Nice, clean cross join in Linq using only extension methods [duplicate]

问题

Possible Duplicate:
Nested “from” LINQ query expressed with extension methods

I'm sure this has been asked before, but I honestly couldn't find anything.

I'm curious what the equivalent syntax would be for the following using only built-in Linq extension methods:

var z1 =
    from x in xs
    from y in ys
    select new { x, y };

I can get the same results with this:

var z2 = xs.SelectMany(x => ys.Select(y => new { x, y }));

But it produces different IL code, and the code is a bit convoluted and hard to understand. Is there a cleaner way to do this with extension methods?

Here's my entire test method as written:

private void Test()
{
    var xs = new[] { 1D, 2D, 3D };
    var ys = new[] { 4D, 5D, 6D };

    var z1 =
        from x in xs
        from y in ys
        select new { x, y };

    var z2 = xs.SelectMany(x => ys.Select(y => new { x, y }));
}

Here's the [Edit: C# interp of the] IL code (using ILSpy):

private void Test()
{
    double[] xs = new double[]
    {
        1.0, 
        2.0, 
        3.0
    };
    double[] ys = new double[]
    {
        4.0, 
        5.0, 
        6.0
    };
    var z =
        from x in xs
        from y in ys
        select new
        {
            x = x,
            y = y
        };
    var z2 = xs.SelectMany((double x) =>
        from y in ys
        select new
        {
            x = x,
            y = y
        });
}

回答1:

One way would be:

var z2 = xs.SelectMany(x => ys, (x, y) => new {x, y});

回答2:

If you really want to use a single LINQ extension method, then another candidate would be Join, with the outerKeySelector and innerKeySelector functions defined such that they will always produce equal values.

var z3 = xs.Join(ys, x => true, y => true, (x, y) => new { x, y });

This will, however, probably give more convoluted IL code than the nested from solution. Incidentally, MSDN uses the nested from in its example for a cross join; look at the first code snippet in How to: Perform Custom Join Operations (C# Programming Guide).

来源：https://stackoverflow.com/questions/9120318/nice-clean-cross-join-in-linq-using-only-extension-methods

标签

linq

join

extension-methods