Is it possible to write join with a boolean evaluation? - linq

Question

I want to write
CompareInfo myCompIntl = CompareInfo.GetCompareInfo( "es-ES" );
var SharedYomi = from ObjA in ClassListA
join ObjB in ClassListB
where CompareInfo.Compare(ObjA.Name, ObjB.Name) == 0
select new {stringA = stringA, string = string};
Linq forces me to write join with equals. I can not pass in a Boolean evaluation.
How can I do that?

Answer 1

You cannot write that using the LINQ lambda query syntax. The join keyword requires you to specify exactly two properties that are compared using the equals keyword, because this maps to the first overload of Join that uses the default comparer to compare keys.
However, there is an overload of Join that accepts an IEqualityComparer that will probably work for you, you just need to use method query syntax.
Since you sound like you're not familiar with the method syntax, here's a good starting article from MSDN:
http://msdn.microsoft.com/en-us/library/vstudio/bb397947.aspx
But, basically, the syntax you think of as "LINQ" is just one way to refer to the LINQ extensions, and is really just syntactic sugar around the IEnumerable extension methods that implement LINQ. So, for example, the query:
from x in y where x.IsActive orderby x.Name select x
it basically identical to
y.Where(x => x.IsActive).OrderBy(x => x.Name).Select(x => x);
For the most part, each query clause maps to a particular overload of a particular IEnumerable method, but those methods have a number of other overloads that take different numbers and types of parameters.
The Join methods are a bit complex, because they take two sequences as input and let you combine individual elements of them using expressions, but the idea is exactly the same. A typical join would look like this:
from x in y
join a in b on x.Id equals a.ParentId
select new { x.Id, x.Name, a.Date }
becomes
y.Join(
b,
x => x.Id,
a => a.ParentId,
(x, a) => new { x.Id, x.Name, a.Date });
This will join a.ParentId and x.Id using the default comparison for their data type (int, string, whatever). The compiler directly translates the query syntax into method syntax, so the two behave exactly the same. (Nitpicking my own answer: Technically, the methods are on the Enumerable class, so you are really calling Enumerable.Join. But as they were implemented as extension methods, you can call them either way and the compiler will figure it out.)
In your case, what you need is to pass in a different comparison method, so you can call string.Compare with the explicit encoding. The other overload of Join lets you supply an implementation of IEqualityComparer<T> to use instead of the default. This will require you to implement IEqualityComparer<string> in a separate class, since there's no easy way to create an anonymous interface implementation (perhaps the only feature I miss from Java). For your example, you want something like this:
public class ComparerWithEncoding : IEqualityComparer<string>
{
private CompareInfo compareInfo
public ComparerWithEncoding ( string encoding )
{
this.compareInfo = CompareInfo.GetCompareInfo(encoding);
}
public bool Equals ( string a, string b )
{
return CompareInfo.Compare(a, b) == 0
}
public int GetHashCode(string a)
{
return a.GetHashCode();
}
}
classListA.Join(
ClassListB,
ObjA => ObjA.Name,
ObjB => ObjB.Name,
(ObjA, ObjB) => new { stringA = ObjA.Foo, stringB = ObjB.Bar },
new ComparerWithEncoding("es-ES"));