Linq Query Syntax and Complex Where Conditions

I want to use a Linq to Objects query to select certain members from a list based on fairly complex selection criteria (for simplification represented here as return true :-)

Using the Where extension method, this works just fine:

    var matches = (from wm in Members
                   select wm).Where(p =>
                   {
                       return true;
                   });

However, when I attempt to do the same thing using the query syntax:

    var matches2 = (from wm in Members
                    where (p =>
                    {
                        return true;
                    })
                   select wm);

The compiler objects

Cannot convert lambda expre开发者_运维百科ssion to type 'bool' because it is not a delegate type

What is going on here behind the scenes, and how can I use a lamda expression with the query syntax?

---

your query gets translated to something like:

Members.Where(member=> p=>{return true;});

...which doesn't compile because (warning: technical errors probably be here) Where() expects a parameter of type Func<TSource, bool>. You are giving it a parameter that would get converted to type Func<TSource, Func<???, bool>>. The error is telling you that a lambda expression cannot be converted to a bool, because lambda expressions can only be converted to delegate types (e.g. Func<...>, and bool is not a delegate type.(/end bumbling explanation)

In query syntax, whatever you put in your where clause goes after the => in the lambda expression for the generated Where() method call. Note that your first snippet can be rewritten as:

Members.Where(p=>true);

which is equivalent to:

from m in members 
where true 
select m;

if you really need a lambda with a statement block in it you can do:

var lam = (Member m)=> {return true;};
from m in members
where lam(m)
select m;

or to inline it:

from m in members
where ((Func<Member, bool>)(Member m)=> {return true;})(m)
select m;

or you can use a regular method instead of an anonymous one:

public bool DoStuff(int i)
{
    return true;
}

...

from m in members
where DoStuff(m)
select m;