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Expression trees are a nice feature, but what are its practical uses? Can they be used for some sort of code generation or metaprogramming or some such?
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Expression Trees represent code as a structure that you can examine, modify, or execute. These tools give you the power to manipulate code during run time. You can write code that examines running algorithms, or injects new capabilities.
The binary expression tree is a binary tree whose leaves are operands, such as constants or variable names, and the other nodes contain operators. For example, the postfix notation a b + c d e + * * results in the following expression tree.
The leaves of an expression tree always contain? Explanation: The leaves of an expression tree always contain the result of a given expression (i.e.) operands.
An expression tree is a representation of expressions arranged in a tree-like data structure. In other words, it is a tree with leaves as operands of the expression and nodes contain the operators. Similar to other data structures, data interaction is also possible in an expression tree.
As Jon notes, I use them to provide generic operators with .NET 3.5. I also use them (again in MiscUtil) to provide fast access to non-default constructors (you can't use Delegate.CreateDelegate with constructors, but Expression works fine).
Other uses of manually created expression trees:
But really, Expression is a very versatile way of writing any dynamic code. Much simpler than Reflection.Emit, and for my money, simpler to understand than CodeDOM. And in .NET 4.0, you have even more options available. I show the fundamentals of writing code via Expression on my blog.
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Marc Gravell has used them to great effect in MiscUtil to implement generic operators.
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