Invoke MethodExpression

[Adriaan Davel]

I'm struggling with a rather elaborate Expression attempt, any help will be appreciated: I have a method: public QueryContext<T> OrderBy(Expression<Func<T, SortColumn>> orderByExpression) that I string onto a number of methods (either the same method or some of my other methods that look similar exposed by QueryContext<T>) in code something like this:

return this.Get<DataStorageLocation>(dsl => !dsl.Deleted)
.IncludeReferencedObject(dsl => new RangedColumn<DataStorageLocation>(dsl.ChildLocations)
.Where<DataStorageLocation>(cdsl => !cdsl.Deleted)
.Where<DataStorageLocation>(cdsl => cdsl.Deleted == false)
.Where<DataStorageLocation>(cdsl => cdsl.Id > 0)
.OrderBy(cdsl => new SortColumn<DataStorageLocation>(cdsl.LocationName)))).ToList();

(this is not very logical code, but gives a good debuggin example) and produces an expression something like this:

new RangedColumn`1(dsl.ChildLocations).Where(cdsl => Not(cdsl.Deleted)).Where(cdsl => (cdsl.Deleted = False)).Where(cdsl => (cdsl.Id > 0)).OrderBy(cdsl => new SortColumn`1(cdsl.LocationName))

Problem comes in when I try to invoke the OrderBy MethodExpression like this:

var args = (from a in methodCallExpression.Arguments select a as Expression).ToList();
methodCallExpression.Method.Invoke(column, args.ToArray());

The Method Name that I have from methodCallExpression.Method.Name is OrderBy and the arguments that I have is cdsl => new SortColumn`1(cdsl.LocationName) which looks correct, yet I get an exception that my expression cannot be cast to 'Expression<Func<T, SortColumn>> orderByExpression' which is my method signature so I almost understand that there is a problem, but what I don't understand is how invoking the method, with the arguments it has for itself gives an error... I've only been playing with Expressions for a (painful) couple of days, if this is a stupid question please answer it before you frag me :) PS: I've tried to write an overload method for OrderBy that takes an Expression object as a parameter, yet the Invoke method insists on calling the other method (which I also understand)

____________________________________________________________ Be brave little warrior, be VERY brave

[Daniel Grunwald]

When your expression is 'x.Where(a => f(a))' and the Where method takes an Expression<Func<T>>, then the expression tree actually is:

MethodCallExpression

• Object: x
• Method: Where
• Arguments:
  • UnaryExpression (Type: Quote)
    • LambdaExpression
      • Parameter: a
      • Body: f(a)

'Quote' is the operator that converts from a lambda to an expression tree (similar to quote in LISP). So you're trying to call Where with an UnaryExpression where it expects an Expression<Func<T>>. Solution: use Expression.Lambda<TDelegate>() to create a new typed lambda expression reusing the parameter and body from the input LambdaExpression.

[Adriaan Davel]

Hi Daniel, Thanks, will give this a try

____________________________________________________________ Be brave little warrior, be VERY brave