug, it is pretty easy to see that value is a read only value, but it is not mark as such. This is one of ReSharper’s suggestions that came in 4.0, and made me aware of the reasons to do so. Because it is not mark as such, I have a way in…

All it requires is a bit of LCG (lightweight code generation), such as this:

public static Action<ConstantExpression, object> CreateSetAccessor()
    var accessor = new DynamicMethod("SetValue", typeof (void), new Type[]
        typeof (ConstantExpression),
        typeof (object)
    }, true);
    var generator = accessor.GetILGenerator();
    return (Action<ConstantExpression, object>)accessor.CreateDelegate(typeof(Action<ConstantExpression,object>));

And now we have a passing test:

var valueSetAccessor = CreateSetAccessor();
var constant = Expression.Constant(5);
var captureReference = constant;

valueSetAccessor(constant, 2);

Debug.Assert(ReferenceEquals(constant, captureReference));

Now, I don’t really recommend doing this. This is unsupported, etc.

But it is a cool trick, and it applies pretty much generically across all of the Expression classes.


Krzysztof Kozmic
05/18/2009 06:06 AM by
Krzysztof Kozmic

Even if the field was marked readonly, you still could do that (not sure view LCG, but certainly via reflection).

readonly is more like a suggestion, not a hard rule, and CLS does not even require a language to support that IIRC.

Ayende Rahien
05/18/2009 06:12 AM by
Ayende Rahien


That is not accurate for LCG

You will get:

Unhandled Exception: System.Security.VerificationException: Operation could dest

abilize the runtime.

at SetValue(MyConstantExpression , Object )

at ConsoleApplication1.Program.Main(String[] args) in c:\temp\ConsoleApplication1\ConsoleApplication1\Program.cs:line 31

Using reflection, it works, but I don't know why

Krzysztof Kozmic
05/18/2009 06:17 AM by
Krzysztof Kozmic

Oh, I see - than my statement about CLS not requiring it probably is wrong.

Peter Morris
05/19/2009 09:10 AM by
Peter Morris

I would be far too scared to use code like this :-) You might have a working product released when a .NET framework service pack comes along and plugs the whole. Ouch!

Although I did find the LCG code very interesting. It's something I've been meaning to look into for ages but never had a reason to allocate the time to :-)

06/24/2009 10:18 AM by

Hi Ayende,

Now .net 4.0 has made the ExpressionVisitor from System.Core public.

That is very nice, say good by to hacks!!!

06/24/2009 12:54 PM by

And by the way, they listened to your recomandation:

the private members are READONLY now for Expression types....

In .NET 4.0

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