在进行数据查询时，经常碰到需要动态构建查询条件。使用LINQ实现这个需求可能会比以前拼接SQL语句更麻烦一些。本文介绍了3种运行时动态构建查询条件的方法。
本文中的例子最终实现的都是同一个功能，从Northwind数据库Customers表中搜索出CompanyName列带有keywords中任意元素的项。keywords是个字符串数组，该数组长度在编译时是不确定的。思路及方法说明写在代码注释中.
1.表达式树
1
public static IEnumerable<Customers> GetCustomersFunc1(string[] keywords)
2
{
3
DataClassesDataContext dc = new DataClassesDataContext();
4
5 //创建一个静态类型为Customers的参数表达式
6 ParameterExpression c = Expression.Parameter(typeof(Customers), "c");
7
8 //创建一个恒等于false的表达式,用于与下面的表达式取并集
9 Expression condition = Expression.Constant(false);
10 foreach (string keyword in keywords)
11
{
12 //该表达式用于判断一个Customers类的CompanyName属性的值是否包含了关键字keyword
13 Expression con = Expression.Call(
14 Expression.Property(c, typeof(Customers).GetProperty("CompanyName")),
15 typeof(string).GetMethod("Contains", new Type[] { typeof(string) }),
16 Expression.Constant(keyword));
17
18 //与之前的condition表达式进行逻辑或运算。
19 //如果要查找的项需要包含keywords中的所有关键字,则可使用Expression.And(con, condition)
20 //并且将Expression condition = Expression.Constant(false);
21 //改成Expression condition = Expression.Constant(true);
22 condition = Expression.Or(con, condition);
23
}
24
25 //创建一个以一个Customers类作为参数并返回bool类型的委托
26 Expression<Func<Customers, bool>> end = Expression.Lambda<Func<Customers, bool>>(condition, new ParameterExpression[] { c });
27
28 //使用刚才构建的条件进行查询
29 var result = dc.Customers.Where(end);
30 return result;
31
}
32
public static IEnumerable<Customers> GetCustomersFunc1(string[] keywords)2
{
3
DataClassesDataContext dc = new DataClassesDataContext();4
5
//创建一个静态类型为Customers的参数表达式6
ParameterExpression c = Expression.Parameter(typeof(Customers), "c");7
8
//创建一个恒等于false的表达式,用于与下面的表达式取并集9
Expression condition = Expression.Constant(false);10
foreach (string keyword in keywords)11
{
12
//该表达式用于判断一个Customers类的CompanyName属性的值是否包含了关键字keyword13
Expression con = Expression.Call( 14
Expression.Property(c, typeof(Customers).GetProperty("CompanyName")),15
typeof(string).GetMethod("Contains", new Type[] { typeof(string) }),16
Expression.Constant(keyword));17
18
//与之前的condition表达式进行逻辑或运算。19
//如果要查找的项需要包含keywords中的所有关键字,则可使用Expression.And(con, condition)20
//并且将Expression condition = Expression.Constant(false);21
//改成Expression condition = Expression.Constant(true);22
condition = Expression.Or(con, condition); 23
}24
25
//创建一个以一个Customers类作为参数并返回bool类型的委托26
Expression<Func<Customers, bool>> end = Expression.Lambda<Func<Customers, bool>>(condition, new ParameterExpression[] { c });27
28
//使用刚才构建的条件进行查询29
var result = dc.Customers.Where(end);30
return result;31
}32
2.使用System.Linq.Dynamic
1 public static IEnumerable<Customers> GetCustomersFunc2(string[] keywords)
2 {
3 //需要引用System.Linq.Dynamic。Dynamic.cs文件可在LinqSamples中找到
4
5 DataClassesDataContext dc = new DataClassesDataContext();
6 string queryString = "";
7 foreach (string keyword in keywords)
8 {
9 //原形为(c=>c.CompanyName.Contains(keyword1)) || (c=>c.CompanyName.Contains(keyword2)) || 
10 queryString += "CompanyName.Contains(\"" + keyword + "\") or ";
11 }
12
13 //与false进行逻辑或运算,为了避免queryString中最后的or出现语法错误
14 queryString += "1=0";
15 return dc.Customers.Where(queryString);
16 }
17
public static IEnumerable<Customers> GetCustomersFunc2(string[] keywords)2
{3
//需要引用System.Linq.Dynamic。Dynamic.cs文件可在LinqSamples中找到4
5
DataClassesDataContext dc = new DataClassesDataContext();6
string queryString = "";7
foreach (string keyword in keywords)8
{9
//原形为(c=>c.CompanyName.Contains(keyword1)) || (c=>c.CompanyName.Contains(keyword2)) || 10
queryString += "CompanyName.Contains(\"" + keyword + "\") or ";11
}12
13
//与false进行逻辑或运算,为了避免queryString中最后的or出现语法错误14
queryString += "1=0";15
return dc.Customers.Where(queryString);16
}17
3.披着Linq的外衣拼接SQL语句
1 public static IEnumerable<Customers> GetCustomersFunc3(string[] keywords)
2 {
3 //这个方法其实是伪Linq,核心还是在拼接SQL语句,所以就不多解释了
4 DataClassesDataContext dc = new DataClassesDataContext();
5 string sqlQuery = "SELECT [CustomerID], [CompanyName], [ContactName], [ContactTitle], [Address], ";
6 sqlQuery += "[City], [Region], [PostalCode],[Country], [Phone], [Fax] FROM [dbo].[Customers] WHERE ";
7 foreach (string keyword in keywords)
8 {
9 sqlQuery += "([CompanyName] LIKE '%" + keyword + "%' ) OR ";
10 }
11 sqlQuery += "(1=0)";
12 return dc.ExecuteQuery<Customers>(sqlQuery);
13 }
14
15
public static IEnumerable<Customers> GetCustomersFunc3(string[] keywords)2
{3
//这个方法其实是伪Linq,核心还是在拼接SQL语句,所以就不多解释了4
DataClassesDataContext dc = new DataClassesDataContext();5
string sqlQuery = "SELECT [CustomerID], [CompanyName], [ContactName], [ContactTitle], [Address], ";6
sqlQuery += "[City], [Region], [PostalCode],[Country], [Phone], [Fax] FROM [dbo].[Customers] WHERE ";7
foreach (string keyword in keywords)8
{9
sqlQuery += "([CompanyName] LIKE '%" + keyword + "%' ) OR ";10
}11
sqlQuery += "(1=0)";12
return dc.ExecuteQuery<Customers>(sqlQuery);13
}14
15
