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Avoid duplication in DataTable query and build

What would be the right way to avoid duplication when querying datatable and then saving it to DataTable. I'm using the pattern below, which gets very error-prone once tables grow. I looked at below hints. With first one copyToDataTable() looks not really applicable and second is for me much too complex for the task. I would like to split the below code into 2 separate methods (first to build the query and second to retrieve the DataTable). Perhaps if I avoid the anonymous type in the query this should be easier to avoid hardcoding all the column names - but I'm somehow lost with this.
Filling a DataSet or DataTable from a LINQ query result set
or
https://msdn.microsoft.com/en-us/library/bb669096%28v=vs.110%29.aspx
public DataTable retrieveReadyReadingDataTable()
{
DataTable dtblReadyToSaveToDb = RetrieveDataTableExConstraints();
var query = from scr in scrTable.AsEnumerable()
from products in productsTable.AsEnumerable()
where(scr.Field<string>("EAN") == products.Field<string>("EAN"))
select
new
{
Date = DateTime.Today.Date,
ProductId = products.Field<string>("SkuCode"),
Distributor = scr.Field<string>("Distributor"),
Price = float.Parse(scr.Field<string>("Price")),
Url = scr.Field<string>("Url")
};
foreach (var q in query)
{
DataRow newRow = dtblReadyToSaveToDb.Rows.Add();
newRow.SetField("Date", q.Date);
newRow.SetField("ProductId", q.ProductId);
newRow.SetField("Distributor", q.Distributor);
newRow.SetField("Price", q.Price);
newRow.SetField("Url", q.Url);
}
return dtblReadyToSaveToDb;
}

Firstly, you have to decide what "duplicate" means in your case. According to your code i would say a duplicate is a row with the same value in column Date, ProductId and Distributor. So add a multi column primary key for those columns first.
Secondly, you should add some sort of code that first queries existing rows and then compares these existing rows to the rows you want to create. If a match is found, then simply just don't insert a new row.

LINQ - Convert LINQ select to DataTable

I have a class that does some validation on the data. I want to reuse that class from a different place.
I cant change that class.
It goes something like:
public static List<string> Validate(DataTable CurrentMasterTable, Dictionary<string, string[]> LocalMasterRuleSet)
{...}
In its own it works just fine.
Issue is in the following. From another place in the project, to use that class, I must send it DataTable object, so it can do its magic.
How can I use LINQ, make selection on the table and send it as DataTable?
I have this code to begin with:
var listOfSinglePropertyNames = (from x in dbContext.tbl_mytable.AsEnumerable() select x);
IEnumerable<tbl_mytable> query = listOfSinglePropertyNames;
How do I convert the query to DataTable object? Reading other posts, I have seen I should use CopyToDataTable();
But I am not having any success.

Using LINQ to datatable. Adapt this code for your needings:
// Query the SalesOrderHeader table for orders placed
// after August 8, 2001.
IEnumerable<DataRow> query =
from order in orders.AsEnumerable()
where order.Field<DateTime>("OrderDate") > new DateTime(2001, 8, 1)
select order;
// Create a table from the query.
DataTable boundTable = query.CopyToDataTable<DataRow>();

How to Create DataTable in LINQ

How to create DataTable in LINQ? In SQL Statements i used
DataTable dt;
dt = con.openDataTableQuery("SELECT TransactionID FROM HeaderTransaction");
How about in LINQ? Can you give an example?

Please see the documentation for LINQ to DataSet, examples can be found on the page.
The following is an example emulating your SQL code.
// Add a reference to System.Data.DataSetExtensions
// Query the headerTransaction table to select all items with TransactionID == 1, as an example, use your own value(s)
IEnumerable<DataRow> query =
from headers in headerTransaction.AsEnumerable()
where order.Field<int>("TransactionID") = 1
select order;
// Create a table from the query.
DataTable headersIdQuery = query.CopyToDataTable<DataRow>();
AsEnumerable() returns IEnumerable. As you asked for a DataTable, we convert IEnumerable to a DataTable, using CopyToDataTable().

Querying data from SQL

I need to query a table from database which has 400 rows and 24 columns. I need to query this table so that on each row and then on each column of row I can perform some C# code ( I can use column information to execute some code).
Now at the moment I am querying each row again and again from table using select statement and storing to a custom list and performing custom operations on it.
Is it best and fastest way of doing it ? or should I just query the whole table one time and store somewhere ? not sure where in a dataset and then run throw custom code to do some operation using information in each row ?

You can fetch the table from database once and store it in datatable and then just use linq to select the column something like this
var data = dt.AsEnumerable().Select(s => s.Field<string>("myColumnName")).ToArray<string>();
and If you don't want to use other columns anywhere in your code then you should select only useful column from the database.
You can also select multiple columns of a database using linq. The values will be stored in anonymous type of object.
var mutipleData = from row
in dt.AsEnumerable()
select new
{ Value1 = row["Column1"].ToString(),
Value2 = row["Column2"].ToString()
};

Assuming that each field is 1000 bytes, the total memory to hold your 400 rows would be 9.6MB. Peanuts! Just read the whole table in a DataTable and process it as you wish.

Select all the records from DB table which are f your concern
Copy the select records into DataTable.
Pseudo Code:
--dt is datatable
foreach(datarow dr in dt.rows)
{
--perform operation
string str=dr["columnname"].tostring
}

400 rows isn't a massive amount, however it depends on the data in each column and how often you are likely to run the query. If all you are going to do is run the query and manipulate the output, use a DataReader instead.

If it's only 400 records, I'd fetch them all at once, store them in a class and iterate over each instance.
Something like:
Class
public class MyTableClass(){
string property1 {get; set;}
string property2 {get; set;}
int property3 {get; set;}
// etc.
}
Logic:
ICollection<MyTableClass> lstMyTableClass = (
from m in db.mytableclass
select m
).ToList();
return lstMyTableClass;
And then a loop:
foreach(var myTableInstance in lstMyTableClass){
myTableInstance.DoMyStuff();
}

If the number of records will always be below thoussand, just query all the records and keep it in a List.
Once the data is in the List you can query the List n number of times using LINQ without hitting the database for each request.

What am I doing wrong with this linq query (querying datatable)

I am connecting my Windows Form app to their Access DB (ugh, I know), and cannot get my linq query to return anything.
var matchDateField = from myRow in boilerDT.AsEnumerable()
where myRow.Field<DateTime>("EntryDate").ToShortDateString() == dateTimePicker1.Value.ToShortDateString()
select myRow;
Any suggestions?

Get all the rows:
IEnumerable<DataRow> dateFieldQuery =
from myRow in boilerDT.AsEnumerable()
select myRow;
Filter by date:
IEnumerable<DataRow> matchDateField =
dateFieldQuery.Where(p => p.Field<DateTime>("EntryDate").Date == dateTimePicker1.Value);
So here you're using deferred execution which enables multiple queries to be combined or a query to be extended. When a query is extended, it is modified to include the new operations, and the eventual execution will reflect the changes.
The first query returns all the rows and the second query extends the first by using Where to return all the rows with specific date.