using System;

using System.Linq;

using System.Text;

public class Program

{

    public static void Main()

    {

        StringBuilder whereClause = new StringBuilder();

        Filter filter = null;

        var array = new [] 

        { 

            new Filter { Name = "Prop0", Value = "null", Operator = "=" },

            new Filter { Name = "Prop1", Value = 2, Operator = "=" },

            new Filter { Name = "Prop2", Value = "text", Operator = "LIKE" }

        };

        for (int i = 0; i < array.Length; i++)

        {

            filter = array[i];

            var exp = ExpressionFactory.GetBy(filter);

            whereClause.Append(exp.GetExpressionStr(filter, i));

            whereClause.Append(" AND ");

        }

        Console.WriteLine(whereClause);

    }

    public static class ExpressionFactory

    {

        private static StandardExpression _standardExpression = new StandardExpression();

        private static ICustomExpression[] _specialExpressions = new []

        {

            (ICustomExpression)new NullExpression(),

            (ICustomExpression)new LikeExpression()

        };

        public static ICustomExpression GetBy(Filter filter)

        {

            var match = _specialExpressions.SingleOrDefault(e => e.ExpressionIsValid(filter));

            if (match == null)

                return _standardExpression;

            return match;

        }

    }

    public class Filter

    {

        private string _operator = "="; // =, !=, <,> ...

​

        public string Name { get; set; }

        public object Value { get; set; }

        public string Operator { get { return _operator; } set { _operator = value; } }

    }

​

    public interface ICustomExpression

    {

        bool ExpressionIsValid(Filter filter);

        string GetExpressionStr(Filter filter, int index);

    }

​

​

    public class StandardExpression : ICustomExpression

    {

        public bool ExpressionIsValid(Filter filter)

        {

            return true;

        }

​

        public string GetExpressionStr(Filter filter, int index)

        {

            return filter.Name + " " + filter.Operator + " @" + index;

        }

    }

​

    public class LikeExpression : ICustomExpression

    {

        public bool ExpressionIsValid(Filter filter)

        {

            return filter.Operator.ToLower().Equals("like");

        }

​

        public string GetExpressionStr(Filter filter, int index)

        {

            return filter.Name + ".Contains(@" + index + ")";

        }

    }

    public class NullExpression : ICustomExpression

    {

        public bool ExpressionIsValid(Filter filter)

        {

            return filter.Value.ToString().ToLower().Equals("null");

        }

​

        public string GetExpressionStr(Filter filter, int index)

        {

            return filter.Name + " " + filter.Operator + " null ";

        }

    }

}