How to Use LINQ methods to compare objects of custom types

Original article:Http://blogs.msdn.com/csharpfaq/archive/2009/03/25/how-to-use-linq-methods-to-compare-objects-of-custom-types.aspx

Translation:Http://www.cnblogs.com/tianfan/

LINQ provides a convenient syntax and incluuseful methods for operating with collections of objects. However, to be correctly processed by LINQ comparison methods such as distinct or intersect, a type must satisfy certain requirements.

Let's take a look at the distinct method, which returns all distinct objects from a collection.

List <int> numbers = new list <int> {1, 1, 2, 3 };

VaR distinctnumbers = numbers. Distinct ();

Foreach (VAR number in distinctnumbers)

Console. writeline (number );

The output is:

1

2

3

But what if you want to use the distinct method for a collection of objects of your own type? For example, like this:

Class number

{

Public int digital {Get; set ;}

Public String textual {Get; set ;}

}

Class Program

{

Static void main (string [] ARGs)

{

List <number> numbers = new list <number> {

New number {digital = 1, textual = "one "},

New number {digital = 1, textual = "one "},

New number {digital = 2, textual = "two "},

New number {digital = 3, textual = "three "},

};

VaR distinctnumbers = numbers. Distinct ();

Foreach (VAR number in distinctnumbers)

Console. writeline (number. Digital );

}

}

The Code compiles, but the output is different:

1

1

2

3

Why did that happen? The answer is in the LINQ implementation details. To be correctly processed by the distinct method, a type must implement the iequatable <t> interface and provide its own equals and gethashcode methods.

So, the number class from the previous example shocould actually look like this:

Class number: iequatable <number>

{

Public int digital {Get; set ;}

Public String textual {Get; set ;}

Public bool equals (number other)

{

// Check whether the compared object is null.

If (object. referenceequals (Other, null) return false;

// Check whether the compared object references the same data.

If (object. referenceequals (this, other) return true;

// Check whether the objects 'properties are equal.

Return digital. Equals (other. Digital )&&

Textual. Equals (other. textual );

}

// If equals returns true for a pair of objects,

// Gethashcode must return the same value for these objects.

Public override int gethashcode ()

{

// Get the hash code for the textual field if it is not null.

Int hashtextual = textual = NULL? 0: Textual. gethashcode ();

// Get the hash code for the digital field.

Int hashdigital = digital. gethashcode ();

// Calculate the hash code for the object.

Return hashdigital ^ hashtextual;

}

}

But what if you cannot modify the type? What if it was provided by a library and you have no way of implementing the iequatable <t> interface in this type? The answer is to create your own algorithm ity comparer and pass it as a parameter to the distinct method.

The specified ity comparer must implement the iequalitycomparer <t> interface and, again, provide gethashcode and equals methods.

Here is how the capacity ity comparer for the original number class might look:

Class numbercomparer: iequalitycomparer <number>

{

Public bool equals (number X, number y)

{

If (object. referenceequals (x, y) return true;

If (object. referenceequals (x, null) |

Object. referenceequals (Y, null ))

Return false;

Return X. Digital = Y. Digital & X. textual = Y. textual;

}

Public int gethashcode (Number number)

{

If (object. referenceequals (number, null) return 0;

Int hashtextual = number. textual = NULL

? 0: Number. textual. gethashcode ();

Int hashdigital = number. Digital. gethashcode ();

Return hashtextual ^ hashdigital;

}

}

And don't forget to pass the comparer to the distinct method:

VaR distinctnumbers = numbers. Distinct (New numbercomparer ());

Of course, these rules don't just apply to the distinct method. for example, the same is true for the contains, except T, intersect, and Union methods. in general, if you see that a LINQ method has an overload that accepts the iequalitycomparer <t> parameter, it probably means that to use it for your own data types you need to either implement iequatable <t> in your class or create your own capacity ity comparer.