Cal*_*nus 997 .net c# linq datatable .net-3.5
我正在尝试对DataTable对象执行LINQ查询,奇怪的是我发现在DataTables上执行此类查询并不简单.例如:
var results = from myRow in myDataTable
where results.Field("RowNo") == 1
select results;
这是不允许的.我如何得到这样的工作？
我很惊讶DataTables上不允许LINQ查询!
Col*_*n K 1250
你不能查询DataTable's Rows集合,因为DataRowCollection没有实现IEnumerable<T>.您需要使用AsEnumerable()扩展名DataTable.像这样:
var results = from myRow in myDataTable.AsEnumerable()
where myRow.Field<int>("RowNo") == 1
select myRow;
正如Keith所说,您需要添加对System.Data.DataSetExtensions的引用
AsEnumerable()回报IEnumerable<DataRow>.如果您需要转换IEnumerable<DataRow>为a DataTable,请使用CopyToDataTable()扩展名.
下面是Lambda Expression的查询,
var result = myDataTable
.AsEnumerable()
.Where(myRow => myRow.Field<int>("RowNo") == 1);
Joe*_*Fan 127
var results = from DataRow myRow in myDataTable.Rows
where (int)myRow["RowNo"] == 1
select myRow
Jon*_*jap 68
并不是故意不允许在DataTables上使用它们,只是DataTables会在可以执行Linq查询的IQueryable和通用IEnumerable构造之前进行预定.
两个接口都需要某种类型安全验证.DataTables不是强类型的.例如,这与人们无法查询ArrayList的原因相同.
要使Linq工作,您需要将结果映射到类型安全的对象,并对其进行查询.
Kei*_*ith 48
正如@ ch00k所说:
using System.Data; //needed for the extension methods to work
...
var results =
from myRow in myDataTable.Rows
where myRow.Field<int>("RowNo") == 1
select myRow; //select the thing you want, not the collection
您还需要添加项目引用 System.Data.DataSetExtensions
小智 37
var query = from p in dt.AsEnumerable()
where p.Field<string>("code") == this.txtCat.Text
select new
{
name = p.Field<string>("name"),
age= p.Field<int>("age")
};
van*_*dsh 33
我意识到这已经回答了几次,但只是为了提供另一种方法,我喜欢使用这种.Cast<T>()方法,它帮助我保持理智,看到定义的显式类型,并且.AsEnumerable()内心深处我认为无论如何都要调用它:
var results = from myRow in myDataTable.Rows.Cast<DataRow>()
where myRow.Field<int>("RowNo") == 1 select myRow;
要么
var results = myDataTable.Rows.Cast<DataRow>()
.FirstOrDefault(x => x.Field<int>("RowNo") == 1);
小智 28
使用LINQ来操作DataSet/DataTable中的数据
var results = from myRow in tblCurrentStock.AsEnumerable()
where myRow.Field<string>("item_name").ToUpper().StartsWith(tbSearchItem.Text.ToUpper())
select myRow;
DataView view = results.AsDataView();
小智 28
//Create DataTable
DataTable dt= new DataTable();
dt.Columns.AddRange(New DataColumn[]
{
new DataColumn("ID",typeOf(System.Int32)),
new DataColumn("Name",typeOf(System.String))
});
//Fill with data
dt.Rows.Add(new Object[]{1,"Test1"});
dt.Rows.Add(new Object[]{2,"Test2"});
//Now Query DataTable with linq
//To work with linq it should required our source implement IEnumerable interface.
//But DataTable not Implement IEnumerable interface
//So we call DataTable Extension method i.e AsEnumerable() this will return EnumerableRowCollection<DataRow>
// Now Query DataTable to find Row whoes ID=1
DataRow drow = dt.AsEnumerable().Where(p=>p.Field<Int32>(0)==1).FirstOrDefault();
//
Moh*_*rma 21
试试这个简单的查询行:
var result=myDataTable.AsEnumerable().Where(myRow => myRow.Field<int>("RowNo") == 1);
Dav*_*ier 15
您可以在Rows集合上使用LINQ到对象,如下所示:
var results = from myRow in myDataTable.Rows where myRow.Field("RowNo") == 1 select myRow;
Mat*_*emp 12
这是一个适合我并使用lambda表达式的简单方法:
var results = myDataTable.Select("").FirstOrDefault(x => (int)x["RowNo"] == 1)
然后,如果你想要一个特定的价值:
if(results != null)
var foo = results["ColName"].ToString()
小智 11
试试这个
var row = (from result in dt.AsEnumerable().OrderBy( result => Guid.NewGuid()) select result).Take(3) ;
xad*_*iel 11
最有可能的是,DataSet,DataTable和DataRow的类已在解决方案中定义.如果是这种情况,您将不需要DataSetExtensions参考.
防爆.DataSet类名 - > CustomSet,DataRow类名 - > CustomTableRow(具有已定义的列:RowNo,...)
var result = from myRow in myDataTable.Rows.OfType<CustomSet.CustomTableRow>()
where myRow.RowNo == 1
select myRow;
或者(我更喜欢)
var result = myDataTable.Rows.OfType<CustomSet.CustomTableRow>().Where(myRow => myRow.RowNo);
小智 9
var results = from myRow in myDataTable
where results.Field<Int32>("RowNo") == 1
select results;
在我的应用程序中,我发现在答案中建议使用LINQ to Datasets和DataTable的AsEnumerable()扩展非常慢.如果您对优化速度感兴趣,请使用James Newtonking的Json.Net库(http://james.newtonking.com/json/help/index.html)
// Serialize the DataTable to a json string
string serializedTable = JsonConvert.SerializeObject(myDataTable);
Jarray dataRows = Jarray.Parse(serializedTable);
// Run the LINQ query
List<JToken> results = (from row in dataRows
where (int) row["ans_key"] == 42
select row).ToList();
// If you need the results to be in a DataTable
string jsonResults = JsonConvert.SerializeObject(results);
DataTable resultsTable = JsonConvert.DeserializeObject<DataTable>(jsonResults);
关于如何实现此目的的示例如下:
DataSet dataSet = new DataSet(); //Create a dataset
dataSet = _DataEntryDataLayer.ReadResults(); //Call to the dataLayer to return the data
//LINQ query on a DataTable
var dataList = dataSet.Tables["DataTable"]
.AsEnumerable()
.Select(i => new
{
ID = i["ID"],
Name = i["Name"]
}).ToList();
对于VB.NET代码如下所示:
Dim results = From myRow In myDataTable
Where myRow.Field(Of Int32)("RowNo") = 1 Select myRow
试试这个...
SqlCommand cmd = new SqlCommand( "Select * from Employee",con);
SqlDataReader dr = cmd.ExecuteReader( );
DataTable dt = new DataTable( "Employee" );
dt.Load( dr );
var Data = dt.AsEnumerable( );
var names = from emp in Data select emp.Field<String>( dt.Columns[1] );
foreach( var name in names )
{
Console.WriteLine( name );
}
IEnumerable<string> result = from myRow in dataTableResult.AsEnumerable()
select myRow["server"].ToString() ;
你可以通过这样的linq让它变得优雅:
from prod in TenMostExpensiveProducts().Tables[0].AsEnumerable()
where prod.Field<decimal>("UnitPrice") > 62.500M
select prod
或者像动态linq这样(AsDynamic直接在DataSet上调用):
TenMostExpensiveProducts().AsDynamic().Where (x => x.UnitPrice > 62.500M)
我更喜欢最后一种方法,而且最灵活.PS:别忘了连接System.Data.DataSetExtensions.dll参考
您可以尝试这样做，但是必须确保每个列的值类型
List<MyClass> result = myDataTable.AsEnumerable().Select(x=> new MyClass(){
Property1 = (string)x.Field<string>("ColumnName1"),
Property2 = (int)x.Field<int>("ColumnName2"),
Property3 = (bool)x.Field<bool>("ColumnName3"),
});