.NET 4.0框架中是否已经存在以下通用方法？

Question

Does the following generic function exist anywhere in the .NET 4.0 framework? I would like to reuse it if it does rather than writing it myself:

public static class Lambda 
{ 
  public static U Wrap<U>(Func<U> f) 
  { 
    return f(); 
  } 
} 

It allows for the following construct (ie, lambda expressions embedded in the select clause of a LINQ query):

string test="12,23,34,23,12";
var res=from string s in test.Split(',') 
        select Lambda.Wrap(() => {string u=s+s; return int.Parse(u);});

Update: To all people questioning this solution, look at Onkelborg's answer to see what it would take to include the lambda without Lambda.Wrap() (while still maintaining query syntax). Please do not eliminate the lambda. This has to work for abitrary (value returning) lambdas . Also, I am looking for a query syntax solution, only. Please do not convert the expression to fluent syntax , thus trivializing it.

Answer 1

You can use the let syntax in your code:

string test = "12,23,34,23,12";
var res = from string s in test.Split(',') 
          let u = s+s
          select int.Parse(u);

Alternately, you could use the LINQ extension methods directly instead of the special syntax:

string test = "12,23,34,23,12";
var res = test.Split(',')
              .Select(s => { var u = s + s; return int.Parse(u); });

Since the question was updated:

I don't mean to be disrespectful, but I think this solution isn't necessary.

Here's a bit of an exploration:

If we want to accept truly "arbitrary" lambdas like you say, then they can come from an outside source, and Wrap does nothing because it's the same as f() :

// with 'f' as some arbitrary lambda, then this:
var res = from string s in test.Split(',')
          select f.Wrap();

// is the same as:
var res = from string s in test.Split(',')
          select f();

But if you do this, f can't depend upon s in any way (for example, this way you can't write your example code):

// e.g. where does s come from?
var f = () => { var u = s+s; return int.Parse(u); }; 

// we can write it like this, as a function of 's':
var f2 = s => { var u = s+s; return int.Parse(u); };
//but then you can just use "select f2(s)" and we're back not needing Wrap any more

What we're really looking for is arbitrary closures over s . For this to happen, the lambdas have to be defined inline, where s is in scope, so you aren't really accepting truly "arbitrary" lambdas any more, and they have to be written directly in the code.

This is why I proposed the let syntax, since any lambda you can come up with can be converted to that syntax, and it goes with the rest of the query syntax. This is what let is designed for! :)

Alternately, you could just use lambdas which take parameters like f2 above.

If you really want to stick with the lambda syntax, I'd suggest using the extension methods. Like I said in my comment, it looks like you're looking for something halfway between query & extension syntax.

I'd be interested in why you want to use the lambda syntax with the query syntax?

Hope this helps :)

Answer 2

How is this different from

var res = test.Split(',').Select(s => {
                                        var u = s + s;
                                        return int.Parse(u);
                                      });

?

Yes, this compiles and runs.

Answer 3

According to Reflector, that method doesn't exist. But I don't see why you would need it :)

string test = "12,23,34,23,12";
var res = from string s in test.Split(',')
          select ((Func<int>)(() => { string u = s + s; return int.Parse(u); }))();

I think this would be exactly the same thing

Answer 4

No, it does not exist. Onkelborg's solution is the best you can do out of the box, so if you do really want to execute an arbitrary code block inside a LINQ expression, then I would author the Wrap function you suggest in order to get C# type inference to kick in and make it less ugly.