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Hi, there. So you have a listbox with, for example, country names, and you want to display in the datagridview a list only of the cities of the country selected in the listbox. You actually don't need a whole new query for this. This is called a master-detail relationship (so you can google for that), or more precisely, a one-to-many relationship.
In the designer, you can set up a relation between the two tables. Then you use the table adapter to load the tables as usual. But you set the data source of the datagridview to the same binding source of the listbox, with the data member set to the relation. So, the master binding source points to the data table in the data set. Then the detail binding source points to the relation in the master binding source.
On the form, once you set the bindings this way, it's all automatic. When the selected item in the list box changes, the datagridview automatically updates.
A different method is this. Set the master binding source to its master table, and the detail binding source to its detail table. Then, when the selected value in the listbox changes, intercept the event and set a filter on the detail binding source (which is bound to the datagridview).
The first method is mostly done in designer, and the second is done mostly in code. You could create a query for this purpose, but these two methods would generally be more efficient. I hope this helps!
modified 7-Jan-22 18:08pm.
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Hello RobertSF,
that was the asnwer I was looking for thanx!
Will any changes in the gridview be adapted in the database too this way?
edit : I spoke too early  when I change the selection in my listbox the application hangs
modified 12-Jan-22 5:53am.
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If the grid is edited, yes, you can propagate the changes to the database. It won't happen automatically. You'll first have to call EndEdit on the binding source and possibly first on the grid, and then use either a table adapter update method or the dataset update method.
It's pretty rare for applications to hang, in my experience. They more often crash with exceptions, though they can also get stuck in infinite loops. Try working with the debugger and going line by line.
Or google for "master detail two datagridview" and try it with two grids instead of listbox and a grid. I don't think I can post link, but the full title of the Microsoft document is "Walkthrough: Creating a Master/Detail Form Using Two Windows Forms DataGridView Controls"
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Maybe this will help. I created an Access database with two tables.
Table: Countries
-string: CountryName
Table: Cities
-string: CountryName
-string: CityName
No relationships created between the two tables. Entered three countries (USA, Canada, Mexico), and three cities in each country.
In Visual Studio, created a new dataset with the two tables from the database. Here, I did create a relationship, linking the Cities table to the Countries table by the CountryName column. The relationship, which exists only in Visual Studio, is called Countries_Cities.
Then I created a form and dropped a ListBox into the form. Setting the DataSource to the Countries table (you must drill down in Designer) automatically creates the countriesBindingSource. Then I set the DisplayMember to CountryName. Here are the binding setting for the ListBox.
this.listBox1.DataSource = this.countriesBindingSource;
this.listBox1.DisplayMember = "CountryName";
Then I dropped a DataGridView into the form. When I clicked the button to select the Data Source, my choices were countriesBindingSource and Other Data Sources. Drill down into countriesBindingSource and select the Countries_Cities relationship. This will automatically create the countriesCitiesBindingSource and will auto-populate the DataGridView with two columns, CountryName and CityName. You can delete the CountryName column. Here are the binding values for the DataGridView.
this.dataGridView1.DataSource = this.countriesCitiesBindingSource;
This is all designer code, so you need not code anything so far. Now run the application. The form should appear, showing the countries in the List Box, but only the cities for the selected country in the DataGridView.
Click on the other countries in the ListBox, and the DataGridView should automatically show only the cities for the selected country. I hope this helps. This stuff can be pretty crazy-making because there are few practical examples to follow.
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This is a program I have been tinkering with for my daughter to help her budget her finances. I understand that there are programs out there, but I wanted something to do and decided to play around with do this.
I am working on the income section, which has income coming in either weekly, semi monthly, bimonthly, monthly or yearly. What I am showing is the weekly section and I am curious as to whether there is a better method, which there usually is to what I am attempting to accomplish.
The "ds.Tables[0].Rows[i][0].ToString()" brings in the test verbiage for whether its weekly, monthly, whatever.
The "f" variable in the for loop is a numerical response provided by the user as to how far out they which to forecast their budget. 4 weeks, 6 weeks, 8 weeks and so forth.
If there is more detail needed, please let me know.
if (ds.Tables[0].Rows[i][0].ToString() == "Weekly (52)")
{
var date = new DateTime(DateTime.Now.Year, DateTime.Now.Month, 1);
for (int x = 1; x < 7; x++)
{
if (ds.Tables[0].Rows[i][1].ToString() != date.ToString("ddd"))
{
DateTime newdate = date.AddDays(1);
date = newdate;
}
else
{
for (int n = 0; n <= ((4*f)-1); n++)
{
list.Add(date);
DateTime newdate = date.AddDays(7);
date = newdate;
}
break;
}
}
}
Thank you,
Richard
Disable Vet
Grandfather
Pain in the @ss
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You refer to your daughter as "the user"? Unless she's a CEO, she could probably get by with Excel; they even have "budget templates".
"Before entering on an understanding, I have meditated for a long time, and have foreseen what might happen. It is not genius which reveals to me suddenly, secretly, what I have to say or to do in a circumstance unexpected by other people; it is reflection, it is meditation." - Napoleon I
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I'd start with something like this:
private static DateTime NextWeekDay(DateTime date, string expectedDayOfWeek, DateTimeFormatInfo dateFormat)
{
int expectedDayIndex = Array.IndexOf(dateFormat.AbbreviatedDayNames, expectedDayOfWeek);
if (expectedDayIndex != -1)
{
int dayOfWeek = (int)date.DayOfWeek;
if (dayOfWeek != expectedDayIndex)
{
int daysToAdd = expectedDayIndex - dayOfWeek;
if (daysToAdd < 0) daysToAdd += 7;
date = date.AddDays(daysToAdd);
}
}
return date;
}
DateTime today = DateTime.Today;
DataTable dt = ds.Tables[0];
DateTimeFormatInfo dateFormat = DateTimeFormatInfo.GetInstance(CultureInfo.CurrentCulture);
foreach (DataRow row in dt.Rows)
{
string budgetType = row[0].ToString();
switch (budgetType)
{
...
case "Weekly (52)":
{
DateTime date = today.AddDays(1 - today.Day);
string expectedDayOfWeek = row[1].ToString();
date = NextWeekDay(date, expectedDayOfWeek, dateFormat);
int max = (4 * f);
for (int n = 0; n < max; n++)
{
list.Add(date);
date = date.AddDays(7);
}
break;
}
...
}
}Further optimisation may be possible depending on what the rest of the code looks like.
"These people looked deep within my soul and assigned me a number based on the order in which I joined."
- Homer
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If the code works and it is clear to you, then don't go changing it just for the sake of changing it. I often hear people talking about "improving" code, but they are vague on what improvement actually means in this case. Sure, there are some minor optimisations that jump out but whether or not they are worth putting in is debatable.
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Hello Guys,
I have a CSV file which looks something like this :
Entry Price, Quantity, SL Level, TP Level, Side
nothing, nothing, nothing, nothing, nothing
As you can see, using the code below, I try to make sure if I have the file already, otherwise I create one with the same content above :
if (!File.Exists(path))
{
sw = File.AppendText(path);
if (writeHeaders) {
sw.WriteLine("Entry Price, " + "Quantity, " + "SL Level, " + "TP Level, " + "Side");
writeHeaders = false;
}
sw.WriteLine("nothing" + "," + "nothing" + "," + "nothing" + "," + "nothing" + "," + "nothing");
sw.Close();
}
but the if a position is taking place, I want to replace "nothing" with real values, depends on each column :
if (File.Exists(path) && PositionAccount.Quantity > 0)
{
List<string> newLines = new List<string>();
string[] existingLines = File.ReadAllLines(path, encode);
foreach (string line in existingLines)
{
string[] columns = line.Split(new char[] {','});
string EntryPrice = columns[0];
string Qtty = columns[1];
string SSLevel = columns[2];
string TTPLevel = columns[3];
string PSide = columns[4];
EntryPrice = EntryPrice.Replace("nothing", PositionAccount.AveragePrice.ToString());
Qtty = Qtty.Replace("nothing", QtyMini.ToString());
SSLevel = SSLevel.Replace("nothing", LongSL.ToString());
TTPLevel = TTPLevel.Replace("nothing", LongTP.ToString());
PSide = PSide.Replace("nothing", "Long");
columns[0] = EntryPrice;
columns[1] = Qtty;
columns[2] = SSLevel;
columns[3] = TTPLevel;
columns[4] = PSide;
string newLine = string.Join(",", columns);
newLines.Add(newLine);
}
File.WriteAllLines(path, newLines);
}
both above codes are working perfectly, and doing what they are supposed to do, EXCEPT THAT, when I try to READ the csv file from another strategy, it shows an Errors Message of "Error Msg : the process cannot access the file used by another process" and it disables everything.
I really tried all possible ways that I came across in all forums to fix this problem, but I was not able to do it, I think that the file is not closed from the first opening this is why it shows this message, so I tried
using () , I tried FileSharing, File Access, I tried FileStream, I tried StreamReader, but in some of them I was not able to readalllines and writealllines, as well I was not able to read only line[1] for example, and modify it and replace it.
using (mystream = File.Open(path, FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite))
{
}
Can you plz tell me how to fix it ? Thank you
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You're doing a "read all", then a "write all", both to the same "path".
"Simple" requires that you "write" to a different file than the one you're reading.
Other scenarios are more advanced.
"Before entering on an understanding, I have meditated for a long time, and have foreseen what might happen. It is not genius which reveals to me suddenly, secretly, what I have to say or to do in a circumstance unexpected by other people; it is reflection, it is meditation." - Napoleon I
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Thanks for bring this up; I wasn't aware they existed.
That's interesting: an immutable class with a value comparison.
That's basically a value type that can be inherited from, which is a big limitation - there are times when it would have been nice to inherit from int to add specific functionality without having to encapsulate it and manually build the comparison operators (since classes always default to a reference equality) - a record does a property-by-property value comparison for you, and is by default immutable. Nice.
Not sure what I'll use them for yet, but they are well worth remembering for future projects.
Good question! 
"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony
"Common sense is so rare these days, it should be classified as a super power" - Random T-shirt
AntiTwitter: @DalekDave is now a follower!
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You're welcome!
Sing the praise when you do! 
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I might be missing the benefit because records, I suspect, target things like data manipulation / EntityFramework, which is as far as possible from what I am doing now, both at work and home....
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I guess they are quite handy to quickly create non trivial dictionary key...
for example:
public static WeakDictionary<TKey, TValue> Get<TKey, TValue>()
where TValue : class
where TKey : notnull
{
var k = new Key(typeof(TKey), typeof(TValue));
if (!caches.TryGetValue(k, out var result))
{
lock (caches)
{
if (!caches.TryGetValue(k, out result))
{
result = new WeakDictionary<TKey, TValue>();
caches[k] = result;
}
}
}
return (WeakDictionary<TKey, TValue>)result;
}
record class Key(Type key, Type value);
readonly static ConcurrentDictionary<Key, IWeakCache> caches = new();
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trying to imagine what goes on in this code: smoke started coming out of my ears 
«The mind is not a vessel to be filled but a fire to be kindled» Plutarch
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Super Lloyd wrote: Though I wonder why it's available only to records.
Not the case since C# 10:
Beginning with C# 10, a left-hand operand of a with expression can also be of a structure type or an anonymous type.
Still not available for regular classes though.
"These people looked deep within my soul and assigned me a number based on the order in which I joined."
- Homer
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You might enjoy this discussion on Reddit: What is the point of records [^].
imho, Records major potential benefit is: immutability.
«The mind is not a vessel to be filled but a fire to be kindled» Plutarch
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Good find Bill, thanks!
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That's the point of many functional building blocks; they're simple by design; expressly so.
In contrast to reference types; a value type, like a record, avoids the guess work that's inherent with state management. On the surface that may appear pointless; why would we want something less powerful and similarly not mutable?
Simply said it's difficult to impossible to mathematically reason about a type whose state is in flux. The functional programming approach is always to build parts that are as simple as they can be; because the more simplistic something is, the greater its potential for composition, and the easier for us to mathematically reason about the outcome of that composition.
Functionally constructed applications are a composition of many simplistic parts:
- their simplicity is what simplifies reasoning
- whereas their ease of composition, is what enables the power.
As example:
The lack of flux in functional programming is what typically makes functional code far easier to multithread than OOP code; because.. for example: all the contention that's typical to referencing is avoided as a default -- by explicit design there is no likelihood for data race conditions between functionally code threads.
As to why "with" is only available to records?
It's because records, unlike reference types, are by design immutable; so without something like "with"; changing values would be far more difficult for record types, and there typically would be no standard approach that everyone could rely upon. With reference types; the standard is; getters and setters. "with" is essentially the setter standard for record types.
Side note:
"With" design is not an entirely new concept; it for the most part is a very simplistic introduction of Functional Optics (e.g. Lenses) to C#. Many of these more complex functional optics can be constructed in C#; although a lot of boiler plate is still required to take full advantage of it. Hopefully this is a gap that could be covered in a future C# release i.e. automatic code generation of functional optics for record types.
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cool.
But you got one thing wrong though. Records are NOT immutable. They can be immutable, as much as any other class can be. 'with' syntax make it easier to use the immutable versions one could say though.
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Not quite...
In its simplest syntactic form i.e. positional syntax; records are immutable by default, for example:
record User(int Id, string LastName);
However where you are correct, is that records defined with nominal creation syntax are mutable as a default, for example:
public record User {
public int Id { get; set; }
public string LastName { get; set; }
}
To stay competitive with other languages e.g. Swift, Microsoft have included additional syntactical flexibility to, e.g. control immutability at a field level, for example:
public record User {
public int Id { get; init; }
public string LastName { get; set; }
}
However, the reason for records according to Microsoft is as follows;:
While records can be mutable, they're primarily intended for supporting immutable data models.
C# 10; again brings more syntactic flexibility around records, e.g. the addition of:
record struct ...
readonly record struct ...
Functional programmers will as a default lean on positional syntax; because its both shorter and guarantees immutability, and is quite similar to other functional languages. Whereas the OOP programmer would be more familiar with the nominal syntax and its behavior.
As for with; its purpose is to provide a non destructive way to do mutation; or stated differently; provide a simplified syntactic solution to create a new copy of a data structure; where some of the field values are changed in process; without of course harming the integrity of the source data structure.
modified 7-Jan-22 16:53pm.
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Quick preamble, I last coded in C# probably 10 years ago. A lot has changed.
I am using SyncFusions drop down box to return an index value from a list.
The variables ddlIndex, and feederDropValue both return the correct values.
<SfDropDownList DataSource = "@lines" TValue="string" TItem="FeederTableQuery" Placeholder="Select a feeder" @bind-Value = "@FeederDropValue" @bind-Index="@ddlIndex">
<DropDownListFieldSettings Text="FeederId" Value="FeederId"></DropDownListFieldSettings>
</SfDropDownList>
<DashboardLayoutPanel Column="2" Row="1" SizeX="2" SizeY="2">
<HeaderTemplate>
<div class="e-header-text">Feeder Details</div>
<div class="header-border"></div>
</HeaderTemplate>
<ContentTemplate>
<div class="panel-content">
<p>Selected value was @FeederDropValue</p>
<p>Selected index was @ddlIndex</p>
@lines.ElementAt(ddlIndex).ToString();
</div>
</ContentTemplate>
</DashboardLayoutPanel>
@code
{
private int? ddlIndex { get; set; } = 0;
public string FeederDropValue;
private IEnumerable <FeederTableQuery> lines { get; set; }
}
I want to the print out what is at that index value, however I get an error when trying select elementAt:
cannot convert from 'int?' to 'System.Index
I admit I not profficent with IEnumerables (probably where I am going wrong), but any help is much appreciated!
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try using lines[(int)ddlIndex]
You can't use nullable vars without casting them.
I probably wouldn't make the ddlIndex nullable, because that means if it happens to be null, it will throw an InvalidOperationException when you try to index a collection.
".45 ACP - because shooting twice is just silly" - JSOP, 2010
-----
You can never have too much ammo - unless you're swimming, or on fire. - JSOP, 2010
-----
When you pry the gun from my cold dead hands, be careful - the barrel will be very hot. - JSOP, 2013
modified 3-Jan-22 8:24am.
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