c# – 错误：LINQ to Entities无法识别DataLength方法

var db = new MyEntities(GetEntityConnectionString());

ObjectContext objectContext = ((IObjectContextAdapter)db).ObjectContext;
ObjectSet<DOCUMENT> objectSet = objectContext.CreateObjectSet<DOCUMENT>();
var results = objectSet.Where("SqlServer.DATALENGTH(it.BINARYCONTENT)>50000");
Assert.IsTrue(results.ToList().Count == 9);

var results2 = objectSet.Where(doc=>System.Data.Objects.SqlClient.SqlFunctions.DataLength( doc.BINARYCONTENT)>50000);
Assert.IsTrue(results2.ToList().Count == 9);

var results3 = db.DOCUMENTS.Where(doc => System.Data.Objects.SqlClient.SqlFunctions.DataLength(doc.BINARYCONTENT) > 50000);
Assert.IsTrue(results3.ToList().Count == 9);

第一个断言成功,那么为什么在执行results2和result 3时会出现以下异常？

An exception of type ‘System.NotSupportedException’ occurred in EntityFramework.SqlServer.dll but was not handled in user code

Additional information: LINQ to Entities does not recognize the method ‘System.Nullable`1[System.Int32] DataLength(Byte[])’ method,and this method cannot be translated into a store expression.

有没有办法让其他断言成功？