創建動態查詢

想在項目中實現一個靈活的動態查詢類，參考http://www.cnblogs.com/lyj/archive/2008/03/25/1122157.html和http://www.cnblogs.com/killuakun/archive/2008/08/03/1259389.html后寫了一段Demo，發現代碼在VS2012 EF4.5中會拋如下異常：

相同的代碼在VS2008 EF3.5中是可以正常運行的：

糾結萬分后找到解決方法，代碼如下：

[csharp] view plaincopyprint?

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";

動態查詢導航屬性

實體關系如圖：

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

[csharp] view plaincopyprint?

??????????? 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")); //先得到導航屬性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";?

?

網上的中文資料很少，我對著Expression.Property方法琢磨了老半天才整明白，寫在這方便大家吧。?

?

執行結果：?

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")); //先得到導航屬性ProvinceExpression leftproperty = Expression.Property(left, "Name"); //再得到Province.NameExpression 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";網上的中文資料很少，我對著Expression.Property方法琢磨了老半天才整明白，寫在這方便大家吧。執行結果：

再貼上自己項目中用的方法

[csharp] view plaincopyprint?

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] view plaincopyprint?

ParameterExpression param = Expression.Parameter(typeof(City), "x");?

Expression left = GetProperty(null, param, "Province.Name"); //得到查詢條件屬性?

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"); //得到查詢條件屬性Expression right = Expression.Constant("湖南省");Expression filter = Expression.Equal(left,right);

轉載于:https://www.cnblogs.com/swarb/archive/2012/10/22/9924379.html

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