很多时候,我们的查询条件有多个,并且参数不一样,得到的查询条件也就不一样。这就要我们动态的生成查询条件,再把这些查询条件合并起来。网上找了这个类,可以很好的达到我们的要求。
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.Or(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.And(expr1.Body, invokedExpr), expr1.Parameters);
}
}
测试一下:
static void LinqAndTest()
{
List<string> list = new List<string>();
list.Add("one");
list.Add("two");
list.Add("three");
list.Add("four");
list.Add("five");
list.Add("six");
//返回字长大于3
Expression<Func<string, bool>> exp = s => s.Length > 3;
var results= list.Where(exp.Compile());
foreach (var result in results)
{
Console.WriteLine(result);
}
Expression<Func<string, bool>> exp2 = s => s.Contains('o');
var exp3 = exp.And(exp2);
//合并两个条件:返回字长大于3且包括o字符
results = list.Where(exp3.Compile());
foreach (var result in results)
{
Console.WriteLine(result);
}
Console.Read();
}