linq动态多条件查询

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

namespace ClassTest
{

public static class PredicateExtensions
{

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>> expression1, Expression<Func<T, bool>> expression2)
{

var invokedExpression = Expression.Invoke(expression2, expression1.Parameters.Cast<Expression>());

return Expression.Lambda<Func<T, bool>>(Expression.Or(expression1.Body, invokedExpression), expression1.Parameters);
}

public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expression1, Expression<Func<T, bool>> expression2)
{

var invokedExpression = Expression.Invoke(expression2, expression1.Parameters.Cast<Expression>());

return Expression.Lambda<Func<T, bool>>(Expression.And(expression1.Body, invokedExpression), expression1.Parameters);

}

}
public class Test
{

struct Product
{
public int productID;
public string productName;
public string productDesc;
}
static void Main()
{
List<Product> products = new List<Product>();
Product product;
product.productID = 1;
product.productName = "aaa";
product.productDesc = "good aaa";
products.Add(product);
product.productID = 1;
product.productName = "bbb";
product.productDesc = "good bbb";
products.Add(product);
product.productID = 3;
product.productName = "ccc";
product.productDesc = "good ccc";
products.Add(product);
product.productID = 4;
product.productName = "ddd";
product.productDesc = "good ddd";
products.Add(product);

var predicate = PredicateExtensions.True<Product>();
int id=1;
string name=null;
string desc=null;

if (id != 0)
predicate=predicate.And(p => p.productID == id);
if (!string.IsNullOrEmpty(name))
predicate=predicate.And(p => p.productName.Contains(name));
if (!string.IsNullOrEmpty(desc))
predicate=predicate.And(p => p.productDesc.Contains(desc));

var proList = products.AsQueryable().Where(predicate).Select(p=>p);

foreach (var prod in proList)
Console.WriteLine("{0} {1} {2}",prod.productID,prod.productName,prod.productDesc);
}

}
}