Question

What does Expression<T> do?

I have seen it used in a method similar to:

private Expression<Func<MyClass,bool>> GetFilter(...)
{
}

Can’t you just return the Func<MyClass,bool> ?

Google and SO searches have failed me due to the < and > signs.

Answer 1

If TDelegate represents a delegate type, then Expression<TDelegate> represents a lambda expression that can be converted to a delegate of type TDelegate as an expression tree. This allows you to programatically inspect a lambda expression to extract useful information.

For example, if you have

var query = source.Where(x => x.Name == "Alan Turing");

then x => x.Name == "Alan Turning" can be inspected programatically if it’s represented as an expression tree, but not so much if it’s thought of as a delegate. This is particularly useful in the case of LINQ providers which will walk the expression tree to convert the lambda expression into a different representation. For example, LINQ to SQL would convert the above expression tree to

 SELECT * FROM COMPUTERSCIENTIST WHERE NAME = 'Alan Turing'

It can do that because of the representation of the lambda expression as a tree whose nodes can be walked and inspected.