Get Object from Delegate Filter Expression Expression>

Question

Using lambda delegate Expression> - where my expression takes a Role object (POCO).

Looking to use that POCO Role object and map it to a data layer Role object with matching properties. To do that, I need to be able to get the Role object from the delegate.

Example:

public List<Role> FindAll(Expression<Func<Role, bool>> filter)

Calling this method like:

FindAll(r => r.Name == role.Name);

r is type Role, and within the FindAll function, I can see that filter has one parameter, as such:

enter image description here

Can I extract that object? And how?

I'm sure it MUST be doable, after all, linq does it internally all the time...

Have you tried casting them to the specialized `Expression`'s? — leppie, Feb 01 '12 at 16:39
@leppie 1) What do you mean, 'Why am I duplicating `Where`? 2) Parameter[0] is of type ParameterExpression - so far I haven't found any properties off of that I can cast to my own type from, but I'm still trying. — The Evil Greebo, Feb 01 '12 at 16:47
The debugger is listing all the types for you already. Just cast to that, and viola, you have the properties you see in the debugger. It is a fairly trivial task, no magic involved :) — leppie, Feb 01 '12 at 16:52
No, it isn't. Filter is of type System.Linq.Expressions.LambdaExpression. Parameter[0] is of type ParameterExpression. The Type property on ParameterExpression is returning the Type of object represented, but not the actual object itself. Can't cast that. — The Evil Greebo, Feb 01 '12 at 16:58
Are you trying to determine the Type of the parameter? The reason why I ask is because you ask about extracting an object and there is no object to extract other than the delegate. — Roy Goode, Feb 01 '12 at 17:05
(Role)filter yields: Cannot convert type 'System.Linq.Expressions.Expression>' to 'Entity.Role'. (Role)filter.Parameters[0] yields: (Role)filter.Parameters[0] Cannot convert type 'System.Linq.Expressions.ParameterExpression' to 'Entity.Role' — The Evil Greebo, Feb 01 '12 at 17:05
@TheEvilGreebo it's a delegate parameter, not an object. You definitely can't cast it to a Role. i.e. there is no Role object. — Roy Goode, Feb 01 '12 at 17:07
@RoyGoode Well, I'm hoping that since the delegate is using a Role object, there's a way to get a reference to the Role object the delegate is using. — The Evil Greebo, Feb 01 '12 at 17:09
Followup: Right - I'm not trying to cast it. That was leppie's suggestion. I'm trying to get to the object that's being used inside the delegate itself. — The Evil Greebo, Feb 01 '12 at 17:10
@TheEvilGreebo It's a delegate method expecting a parameter of type Role. Surely there's no Role object in existence until the method is invoked? e.g. Role myRoleObjectToInvokeTheMethod = new Role(); var compiledFilter = filter.Compile(); var resultValue = compiledFilter(myRoleObjectToInvokeTheMethod); — Roy Goode, Feb 01 '12 at 17:11
@TheEvilGreebo You didn’t read properly. Nowhere did leppie say that you should cast to `Role` – of course you should cast to the appropriate type, which the debugger tells you (`ParameterExpression`). — Konrad Rudolph, Feb 01 '12 at 17:19
@RoyGoode Sure but at invocation, the delegate method DOES have a parameter expression that knows about the underlying type its using (as in the T of Func). That underlying type has been instantiated in the invocation of the delegate - so I'm trying to get to it, if at all possible. — The Evil Greebo, Feb 01 '12 at 17:20
@KonradRudolph You're right - I misunderstood leppie. Re-reading it, I guess I don't understand - what kind of specialized expression would I cast it to? — The Evil Greebo, Feb 01 '12 at 17:23
@TheEvilGreebo OK, so you want to find out the Type in code: Type delegateParameterType = filter.Parameters[0].Type; — Roy Goode, Feb 01 '12 at 17:25
@RoyGoode No, that wasn't what I was going for - but Bryan's answer crystallized it for me. Thanks anyway. — The Evil Greebo, Feb 01 '12 at 17:35
@TheEvilGreebo No problem, I can see your response to Bryan's answer provides us with a better explanation of what you were trying to do. — Roy Goode, Feb 01 '12 at 17:36

score 2 · Accepted Answer · answered Feb 01 '12 at 17:26

There are two roles here: r, which represents the filter parameter, and role, which is an object that is closed over by the lambda expression. I assume you mean you want a reference to the role object, since you already found the ParameterExpression which represents r.

That object will be a ConstantExpression whose type is Role, and it will be the value of the Expression property of the MemberAccessExpression which represents role.Name. That will be the right-hand side of the BinaryOperator expression representing the equality test, which serves as the Body of the lambda expression.

Is that what you need?

Yes and no. No, it's not what I really wanted - but reading your explanation has helped crystallize why it won't work. (r=>r.name = role.name) is only one type of possible filters. I could just as easily say (r=>r.name="Fred") - which would never have a role object so of course, why would that object *be* there... — The Evil Greebo, Feb 01 '12 at 17:34

Get Object from Delegate Filter Expression Expression>

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