使用 var 匿名对象获取 LINQ 的 InvalidCastException
我在尝试使用 foreach 构造中的 var 变量循环查询对象时遇到错误。出于明确的原因，我将 xml 加载到数据集中并执行 Dataset to Linq。请纠正我的错误。这是我的代码：
ds.ReadXml(fsReadXml);
DataTable myDataTable = ds.Tables[0];
var ordersQuery = myDataTable.AsEnumerable();
var subQuery = (from Geo03 in ordersQuery
select new
{
PARENTTYPE = Geo03.Field<String>("PARENTTYPE"),
PARENTINSTANCE = Geo03.Field<int>("PARENTINSTANCE"),
CHILDTYPE = Geo03.Field<String>("CHILDTYPE"),
CHILDINSTANCE = Geo03.Field<String>("CHILDINSTANCE"),
CHILDPOS = Convert.ToInt32(Geo03.Field<int>("CHILDPOS"))
});
XDocument doc = null;
foreach (var cin in subQuery)
{
}
这是生成的错误：
System.InvalidCastException: Specified cast is not valid.
at System.Data.DataRowExtensions.UnboxT`1.ValueField(Object value)
at System.Data.DataRowExtensions.Field[T](DataRow row, String columnName)
at SpectraWorkon.Program.<Main>b__0(DataRow Geo03) in c:\users\arun\documents\visual studio 2010\Projects\SpectraWorkon\SpectraWorkon\Program.cs:line 29
at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
at SpectraWorkon.Program.Main(String[] args) in c:\users\arun\documents\visual studio 2010\Projects\SpectraWorkon\SpectraWorkon\Program.cs:line 38
我收到 foreach (var cin in subQuery) 行错误，说它无法转换。
I am getting error while trying to loop through query object using var variable in foreach construct. I am loading xml in to dataset and performing Dataset to Linq for explicit reasons. Please correct my mistake. This is my code:
ds.ReadXml(fsReadXml);
DataTable myDataTable = ds.Tables[0];
var ordersQuery = myDataTable.AsEnumerable();
var subQuery = (from Geo03 in ordersQuery
select new
{
PARENTTYPE = Geo03.Field<String>("PARENTTYPE"),
PARENTINSTANCE = Geo03.Field<int>("PARENTINSTANCE"),
CHILDTYPE = Geo03.Field<String>("CHILDTYPE"),
CHILDINSTANCE = Geo03.Field<String>("CHILDINSTANCE"),
CHILDPOS = Convert.ToInt32(Geo03.Field<int>("CHILDPOS"))
});
XDocument doc = null;
foreach (var cin in subQuery)
{
}
This is the generated error:
System.InvalidCastException: Specified cast is not valid.
at System.Data.DataRowExtensions.UnboxT`1.ValueField(Object value)
at System.Data.DataRowExtensions.Field[T](DataRow row, String columnName)
at SpectraWorkon.Program.<Main>b__0(DataRow Geo03) in c:\users\arun\documents\visual studio 2010\Projects\SpectraWorkon\SpectraWorkon\Program.cs:line 29
at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
at SpectraWorkon.Program.Main(String[] args) in c:\users\arun\documents\visual studio 2010\Projects\SpectraWorkon\SpectraWorkon\Program.cs:line 38
I am getting error for the line foreach (var cin in subQuery), saying it is unable to cast.
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您正在对非
int字段调用Field。You're calling
Field<int>on a field that isn't anint.请记住，直到您实际使用 linq 查询（例如在 foreach 循环中，或者您调用 .ToList()）时，它才会执行。因此，在您的情况下，例外是在您构建查询的部分。
InvaliCastException表明您正在填写的字段不是您期望的类型。因此，我建议您检查查询中Field的类型。Have in mind that the linq query isn't executed until you actually use it(for example in a foreach loop, or you call .ToList()). So in your case the exception is in the part where you build your query. The
InvaliCastExceptionsuggests that you are filling a field that is not the type that you expect it to be. So I would suggest that you check the types of theField<T>in your query.