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LINQ Extensions - C#

Overview¶

This article provides the code for two extension methods for use when building expressions, commonly used with LINQ. These functions allow you to build Boolean expressions on the fly.

• OrElse()
• AndAlso()
  

Sample Usage¶

The CustomerOrder is only used as an example; it can be any class.

Expression<Func<CustomerOrder, bool>> expr = x => false;

if (someCondition)
{
    expr = expr.OrElse(a => a.OrderAmount >= 1000);
}

if (someOtherCondition)
{
    expr = expr.OrElse(a => a.OrderAmount <= 5000);
}

var items = _db.CustomerOrders.Where(expr.AndAlso(a => !a.IsDeleted));

Extension Methods¶

using System;
using System.Linq.Expressions;

public static class LinqExtensions
{
    /* Adapted from
        * https://stackoverflow.com/questions/29565373/combine-listexpressionfunct-bool-to-an-or-clause-linq
        */
    public static Expression<Func<T, bool>> OrElse<T>(
        this Expression<Func<T, bool>> a,
        Expression<Func<T, bool>> b)
    {
        var bodyB = b.Body.Replace(b.Parameters[0], a.Parameters[0]);

        return Expression.Lambda<Func<T, bool>>(Expression.OrElse(a.Body, bodyB), a.Parameters);
    }


    public static Expression<Func<T, bool>> AndAlso<T>(
        this Expression<Func<T, bool>> a,
        Expression<Func<T, bool>> b)
    {
        var bodyB = b.Body.Replace(b.Parameters[0], a.Parameters[0]);

        return Expression.Lambda<Func<T, bool>>(Expression.AndAlso(a.Body, bodyB), a.Parameters);
    }


    private static Expression Replace(this Expression expression, Expression searchEx, Expression replaceEx)
    {
        return new ReplaceVisitor(searchEx, replaceEx).Visit(expression);
    }


    private class ReplaceVisitor : ExpressionVisitor
    {
        private readonly Expression _from, _to;
        public ReplaceVisitor(Expression from, Expression to)
        {
            _from = from;
            _to = to;
        }
        public override Expression Visit(Expression node)
        {
            return node == _from ? _to : base.Visit(node);
        }
    }
}