转载泛型方法动态生成表达式树 Expression

        public string GetGridJSON(TraderInfo model)
        {
            IQueryable<TraderInfo> Temp = db.TraderInfo;
            if (model.LoginAccount != null)
            {
                Temp = Temp.Where(X => X.LoginAccount == model.LoginAccount);
            }
            if (model.ShopName != null)
            {
                Temp = Temp.Where(X => X.ShopName == model.ShopName);
            }
            return JsonConvert.SerializeObject(Temp.ToList(), Formatting.Indented, new JsonSerializerSettings() { DateFormatHandling = 0 });
        }

这是MVC 项目。  点击查询 执行 GetGridJSON 方法。由于 MVC 规范  name属性可以匹配 模型，如果 不填写 账号 、名称 ，返回的  model 里面 的 账号 、名称属性为 null。 每次都要判断 是否 是 null ,如果是 ，就 查询 所有，否  才能匹配where。

这很不好，如果我的条件 很多 呢？  这样的判断  就 很烦了。

我做了如下如下封装

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Reflection;
using System.Linq.Expressions;

namespace EF_DAL
{
/// <summary>
/// 动态生成 表达式树
/// </summary>
public class Custom_Expression
{
/// <summary>
///
/// </summary>
/// <param name="left"></param>
/// <param name="right"></param>
/// <returns>Lambda表达式树</returns>
public delegate Expression ExpressionEventHandler(Expression left, Expression right);
private static BinaryExpression filter = Expression.Equal(Expression.Constant(1), Expression.Constant(1));

/// <summary>
/// 自定义Equal方法(允许value为null)，value为null 的时候，该查询条件 不会生成
/// </summary>
/// <typeparam name="T">实体数据类型</typeparam>
/// <param name="columnNames">以逗号分割的列名称</param>
/// <param name="values">这些列对应的值</param>
/// <returns>返回Lambda表达式，eg:where(Lambda)</returns>
public Expression<Func<T, bool>> Custom_Equal<T>(string columnNames, params object[] values)
{
return Custom_Expression_Common<T>(Equal_result, columnNames, values);
}
/// <summary>
/// 初始化where 1=1
/// </summary>
private void Init()
{
filter = Expression.Equal(Expression.Constant(1), Expression.Constant(1));
}

/// <summary>
/// 自定义Contains方法(允许value为null)，value为null 的时候，该查询条件 不会生成
/// </summary>
/// <typeparam name="T">实体数据类型</typeparam>
/// <param name="columnNames">以逗号分割的列名称</param>
/// <param name="values">这些列对应的值</param>
/// <returns>返回Lambda表达式，eg:where(Lambda)</returns>
public Expression<Func<T, bool>> Custom_Contain<T>(string columnNames, params object[] values)
{
return Custom_Expression_Common<T>(Contains_result, columnNames, values);
}

private Expression Contains_result(Expression left, Expression right)
{
return Expression.Call(left, typeof(string).GetMethod("Contains"), right);
}

private Expression Equal_result(Expression left, Expression right)
{
return Expression.Equal(left, right);
}

/// <summary>
///
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="Handler">可以是 Equal、Contains</param>
/// <param name="columnNames"></param>
/// <param name="values"></param>
/// <returns></returns>
private Expression<Func<T, bool>> Custom_Expression_Common<T>(ExpressionEventHandler handler, string columnNames, params object[] values)
{
Init();
var columns = columnNames.Split(‘,‘);
var param = Expression.Parameter(typeof(T));
for (int i = 0; i < columns.Length; i++)
{
if (values[i] == null) continue;
string columnName = columns[i].ToString();
var value = values[i];
Expression left = Expression.Property(param, typeof(T).GetProperty(columnName));
Expression right = Expression.Constant(value, value.GetType());
Expression result = handler(left, right);
filter = Expression.And(filter, result);// where 条件 拼接
}
return Expression.Lambda<Func<T, bool>>(filter, param);
}
}
}

直接这样 调用啦！

public string GetGridJSON(TraderInfo model)
{
EF_DAL.Custom_Expression CE = new EF_DAL.Custom_Expression();
var traderInfoList = db.TraderInfo.Where(CE.Custom_Equal<TraderInfo>("LoginAccount,ShopName", model.LoginAccount, model.ShopName)).ToList();
return JsonConvert.SerializeObject(traderInfoList, Formatting.Indented, new JsonSerializerSettings() { DateFormatHandling = 0 });
}