linq 扩展方法

PredicateExtensions

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Linq.Expressions;
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);

}

}
1：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

2：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
调用：
public Expression<Func<Orders, bool>> GetWhere()

{

Expression<Func<Orders, bool>> expression = PredicateExtensions.True<Orders>();

if (!string.IsNullOrEmpty(txtCustomerId.Text.Trim()))

{

string str = txtCustomerId.Text.Trim();

expression = expression.And(o => o.CustomerID.Contains(str));

}

if (!string.IsNullOrEmpty(txtEmplyeeId.Text.Trim()))

{

string str = txtEmplyeeId.Text.Trim();

expression = expression.And(o => o.EmployeeID.HasValue && o.EmployeeID.Value.Equals(int.Parse(str)));

}

if (txtOrderDateStart.SelectedDate.HasValue)

{

DateTime dt = txtOrderDateStart.SelectedDate.Value;

expression = expression.And(o => o.OrderDate.HasValue && o.OrderDate.Value >= dt);

}
if (txtOrderDateEnd.SelectedDate.HasValue)

{

DateTime dt = txtOrderDateEnd.SelectedDate.Value;

expression = expression.And(o => o.OrderDate.HasValue && o.OrderDate.Value <= dt);

}

return expression;

}

分类: Linq