C# Linq where clause as a variable

Question

I am trying to make a LINQ statement where the where clause comes from a variable. For example:

string whereClause = address.zip == 23456;
var x = from somet         


        

Answer 1

Can try:

var lstQ_Buffer = new List<Q_Buffer>();

Get

lstQ_Buffer = unitOfWork.Q_BufferRepository.Get().ToList();

Then apply where

if (lstQ_Buffer.Count > 0)
{
    lstQ_Buffer = lstQ_Buffer.Where(q => q.fkProgramId == programId && q.fkYearId == yearId && q.fkSemesterId == semesterId && q.fkCourse_ModuleId == courseModuleId && q.fkSubject_SpecialtyId == subjectSpecialtyId && q.fkSubSpecialtyId == subSpecialtyId && q.fkTopicId == topicId && q.fkSubTopicId == subTopicId && q.fkDifficultyLevelId == diffucultyLevelId).ToList();
}

Answer 2

You need to assembly an Expression<Func<T, bool>> and pass it to the Where() extension method:

Expression<Func<T, bool>> whereClause = a => a.zip == 23456;
var x = frSomeList.Where(whereClause);

EDIT: If you're using LINQ to Objects, remove the word Expression to create an ordinary delegate.

Answer 3

As Richard has pointed out, the Dynamic Query Library can be used to build dynamic filter expressions. When using Linq-To-Objects make sure to convert your IEnumerable<T> to a IQueryable<T> first. Here is an (incomplete) example:

using System.Linq.Dynamic;

namespace System.Linq.Dynamic
{
  public class Example
  {
   // Assuming some value is assigned to below field somewhere... 
   private IEnumerable<Address> m_Addresses;

   public void FilterByZipCode(string zipCode)
   {
      var x = m_Addresses.AsQueryable().Where("Zip == @0", zipCode);
      dowork(x);
   }
  }

  public class Address
  {  
     public String Zip { get; set; }

     // More Properties...  
  }
}

Answer 4

That's a built-in Feature of LINQ. Just use the Where extension method.

See LINQ Query Syntax versus Method Syntax (C#) for more information.

Answer 5

This:

var query = from something in someList where whereClause;

is shorthand for:

var query = someList.Where(something => whereClause);

Assuming someList is an IEnumerable<Address>, Where refers to the Enumerable.Where Extension Method. This method expects a Func<Address, bool> which you can define as follows:

Func<Address, bool> whereClause = address => address.Zip == 23456;
var query = someList.Where(whereClause);