【转】关于LinQ的动态Or查询

/// <summary>
/// 构造函数使用True时：单个AND有效，多个AND有效；单个OR无效，多个OR无效；混合时写在AND后的OR有效
/// 构造函数使用False时：单个AND无效，多个AND无效；单个OR有效，多个OR有效；混合时写在OR后面的AND有效
/// </summary>
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);
    }
}

调用方法：

HUBDataDataContext db = new HUBDataDataContext();
var predicate = PredicateExtensions.False <T_DISPATCH>();
predicate = predicate.Or(c => c.ToHUBID == 1);
var dispatch = db.T_DISPATCH.Where(predicate).Where(c => c.State == 3); 

转自：http://www.cnblogs.com/ejiyuan/archive/2009/07/14/1523515.html