Possible to make Array.Find Predicate match method more universal? Totake multiple parameters?

[raylopez99]

I suspect the answer to this question is that it's impossible, but how
do I make the below code work, at the point where it breaks (marked
below). See error CS0411

This is the complete code. Copy and paste it into VS2008 for a
Windows Forms application.

With some effort I can do the job with a user defined function, but my
question is whether I can do it using the built in library function
"Array.Find" and a predicate that takes multiple parameters rather
than one.

Thanks

RL

//////////////
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace Forms01
{
public partial class Form1 : Form
{

Point[] points;

public Form1()
{
points = new Point[]{ new Point(100, 200),
new Point(150, 250), new Point(250, 375),
new Point(275, 395), new Point(295, 450) };

InitializeComponent();

}



private void button1_Click(object sender, EventArgs e)
{

Point first = new Point(11,11);

try
{
first = Array.Find(points, ProductGT10);
MessageBox.Show("Point is: " + first.X.ToString());
}
catch (ArgumentNullException)
{
MessageBox.Show("null");
}

// this first try block works, as expected, when commenting out the
second try block below,
// to give output ‘Point is: 150’


Rectangle R2 = new Rectangle(100, 100, 200, 200);
Point second = new Point(12, 12);

//second try block does not work (does not compile)


try
{
first = Array.Find(points, ProductGT10_2(second, R2));

/*
//error CS0411: The type arguments for method
'System.Array.Find<T>(T[], System.Predicate<T>)' cannot be inferred
from the usage. Try specifying the type arguments explicitly

*/

MessageBox.Show("Point2 is: " + second.X.ToString());
}
catch (ArgumentNullException)
{
MessageBox.Show("null2");
}



}


///////////////////
private static bool ProductGT10(Point p)
{

Rectangle R = new Rectangle(101, 101, 200, 200);

//hard code the rectangle R
//can we avoid having to hard code the rectangle here? Can we pass
it? See below

if (R.Contains(p))
{
return true;
}
else
{
return false;
}
}

//this doesn't compile
private static bool ProductGT10_2(Point P, Rectangle R) //
passing the R
{
if (R.Contains(P))
{
return true;
}
else
{
return false;
}
}


}//end of Form1

} //end of Forms01 namespace

 

[Göran Andersson]

I suspect the answer to this question is that it's impossible, but how
do I make the below code work, at the point where it breaks (marked
below). See error CS0411

This is the complete code. Copy and paste it into VS2008 for a
Windows Forms application.

With some effort I can do the job with a user defined function, but my
question is whether I can do it using the built in library function
"Array.Find" and a predicate that takes multiple parameters rather
than one.

8< snip

Rectangle R2 = new Rectangle(100, 100, 200, 200);
Point second = new Point(12, 12);

//second try block does not work (does not compile)


try
{
first = Array.Find(points, ProductGT10_2(second, R2));

/*
//error CS0411: The type arguments for method
'System.Array.Find<T>(T[], System.Predicate<T>)' cannot be inferred
from the usage. Try specifying the type arguments explicitly

*/

The Predicate<T> delegate only takes a single parameter, so you can't
pass a method that takes two parameters.

You can create a class for the method, with a member variable for the
rectangle:

public class ProductGT10Comparer {

private Rectangle _r;

public ProductGT10Comparer(Rectangle r) {
_r = r;
}

public bool ProductGT10(Point p) {
return _r.Contains(p);
}

}

Create an instance of the class passing the R2 value, and use it's method:

first = Array.Find<Point>(points, new ProductGT10Comparer(R2).ProductGT10);


Another alternative is to use an anonymous method. Then the delegate
will contain the value of R2 so that the code can use it (so it's pretty
much the same as storing the R2 value in a class instance):

first = Array.Find<Point>(points, delegate(Point p) { return
R2.Contains(p); });

 

[raylopez99]

Create an instance of the class passing the R2 value, and use it's method:

first = Array.Find<Point>(points, new ProductGT10Comparer(R2).ProductGT10);

Another alternative is to use an anonymous method. Then the delegate
will contain the value of R2 so that the code can use it (so it's pretty
much the same as storing the R2 value in a class instance):

first = Array.Find<Point>(points, delegate(Point p) { return
R2.Contains(p); });

Thanks Goran! I understood the first example, but not the 'anonymous
delegate' example (I'm not used to that format, but it looks
compact). In any event, I appreciate your answer, it's a nice trick
that I'll store in my library, and I'll report back if I have any
problems.

Sincerely,
RL

 

[Jon Skeet [C# MVP]]

Thanks Goran! I understood the first example, but not the 'anonymous
delegate' example (I'm not used to that format, but it looks
compact). In any event, I appreciate your answer, it's a nice trick
that I'll store in my library, and I'll report back if I have any
problems.

This is basically the point behind closures - they allow you to carry
around some of your environment (including local variables) within
delegates. See
http://csharpindepth.com/Articles/Chapter5/Closures.aspx for other
examples.