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Select distinct DataTable rows

I have a DataTable where sometimes values in all columns in two or more rows repeat. I would like to get distinct DataTable. The solutions from here and here don't work for me because I have many columns and depending on some conditions, the number of columns changes.
I was thinking maybe something like this
System.Data.DataTable table = new System.Data.DataTable(); // already fulfilled table
DataView view = new DataView(table);
var tableDistinct = view.ToTable(true, table.Columns);
But I can't pass table.Columns as an argument.

I don't know what's going wrong because you haven't said what's not working. However, you could use LINQ(-TO-DataTable):
table = table.AsEnumerable()
.GroupBy(r => new{ Col1 = r["Col1"], Col2 = r["Col2"], Col3 = r["Col3"] })
.Select(g => g.First())
.CopyToDataTable();
Change the columns in the anonymous type according to your column-list.

The ToTable access a list of string params, the following should convert all your columns to array of string so you don't have to enter them manually
System.Data.DataTable table = new System.Data.DataTable(); // already fulfilled table
DataView view = new DataView(table);
var tableDistinct = view.ToTable(true, table.Columns.Cast<DataColumn>().Select(z=>z.ColumnName).ToArray());

Specify Which Columns Are In A Datatable

I have a stored procedure that returns extra columns. I don't have control of the stored procedure. I would like to use the following form to generate my worksheet:
ws.Cells.LoadFromDataTable(dt, true, OfficeOpenXml.Table.TableStyles.Light8)
How can I output just the columns I want?
I tried to figure out a Linq query but the problem is that the column names are inaccessible so I can't specify which columns I would like.
SqlConnection cx = new SqlConnection(util.GetConnectionString());
SqlCommand cmd = new SqlCommand("StoredRept", cx);
cmd.CommandType = CommandType.StoredProcedure;
SqlDataAdapter ta = new SqlDataAdapter();
ta.SelectCommand = cmd;
DataTable dt = new DataTable();
ta.Fill(dt);
FileInfo newFile = new FileInfo("c:\temp");
ExcelPackage epp = new ExcelPackage(newFile);
var ws = epp.Workbook.Worksheets.Add("WS");
// here is where I would like to copy certain columns off into another data table but I haven't been able to figure out how
ws.Cells.LoadFromDataTable(dt, true, OfficeOpenXml.Table.TableStyles.Light8);
Any help would be greatly appreciated.

If removing some columns from the table is the only problem, that is easy to resolve. Try something like this:
var dt = new DataTable();
dt.Columns.Add("First", typeof(string));
dt.Columns.Add("Second", typeof(string));
dt.Columns.Add("Third", typeof(string));
dt.Columns.Add("Fourth", typeof(string));
dt.Columns.Add("Fifth", typeof(string));
for (var i = 1; i < 6; i++)
{
dt.Rows.Add($"First {i}", $"Second {i}", $"Third {i}",$"Fourth {i}",$"Fifth {i}");
}
//dt.Dump();//If you have linqpad this is handy to dump table to output
//REMOVE THE COLUMNS.
dt.Columns.RemoveAt(1);
dt.Columns.RemoveAt(2);
//dt.Dump();//again in linqpad this dumps table with remaining 3 columns
You can use following method to find column by name and remove it:
var col=dt.Columns["Second"];
dt.Columns.Remove(col);
Here is linq query to get list with desired columns.
var lq = (from DataRow r in dt.Rows
select new { First = r[0], Second=r[1], Third=r["Fifth"]}
).ToList();
lq.Dump();
Note how you can use both column index or name to get value from row object.

Can you load your spreadsheet with IEnumerable<T> instead of DataTable? If so:
var someColumns = dt
.AsEnumerable()
.Select(row => new
{
When = row.Field<DateTime>("When"),
What = row.Field<string>("What"),
HowMany = row.Field<int>("ColumnNameInDatabase"),
});
If you need DataTable, you can convert IEnumerable to it via any method, like this or this.
BUT
Frankly, I answered your problem, but IMO you've a greater problem at hand, which is using those antiquated DataTable objects instead of IEnumerable<T>. If you consider, for example, Dapper library, you could do this:
util.GetConnectionString()
.Query("StoredRept", commandType: CommandType.StoredProcedure) // Extension method from Dapper
.Select(dyn => new
{
When = (DateTime)dyn.When,
What = (string)dyn.What,
HowMany = (int)dyn.ColumnNameInDatabase
})
I see you're using EPPlus library for Excel manipulation. It can load IEnumerable<T> data, you are not restricted to DataTable.

Linq on DataTable: select specific column into datatable, not whole table

I'm running a LINQ query on a datatable in c#.
I want to select specific columns rather than the whole row and enter the result into a datatable. How can i do that??
My Code:
public DataTable getConversions(string c_to, string p_to)
{
var query = from r in matrix.AsEnumerable()
where r.Field<string>("c_to") == c_to &&
r.Field<string>("p_to") == p_to
select r;
DataTable conversions = query.CopyToDataTable();

If you already know beforehand how many columns your new DataTable would have, you can do something like this:
DataTable matrix = ... // get matrix values from db
DataTable newDataTable = new DataTable();
newDataTable.Columns.Add("c_to", typeof(string));
newDataTable.Columns.Add("p_to", typeof(string));
var query = from r in matrix.AsEnumerable()
where r.Field<string>("c_to") == "foo" &&
r.Field<string>("p_to") == "bar"
let objectArray = new object[]
{
r.Field<string>("c_to"), r.Field<string>("p_to")
}
select objectArray;
foreach (var array in query)
{
newDataTable.Rows.Add(array);
}

Try Access DataTable easiest way which can help you for getting perfect idea for accessing DataTable, DataSet using Linq...
Consider following example, suppose we have DataTable like below.
DataTable ObjDt = new DataTable("List");
ObjDt.Columns.Add("WorkName", typeof(string));
ObjDt.Columns.Add("Price", typeof(decimal));
ObjDt.Columns.Add("Area", typeof(string));
ObjDt.Columns.Add("Quantity",typeof(int));
ObjDt.Columns.Add("Breath",typeof(decimal));
ObjDt.Columns.Add("Length",typeof(decimal));
Here above is the code for DatTable, here we assume that there are some data are available in this DataTable, and we have to bind Grid view of particular by processing some data as shown below.
Area | Quantity | Breath | Length | Price = Quantity * breath *Length
Than we have to fire following query which will give us exact result as we want.
var data = ObjDt.AsEnumerable().Select
(r => new
{
Area = r.Field<string>("Area"),
Que = r.Field<int>("Quantity"),
Breath = r.Field<decimal>("Breath"),
Length = r.Field<decimal>("Length"),
totLen = r.Field<int>("Quantity") * (r.Field<decimal>("Breath") * r.Field<decimal>("Length"))
}).ToList();
We just have to assign this data variable as Data Source.
By using this simple Linq query we can get all our accepts, and also we can perform all other LINQ queries with this…

Here I get only three specific columns from mainDataTable and use the filter
DataTable checkedParams = mainDataTable.Select("checked = true").CopyToDataTable()
.DefaultView.ToTable(false, "lagerID", "reservePeriod", "discount");

LINQ is very effective and easy to use on Lists rather than DataTable. I can see the above answers have a loop(for, foreach), which I will not prefer.
So the best thing to select a perticular column from a DataTable is just use a DataView to filter the column and use it as you want.
Find it here how to do this.
DataView dtView = new DataView(dtYourDataTable);
DataTable dtTableWithOneColumn= dtView .ToTable(true, "ColumnA");
Now the DataTable dtTableWithOneColumn contains only one column(ColumnA).

Your select statement is returning a sequence of anonymous type , not a sequence of DataRows. CopyToDataTable() is only available on IEnumerable<T> where T is or derives from DataRow. You can select r the row object to call CopyToDataTable on it.
var query = from r in matrix.AsEnumerable()
where r.Field<string>("c_to") == c_to &&
r.Field<string>("p_to") == p_to
select r;
DataTable conversions = query.CopyToDataTable();
You can also implement CopyToDataTable Where the Generic Type T Is Not a DataRow.

How to pass DataTable.Select() result to a new DataTable?

I have a DataTable named dt2 with data. I am calling its Select method to get some specific rows.
DataRow[] foundRows;
expression = "parent_id=1";
foundRows = dt2.Select(expression);
How can I pass the Select-method result to a new DataTable – say FilteredData?

You can use CopyToDataTable, available on IEnumerable<DataRow> types.
var filteredData = dt2.Select(expression).CopyToDataTable();

Just for clarity, the Select method returns an array of type DataRow. That's why we need to use CopyToDataTable(). Alex's answer is good. However, if the Select didn't return any rows, CopyToDataTable() will throw an InvalidOperationException.
So test that there is at least one DataRow before using the CopyToDataTable().
var filteredDataRows = dt2.Select(expression);
var filteredDataTable = new DataTable();
if(filteredDataRows.Length != 0)
filteredDataTable = filteredDataRows.CopyToDataTable();

Why not use a DataView instead?
DataView view = new DataView(dt2);
view.RowFilter = "parent_id = 1";
DataView will behave in very much the same way that a DataTable would with the added benefit that any change(s) to the underlying DataTable (dt2 in this case) would be automatically reflected in the DataView.

LINQ query on a DataTable

I'm trying to perform a LINQ query on a DataTable object and bizarrely I am finding that performing such queries on DataTables is not straightforward. For example:
var results = from myRow in myDataTable
where results.Field("RowNo") == 1
select results;
This is not allowed. How do I get something like this working?
I'm amazed that LINQ queries are not allowed on DataTables!

You can't query against the DataTable's Rows collection, since DataRowCollection doesn't implement IEnumerable<T>. You need to use the AsEnumerable() extension for DataTable. Like so:
var results = from myRow in myDataTable.AsEnumerable()
where myRow.Field<int>("RowNo") == 1
select myRow;
And as #Keith says, you'll need to add a reference to System.Data.DataSetExtensions
AsEnumerable() returns IEnumerable<DataRow>. If you need to convert IEnumerable<DataRow> to a DataTable, use the CopyToDataTable() extension.
Below is query with Lambda Expression,
var result = myDataTable
.AsEnumerable()
.Where(myRow => myRow.Field<int>("RowNo") == 1);

var results = from DataRow myRow in myDataTable.Rows
where (int)myRow["RowNo"] == 1
select myRow

It's not that they were deliberately not allowed on DataTables, it's just that DataTables pre-date the IQueryable and generic IEnumerable constructs on which Linq queries can be performed.
Both interfaces require some sort type-safety validation. DataTables are not strongly typed. This is the same reason why people can't query against an ArrayList, for example.
For Linq to work you need to map your results against type-safe objects and query against that instead.

As #ch00k said:
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
You also need to add a project reference to System.Data.DataSetExtensions

I realize this has been answered a few times over, but just to offer another approach:
I like to use the .Cast<T>() method, it helps me maintain sanity in seeing the explicit type defined and deep down I think .AsEnumerable() calls it anyways:
var results = from myRow in myDataTable.Rows.Cast<DataRow>()
where myRow.Field<int>("RowNo") == 1 select myRow;
or
var results = myDataTable.Rows.Cast<DataRow>()
.FirstOrDefault(x => x.Field<int>("RowNo") == 1);
As noted in comments, does not require System.Data.DataSetExtensions or any other assemblies (Reference)

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")
};
the name and age fields are now part of the query object and can be accessed like so: Console.WriteLine(query.name);

Using LINQ to manipulate data in DataSet/DataTable
var results = from myRow in tblCurrentStock.AsEnumerable()
where myRow.Field<string>("item_name").ToUpper().StartsWith(tbSearchItem.Text.ToUpper())
select myRow;
DataView view = results.AsDataView();

//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();
//

Try this simple line of query:
var result=myDataTable.AsEnumerable().Where(myRow => myRow.Field<int>("RowNo") == 1);

You can use LINQ to objects on the Rows collection, like so:
var results = from myRow in myDataTable.Rows where myRow.Field("RowNo") == 1 select myRow;

This is a simple way that works for me and uses lambda expressions:
var results = myDataTable.Select("").FirstOrDefault(x => (int)x["RowNo"] == 1)
Then if you want a particular value:
if(results != null)
var foo = results["ColName"].ToString()

Try this
var row = (from result in dt.AsEnumerable().OrderBy( result => Guid.NewGuid()) select result).Take(3) ;

Most likely, the classes for the DataSet, DataTable and DataRow are already defined in the solution. If that's the case you won't need the DataSetExtensions reference.
Ex. DataSet class name-> CustomSet, DataRow class name-> CustomTableRow (with defined columns: RowNo, ...)
var result = from myRow in myDataTable.Rows.OfType<CustomSet.CustomTableRow>()
where myRow.RowNo == 1
select myRow;
Or (as I prefer)
var result = myDataTable.Rows.OfType<CustomSet.CustomTableRow>().Where(myRow => myRow.RowNo);

var results = from myRow in myDataTable
where results.Field<Int32>("RowNo") == 1
select results;

In my application I found that using LINQ to Datasets with the AsEnumerable() extension for DataTable as suggested in the answer was extremely slow. If you're interested in optimizing for speed, use James Newtonking's Json.Net library (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);

Example on how to achieve this provided below:
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();

For VB.NET The code will look like this:
Dim results = From myRow In myDataTable
Where myRow.Field(Of Int32)("RowNo") = 1 Select myRow

IEnumerable<string> result = from myRow in dataTableResult.AsEnumerable()
select myRow["server"].ToString() ;

Try this...
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 );
}

You can get it work elegant via linq like this:
from prod in TenMostExpensiveProducts().Tables[0].AsEnumerable()
where prod.Field<decimal>("UnitPrice") > 62.500M
select prod
Or like dynamic linq this (AsDynamic is called directly on DataSet):
TenMostExpensiveProducts().AsDynamic().Where (x => x.UnitPrice > 62.500M)
I prefer the last approach while is is the most flexible.
P.S.: Don't forget to connect System.Data.DataSetExtensions.dll reference

you can try this, but you must be sure the type of values for each Column
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"),
});

I propose following solution:
DataView view = new DataView(myDataTable);
view.RowFilter = "RowNo = 1";
DataTable results = view.ToTable(true);
Looking at the DataView Documentation, the first thing we can see is this:
Represents a databindable, customized view of a DataTable for sorting, filtering, searching, editing, and navigation.
What I am getting from this is that DataTable is meant to only store data and DataView is there enable us to "query" against the DataTable.
Here is how this works in this particular case:
You try to implement the SQL Statement
SELECT *
FROM myDataTable
WHERE RowNo = 1
in "DataTable language". In C# we would read it like this:
FROM myDataTable
WHERE RowNo = 1
SELECT *
which looks in C# like this:
DataView view = new DataView(myDataTable); //FROM myDataTable
view.RowFilter = "RowNo = 1"; //WHERE RowNo = 1
DataTable results = view.ToTable(true); //SELECT *