FileInfo Arrya Contains Property

dpkbahuguna · #1 (**permalink**) November 9th, 2008, 06:44 AM

Hello friends,

     I have a FIleInfo collection and I want to search a particular file in this collection but I am unable to get the result...

     I am doing something like this please tell what I am doing wrong here my code is here below:

    In this code there are tow values are coming in FileInfo Ccollection and I have to search anyone according to condition

========================================
          var Qry = from x in FI where (x.ToString().ToLower()).StartsWith(fname.ToLower( )) select x;

        FileInfo RowTextFile = new FileInfo("FIleName.txt");

        FileInfo[] fileName = Qry.ToArray<FileInfo>();

//this FILENAME>TXT is available into FileInfo array so this IF CONTDITION should return TRUE value but condition returning FALSE.
        if(fileName.Contains<FileInfo>(RowTextFile))
    {
        dosomething;
    }
=================================

thanks in advance!!

DPK..

Old Pedant · #2 (**permalink**) November 9th, 2008, 03:54 PM

What language is this supposed to be????

You have var Qry, so it looks like JavaScript. Except it's not like any legal JavaScript that I have ever seen.

And then the rest of the code looks like C#.

Bob Bedell · #3 (**permalink**) November 9th, 2008, 08:32 PM

The language is C# 3.0 with implicitly typed local variable and LINQ query operations support.

The problem is that, since Object.Equals isn't overriden in your FileInfo instances, your comparison is using normal reference type equality semantics. The comparison instances aren't the same object, so the comparison returns false.

Here's is a little custom equality comparer that will allow you to compare you FileInfo object's Name property:

Code:

public class FileNameComparer : EqualityComparer<FileInfo>
    {
        public override bool Equals(FileInfo x, FileInfo y)
        {
            return x.Name == y.Name;
        }

        public override int GetHashCode(FileInfo obj)
        {
            return (obj.Name).GetHashCode();
        }
    }

Now in your code, instantiate a FileNameComparer, then use the Enumerable.Contains<TSource> overload that accepts two parameters: an IEnumerable<TSource>, and a IEqualityComparer<TSource>, like:

Code:

FileInfo[] fileName = Qry.ToArray<FileInfo>();
            FileNameComparer fileNameComparer = new FileNameComparer();
            if (fileName.Contains<FileInfo>(RowTextFile, fileNameComparer))
            {
                return;
            }

HTH,

Bob

Old Pedant · #4 (**permalink**) November 11th, 2008, 03:56 AM

Ahhh...makes eminent sense, now.

I think I also got hung up on the inefficiency of the SQL query. Doing toLowerCase() on a field in SQL pretty much guarantees that you can't get the best performance if the field in question is indexed. Since most SQL operations are case-insensitive, you'd be much better off with

var Qry = from x in FI where x LIKE fname+'%' select x;

Assuming that's legal LINQ syntax. But in any case, finding a way to use LIKE would presumably give better efficiency. Clearly wouldn't give worse.

dpkbahuguna · #5 (**permalink**) November 11th, 2008, 09:10 AM

Ya thanks to reply Old Pedant,

using LINQ i have done it..but just wanted to know why is it not running as I was comparing it.

Thanks again.

Quote:

quote:Originally posted by Old Pedant
Ahhh...makes eminent sense, now.

I think I also got hung up on the inefficiency of the SQL query. Doing toLowerCase() on a field in SQL pretty much guarantees that you can't get the best performance if the field in question is indexed. Since most SQL operations are case-insensitive, you'd be much better off with

var Qry = from x in FI where x LIKE fname+'%' select x;

Assuming that's legal LINQ syntax. But in any case, finding a way to use LIKE would presumably give better efficiency. Clearly wouldn't give worse.

DPK..

dpkbahuguna · #6 (**permalink**) November 11th, 2008, 09:23 AM

Thanks Bob Bedell,

Nice explanation..thank you very much...

cheers :)!!

Quote:

quote:Originally posted by Bob Bedell
The language is C# 3.0 with implicitly typed local variable and LINQ query operations support.

The problem is that, since Object.Equals isn't overriden in your FileInfo instances, your comparison is using normal reference type equality semantics. The comparison instances aren't the same object, so the comparison returns false.

Here's is a little custom equality comparer that will allow you to compare you FileInfo object's Name property:

Code:

public class FileNameComparer : EqualityComparer<FileInfo>
    {
        public override bool Equals(FileInfo x, FileInfo y)
        {
            return x.Name == y.Name;
        }

        public override int GetHashCode(FileInfo obj)
        {
            return (obj.Name).GetHashCode();
        }
    }

Now in your code, instantiate a FileNameComparer, then use the Enumerable.Contains<TSource> overload that accepts two parameters: an IEnumerable<TSource>, and a IEqualityComparer<TSource>, like:

Code:

FileInfo[] fileName = Qry.ToArray<FileInfo>();
            FileNameComparer fileNameComparer = new FileNameComparer();
            if (fileName.Contains<FileInfo>(RowTextFile, fileNameComparer))
            {
                return;
            }

HTH,

Bob

DPK..

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