EntityFramework4.5使用Expression类创建动态查询及动态查询导航属性

创建动态查询

想在项目中实现一个灵活的动态查询类，参考http://www.cnblogs.com/lyj/archive/2008/03/25/1122157.html和http://www.cnblogs.com/killuakun/archive/2008/08/03/1259389.html后写了一段Demo，发现代码在VS2012 EF4.5中会抛如下异常：



相同的代码在VS2008 EF3.5中是可以正常运行的：



纠结万分后找到解决方法，代码如下：

[csharp]
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OscarEntities db = new OscarEntities();

IQueryable<City> cities = db.Citys;
ParameterExpression param = Expression.Parameter(typeof(City),
"c");
Expression left = Expression.Property(param, typeof(City).GetProperty("Name"));

Expression right = Expression.Constant("北京市");

Expression filter = Expression.Equal(left, right);
//Expression pred = Expression.Lambda(filter, param);

//Expression expr = Expression.Call(typeof(Queryable), "Where", new Type[] { typeof(City) },

// Expression.Constant(cities), pred);

//var result = db.Citys.AsQueryable().Provider.CreateQuery<City>(expr);

var result = db.Citys.Where(Expression.Lambda<Func<City,
bool>>(filter, param));
list.DataSource = result.ToList();
list.DisplayMember = "Name";

OscarEntities db = new OscarEntities();
IQueryable<City> cities = db.Citys;
ParameterExpression param = Expression.Parameter(typeof(City), "c");
Expression left = Expression.Property(param, typeof(City).GetProperty("Name"));
Expression right = Expression.Constant("北京市");
Expression filter = Expression.Equal(left, right);
//Expression pred = Expression.Lambda(filter, param);
//Expression expr = Expression.Call(typeof(Queryable), "Where", new Type[] { typeof(City) },
//    Expression.Constant(cities), pred);
//var result = db.Citys.AsQueryable().Provider.CreateQuery<City>(expr);
var result = db.Citys.Where(Expression.Lambda<Func<City, bool>>(filter, param));
list.DataSource = result.ToList();
list.DisplayMember = "Name";

动态查询导航属性

实体关系如图：



如何拼接出 db.Citys.Where(x => x.Province.Name == "湖南省") 呢？，代码如下：

[csharp]
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OscarEntities db = new OscarEntities();

IQueryable<City> cities = db.Citys;
ParameterExpression param = Expression.Parameter(typeof(City),
"c");
Expression left = Expression.Property(param,
typeof(City).GetProperty("Province"));
//先得到导航属性Province
Expression leftproperty = Expression.Property(left,
"Name"); //再得到Province.Name

Expression right = Expression.Constant("湖南省");

Expression filter = Expression.Equal(leftproperty, right);

var result = db.Citys.Where(Expression.Lambda<Func<City,
bool>>(filter, param));
list.DataSource = result.ToList();
list.DisplayMember = "Name";

网上的中文资料很少，我对着Expression.Property方法琢磨了老半天才整明白，写在这方便大家吧。

执行结果：

OscarEntities db = new OscarEntities();
IQueryable<City> cities = db.Citys;
ParameterExpression param = Expression.Parameter(typeof(City), "c");
Expression left = Expression.Property(param, typeof(City).GetProperty("Province")); //先得到导航属性Province
Expression leftproperty = Expression.Property(left, "Name"); //再得到Province.Name
Expression right = Expression.Constant("湖南省");
Expression filter = Expression.Equal(leftproperty, right);
var result = db.Citys.Where(Expression.Lambda<Func<City, bool>>(filter, param));
list.DataSource = result.ToList();
list.DisplayMember = "Name";

网上的中文资料很少，我对着Expression.Property方法琢磨了老半天才整明白，写在这方便大家吧。

执行结果：

再贴上自己项目中用的方法

[csharp]
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public Expression GetProperty(Expression source, ParameterExpression para,
string Name)
{
string[] propertys = Name.Split('.');

if (source == null)

{
source = Expression.Property(para, typeof(City).GetProperty(propertys.First()));

}
else source = Expression.Property(source, propertys.First());

foreach (var item
in propertys.Skip(1))
{
source = GetProperty(source , para, item);

}
return source;

}

public Expression GetProperty(Expression source, ParameterExpression para, string Name)
{
string[] propertys = Name.Split('.');
if (source == null)
{
source = Expression.Property(para, typeof(City).GetProperty(propertys.First()));
}
else source = Expression.Property(source, propertys.First());
foreach (var item in propertys.Skip(1))
{
source = GetProperty(source , para, item);
}
return source;
}

使用方法：

[csharp]
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ParameterExpression param = Expression.Parameter(typeof(City),
"x");
Expression left = GetProperty(null, param,
"Province.Name"); //得到查询条件属性

Expression right = Expression.Constant("湖南省");

Expression filter = Expression.Equal(left,right);

ParameterExpression param = Expression.Parameter(typeof(City), "x");
Expression left = GetProperty(null, param, "Province.Name"); //得到查询条件属性
Expression right = Expression.Constant("湖南省");
Expression filter = Expression.Equal(left,right);