CODEWITHSUNDEEP
c#dynamicexpression
nppatel
nppatel

Reputation: 33

Combining expressions from list of expressions

I want to create dynamic query builder using LINQ to SQL

For that, I created my interface that add each dynamic condition in 

List<Expression<Func<T,bool>>>

Interface looks like :

public interface IExpression<T>
{
    IExpression<T> AddWhere(Expression<Func<T,bool>> whereCriteria);    
}

Now I want to combine all expression in list and construct where clause with "and" condition and execute query.

I tried combining expression but not succeeded in that attempt.

Can anyone please help ? or please suggest any other alternative.

Upvotes: 3

Views: 2712

Answers (1)

Luaan
Luaan

Reputation: 63722

The easiest way is to use PredicateBuilder: http://www.albahari.com/nutshell/predicatebuilder.aspx

Basically, all you have to do is make use of this helper class:

using System;
using System.Linq;
using System.Linq.Expressions;
using System.Collections.Generic;

public static class PredicateBuilder
{
  public static Expression<Func<T, bool>> True<T> ()  { return f => true;  }
  public static Expression<Func<T, bool>> False<T> () { return f => false; }

  public static Expression<Func<T, bool>> Or<T> (this Expression<Func<T, bool>> expr1,
                                                      Expression<Func<T, bool>> expr2)
  {
    var invokedExpr = Expression.Invoke (expr2, expr1.Parameters.Cast<Expression> ());
    return Expression.Lambda<Func<T, bool>>
          (Expression.OrElse (expr1.Body, invokedExpr), expr1.Parameters);
  }

  public static Expression<Func<T, bool>> And<T> (this Expression<Func<T, bool>> expr1,
                                                       Expression<Func<T, bool>> expr2)
  {
    var invokedExpr = Expression.Invoke (expr2, expr1.Parameters.Cast<Expression> ());
    return Expression.Lambda<Func<T, bool>>
          (Expression.AndAlso (expr1.Body, invokedExpr), expr1.Parameters);
  }
}

And then you can use it like so:

  public static Expression<Func<Product, bool>> ContainsInDescription (
                                                params string[] keywords)
  {
    var predicate = PredicateBuilder.False<Product>();
    foreach (string keyword in keywords)
    {
      string temp = keyword;
      predicate = predicate.Or (p => p.Description.Contains (temp));
    }
    return predicate;
  }

(both the code and the example are taken from the link above, I just posted it here in case the link doesn't work sometime).

Your particular scenario is somewhat complicated by the fact that your interface doesn't use generics. Could you show a bit more of the relevant code, so that I can help tailor this solution better to your actual needs?

Upvotes: 7

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