PredicateBuilder类（linq多条件组合查询）

PredicateBuilder类如下：

 public static class PredicateBuilder
    {

/// <summary>
/// 机关函数应用True时：单个AND有效，多个AND有效；单个OR无效，多个OR无效；混应时写在AND后的OR有效  
/// </summary>
/// <typeparam name="T"></typeparam>
/// <returns></returns>
public static Expression<Func<T, bool>> True<T>() { return f => true; }

/// <summary>
/// 机关函数应用False时：单个AND无效，多个AND无效；单个OR有效，多个OR有效；混应时写在OR后面的AND有效  
/// </summary>
/// <typeparam name="T"></typeparam>
/// <returns></returns>
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);
        } 
    }

　　多条件查询的代码：

/// <summary>
/// 多条件查询
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
protected void btnSearch_Click(object sender, EventArgs e)
    {
using(LinqDBDataContext db = new LinqDBDataContext())
        {
var list = db.StuInfo;
var where = PredicateBuilder.True<StuInfo>();
if(this.txtName.Text.Trim().Length!=0)
            {
where = where.And(p => p.StuName.Contains(this.txtName.Text.Trim()));
            }
if(this.txtAge.Text.Trim().Length!=0)
            {
where = where.And(p => p.StuAge == Convert.ToInt32(this.txtAge.Text.Trim()));
            }
var result = list.Where(where).ToList();
this.repStuInfo.DataSource = result;
this.repStuInfo.DataBind();
        }
    }

  上面代码中，txtName是姓名文本框，txtAge是年龄文本框，因为要进行and条件查询所以一开始使用PredicateBuilder.True<StuInfo>()来创建初始为true的where条件，

如果进行or多条件查询，就应该使用PredicateBuilder.False<StuInfo>()来创建初始为false的where条件