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　　使用DataContext扩展方法Find<TEntity>(TEntity obj) 遇到一个问题，如果字段可以为空，提示错误信息：

没有为类型“System.Nullable`1[System.Int32]”和“System.Int32”定义二进制运算符 Equal。 
不知到是什么问题，如果字段不为空的话，查询一切正常。希望大家能帮助解决一下。

Find<TEntity>(TEntity obj)方法用途：通过定义实体类，查询符合它的所有记录。

代码

  Expression right = Expression.Constant(p.GetValue(obj, null));
                                    Expression left = Expression.Property(param, p.Name);
                                    Expression filter = Expression.Equal(left, right);

　　主要问题在于 left.Type 是System.Nullable<System.Int32> ,而 right.Type 是System.Int32 。提示错误在：Expression filter = Expression.Equal(left, right);
Expression 的Type为只读。怎样能让两个类型统一，或者能采用其他的方法来避开这个问题。

执行过程：Aritle art=new Article();art.Status=1; var list=db.Find<Article>(art);

Find<TEntity>(TEntity obj) 代码：

 1   /// <summary>
 2         /// 实现查找
 3         /// </summary>
 4         /// <typeparam name="TEntity"></typeparam>
 5         /// <param name="obj"></param>
 6         /// <returns></returns>
 7         public IQueryable<TEntity> Find<TEntity>(TEntity obj) where TEntity : class
 8         {
 9             //获得所有property的信息
10             PropertyInfo[] properties = obj.GetType().GetProperties(BindingFlags.Public | BindingFlags.Instance);
11             //构造初始的query
12 
13             IQueryable<TEntity> query = this.GetTable<TEntity>().AsQueryable<TEntity>();
14             //遍历每个property
15             foreach (PropertyInfo p in properties)
16             {
17                 if (p != null)
18                 {
19                     Type t = p.PropertyType;
20                     //加入object，Binary，和XDocument， 支持sql_variant，imager 和xml等的影射。
21                     if (t.IsValueType || t == typeof(string) || t == typeof(System.Byte[])
22                       || t == typeof(object) || t == typeof(System.Xml.Linq.XDocument)
23                       || t == typeof(System.Data.Linq.Binary))
24                     {
25                         //如果不为null才算做条件
26 
27                         if (p.GetValue(obj, null) != null && p.GetValue(obj, null).ToString() != "0001/1/1 0:00:00" && p.GetValue(obj, null).ToString() != "0001-01-01 0:00:00" && p.GetValue(obj, null).ToString() != "0")
28                         {
29                             if (((ColumnAttribute)(p.GetCustomAttributes(typeof(ColumnAttribute), true)[0])).IsPrimaryKey && Convert.ToInt32(p.GetValue(obj, null)) == 0)
30                             {
31 
32                             }
33                             else
34                             {
35                                 ParameterExpression param = Expression.Parameter(typeof(TEntity), "c");
36                                
37                                 if (t != typeof(string))
38                                 {
39                                     Expression right = Expression.Constant(p.GetValue(obj, null));
40                                     Expression left = Expression.Property(param, p.Name);
41                                     Expression filter = Expression.Equal(left, right);
42                                     Expression<Func<TEntity, bool>> pred = Expression.Lambda<Func<TEntity, bool>>(filter, param);
43                                   //  Expression SecondCondition = Expression.Call(
44                                   //Expression.Property(param, p.Name)
45                                   //, p.PropertyType.GetMethod("Equals")
46                                   //, Expression.Constant(p.GetValue(obj, null), typeof(string)));
47                                   //  Expression<Func<TEntity, bool>> pred = Expression.Lambda<Func<TEntity, bool>>(SecondCondition, param);
48                                     query = query.Where(pred);
49                                 }
50                                 else
51                                 {
52                                     Expression SecondCondition = Expression.Call(
53                                     Expression.Property(param, p.Name)
54                                     , typeof(string).GetMethod("Contains")
55                                     , Expression.Constant(p.GetValue(obj, null), typeof(string)));
56                                     Expression<Func<TEntity, bool>> pred = Expression.Lambda<Func<TEntity, bool>>(SecondCondition, param);
57                                     query = query.Where(pred);
58                                 }
59 
60                             }
61                         }
62                     }
63                 }
64             }
65             return query;
66         }