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;
    }