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njgroup

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how to select top 1 in linq?

hi,

how to select top 1 in linq?
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DarkoLord
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If you're using LINQ Extension Methods, First() or FirstOrDefault() method. First throws an exception if there are no results, while FirstOrDefaults returns the default instance of the object.
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Hi njgroup;

You can use the Linq Take() method As Follows: Let say that the results of the query is in a collections called customers,

var cust = customers.Take(1);

You can change 1 to any number of elements to return from the top of the collection. So replacing 1 with 7 will return the first 7 elements.

Fernando
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njgroup

ASKER

that interesting,

I have this function to select data, I wanna convert it linq, how can I do that??
Can you paste that function here?
Please post the function and a sample of input data and what the fustian is to return.
As a clarification:

First() or FirstOrDefault() would be equivalent to "SELECT TOP 1 FROM tbl WHERE ..."

Take(1) would be equivalent to "SELECT TOP 1 FROM tbl"
Take(1), First() or FirstOrDefault() all send a TSQL statement to the server in the form of :

SELECT TOP (1) [t0].[Column1], [t0].[Column2], ..., [t0].[Column3]
FROM [dbo].[TableName] AS [t0]
WHERE [t0].[CompareColumn] = @p0

The Take method I believe is more flexable if the number of elements changes because the number is the only thing to change.
I was speaking more in terms of First and FirstOrDefault taking a predicate parameter, which could be equated to a WHERE clause. Perhaps with Linq-to-Sql this would not be of consequence, but if you were doing Linq-to-Objects or Linq-to-Xml, then the difference may be more important  :)

I'm sure you could construct a predicate to perform the same job that Take() accomplishes, though.
I see what you are stating now although I wanted to make sure that njgroup understands that they both can do the same thing just in a different way.
Agreed  :)
Well, there are also some other small but very important differences between First/FirstOrDefault and Take, besides the fact that First/FirstOrDefault take a predicate parameter.

The first is that the Take is implemented using deferred execuction, which means that the actual query is not executed immediately but rather when the data is first accessed.

And the second, which makes Take a little less practical for single element retrieval is the fact that it always returns IEnumerable<TSource>, while the First and FirstOrDefault return just TSource; this means you always have to access the element via the indexer instead of using it directly.

Also they differ a bit in the case if no element is returned: First throws an exception, Take returns an empty IEnumerable<TSource> and FirstOrDefault returns the default instance of TSource (usually null)
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ASKER

sorry for late, I was having something, here is the function:

private DataTable GetSelectedCountry(long formattedIP)
        {
            string Conn_String = System.Configuration.ConfigurationManager.AppSettings.Get("ConnString");
            SqlConnection Conn = new SqlConnection(Conn_String);
            DataTable dt = new DataTable();
            string sql = "SELECT TOP 1 * FROM ip_group_city where ip_start <= " + formattedIP + " order by ip_start desc";
            SqlDataAdapter DA = new SqlDataAdapter(sql, Conn);
            DA.Fill(dt);
            Conn.Close();
            return dt;
        }
ASKER CERTIFIED SOLUTION
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Fernando Soto
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ASKER

sorry, I didnt understand very well, the DataClasses1DataContext is similar to HelperUtilities
 class??
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ASKER

moreover, what is the dll for this class: DataClasses1DataContext?
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ASKER

because I got missing assembly for it!
Hi njgroup;

The DataClasses1DataContext is the Data Context that makes up code that maps the classes in your code to the tables in your database. If you have not created these class you can use the automated tool in Visual Studio to do so. The following link will show you how.

http://weblogs.asp.net/scottgu/archive/2007/05/29/linq-to-sql-part-2-defining-our-data-model-classes.aspx

Because Linq to SQL does not support directly filling a DataTable from a Linq query the HelperUtilities / CreateDataTable<T> will convert the results of the query into a DataTable.

Fernando
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ASKER

thanks Fernando, your solution is really great :))
Not a problem, glad I was able to help.  ;=)