LINQ 联合查询datatable 和xml

Question

            string connstr = "server=.;database=hospital;uid=sa;pwd=123";
            DBHelper.SQLHelper db = new DBHelper.SQLHelper(connstr);
            DataTable dt = db.ExcuteToDataTable("select * from employee");
            DataTable td = db.ExcuteToDataTable("select * from hospital");
            var data = dt.AsEnumerable();
            var tada = td.AsEnumerable();
            var result = from da in data
                         join ta in tada
                         on da.Field<string>("hptid") equals ta.Field<string>("hptid")
                         where da.Field<Int32>("id") < 500
                         select new
                         { 
                           cardID=da.Field<Int32>("cardid"),
                           empName=da.Field<string>("empname"),
                           empSex=da.Field<string>("empsex"),
                           hptAddress=ta.Field<string>("hptname")
                         };
说什么  指定的转换无效

Answer 1

帖出具体的错误信息
单步调试，查看 
var data = dt.AsEnumerable();
var tada = td.AsEnumerable();
是否有数据

Answer 2

var data = dt.AsEnumerable();
var tada = td.AsEnumerable();
这两个都有数据的

Answer 3

result的结果视图里面显示的  "指定的转换无效"

Answer 4

未处理 System.InvalidCastException
  Message="指定的转换无效。"
  Source="System.Data.DataSetExtensions"
  StackTrace:
       在 System.Data.DataRowExtensions.UnboxT`1.ValueField(Object value)
       在 System.Data.DataRowExtensions.Field[T](DataRow row, String columnName)
       在 LinqDemo01.Db.<Excute>b__5(<>f__AnonymousType0`2 <>h__TransparentIdentifier0) 位置 D:\Projects\LINQ\LinqDemo01\Program.cs:行号 34
       在 System.Linq.Enumerable.<SelectIterator>d__d`2.MoveNext()
       在 LinqDemo01.Db.Excute() 位置 D:\Projects\LINQ\LinqDemo01\Program.cs:行号 41
       在 LinqDemo01.Program.Main(String[] args) 位置 D:\Projects\LINQ\LinqDemo01\Program.cs:行号 15
       在 System.AppDomain._nExecuteAssembly(Assembly assembly, String[] args)
       在 System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       在 Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       在 System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       在 System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       在 System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

Answer 5

var result = from da in data
                         join ta in tada
                         on da.Field<string>("hptid") equals ta.Field<string>("hptid")
                         where da.Field<int>("id") < 500
                         select new
                         { 
                           cardID=da.Field<int>("cardid")??0,
                           empName=da.Field<string>("empname")??"",
                           empSex=da.Field<string>("empsex")??"",
                           hptAddress=ta.Field<string>("hptname")??""
                         };

Answer 6

??运算符无法用于int和int类型的操作数。 这什么意思啊

Answer 7

            XDocument xd = XDocument.Load("name.xml");
            XDocument dx = XDocument.Load("price.xml");
            var result = from x in xd.Elements()
                         join d in dx.Elements()
                         on x.Element("pri") equals d.Element("pri")
                         select new
                         {
                             pre = d.Element("pric").Value,
                             name = x.Element("name").Value
                         };result 无结果。

Answer 8

price.xml<?xml version="1.0" encoding="gb2312" ?>
<root>
<price>
<pric>01</pric>
<pri>01</pri>
</price><price>
<pric>02</pric>
<pri>02</pri>
</price><price>
<pric>03</pric>
<pri>03</pri>
</price><price>
<pric>04</pric>
<pri>04</pri>
</price>
</root>
name.xml<?xml version="1.0" encoding="gb2312" ?>
<root>
<price>
<pri>i01</pri>
<name>第一个</name>
</price><price>
<pri>02</pri>
<name>第二个</name>
</price>
<price>
<pri>03</pri>
<name>第三个</name>
</price>
<price>
<pri>04</pri>
<name>第四个</name>
</price></root>

Answer 9

你在result之前加个打印语句，看xd和dx读到文件信息没