How to use LINQ in SharePoint

LINQ : Language Integrated Query

If you are writing in-line server code (C# and HTML code in same ASPX file), then you have to follow next 2 steps:

1) Page level directive

<%@ Assembly Name="System.Core, Version=3.5.0.0, Culture=neutral, 
       PublicKeyToken=B77A5C561934E089" %>
<%@ Import Namespace="System.Linq" %>
<%@ Import Namespace="System.Collections.Generic" %>

2) Modify application's web.config file
Add following lines just below </system.Web> node

  
  </system.web><!-- Reference point -->

  <system.codedom>
    <compilers>
      <compiler language="c#;cs;csharp" extension=".cs" warningLevel="4" 
          type="Microsoft.CSharp.CSharpCodeProvider, System, Version=2.0.0.0, 
          Culture=neutral, PublicKeyToken=b77a5c561934e089">
        <providerOption name="CompilerVersion" value="v3.5"/>
        <providerOption name="WarnAsError" value="false"/>
      </compiler>
      <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb" 
          warningLevel="4" type="Microsoft.VisualBasic.VBCodeProvider, System, 
          Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
        <providerOption name="CompilerVersion" value="v3.5"/>
        <providerOption name="OptionInfer" value="true"/>
        <providerOption name="WarnAsError" value="false"/>
      </compiler>
    </compilers>
  </system.codedom>

3) LINQ code

If you writing C# in seperate file (Code-behind) then you can skip first two steps.

SPList oList;
//To get a particular SPView specific SPListItems 
SPView spView = oList.Views["MySPView_Name"];
SPListItemCollection itemCollec = oList.GetItems(spView);


if (itemCollec != null)
{
 //You can also run following query directly 
 //over SPList (by replacing itemColle with oList) 
 List<SPListItem> items 
        = (from spLItem in itemCollec.OfType<SPListItem>()
              where spLItem["Employee Name"] != null
                 && Convert.ToString(spLItem["Employee Name"]) == "Avi"
              select spLItem).ToList<SPListItem>();

  //Write total items count
 Response.Write(items.Count);
 
 //To get individual SPListItem you can either use 
 //Foreach iteration or indexer e.g. items[i]
}

References:

1) SharePoint Development with LINQ
2) Use Linq inside ...
3) Using LINQ to query a SharePoint List
4) Linq in Sharepoint and querying listitems