C# - WPF UI To Allow User To Build A Complex Query And Output An Expression Tree

What I am about to do build is a UI that will allow a power user to build complex queries.

This is non-trivial, but very common. So before I re-invent the wheel, I would like to see if anybody can point me to some already-written free code or release some of their own.

Required:

To be able to specify some 'where-clause' type logic like this: Age>21 and (Citizen=True or HasGreeCard=True) but without having to type the query code, instead, use a UI with constrained options and on-the-fly syntax checking (e.g., no unclosed parenthesis or ending a term with an Operator).

I've seen this type of thing in many line-of-business apps, where you can say 'add condition' and another line appears on the UI like this:

Dropdown of fields Dropdown of ops (=, >, etc) Blank box for entry

And you can add more lines, and the lines are all and'ed, but you can also decide to OR a few, or insert parenthesis (explicity or via indenting), NOT a line, insert, delete, and move lines around, etc.

Not required but nice:

WPF - I could convert from winforms. Extensibility using OO constructs. Validate the sanity of the query. Emit a System.Linq.Expressions expression tree - or similar data structure. If it attempts to execute the query I don't need that; but I don't mind removing it.