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My eyes just played a trick on me.
I've corrected the reply to the right person now...
Certainty of death, small chance of success...
What are we waiting for?
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Thanks for the reasoned and well thought out answer Dave. In showing you only a tiny patch of my code, I actually missed out the whole structure that this little method sits in.
This method, as with all my other calls to EF sits in a service class written to handle all the calls to a particular table. for example, all the calls to the purchase table are made from the clsPurchaseHandler class, thus separating it from the UI.
Your answer is exactly what I wanted. as I explained I am a freelance programmer running my own little operation, and I haven't got the luxury of being able to discuss code with anyone, and there's always the niggle at the back of my mind that 'I know it works, but is it the most efficient'. Your code detailing the disposal of entities after use and the find method is invaluable.
As for my naming of the method "ReturnPurchaseItem", that's my usual practise, as that's what the method does, I take your point that it may sound vague, but I've got a lot of code out in the wild that uses this notation, so for now, I'll stick with it.
Thanks again Dave, much appreciated.
George
George
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Sorry to be a pest Dave, but the 'find' method you suggest isn't available to my version of EF. I've read up on it and it appears to come in after EF 4.0. I appear to be running EF 6.0, but I still can't see it. What am I doing wrong?
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You have to import the System.Data.Entity namespace at the top of the code file.
If you get the red squiggly under Find, right click the word and then you should be able to pick Resolve -> System.Data.Entity.
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I've done all of that Dave, and I still get the red squiggly. It's got to be something to do with the version of the System.data.entity dll that I'm importing. EF reports that it's using version 6, but the DLL reports version 4. I'll keep trying, but until then, I'll use the version that works, but I can't help feeling that i's going to get up and bite me one day.
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If you've got a mismatch like that, something went terribly wrong.
Go to the Package Manager Console and do a Uninstall-Package EntityFramework. Then go through the References in the project and remove the EntityFramework reference if it's still there.
Once that's done, you can go back to the Package Manager Console and do a
Install-Package EntityFramework".<br />
<br />
That should take care of any mismatches.<br />
<div class="signature"><small><a href="http:
<font color="Blue">Dave Kreskowiak</font><br />
</div>
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I've been battling since Tuesday to fix this Dave. (sorry for taking so much of your time), I've done what you detailed above, but still the 'find' method isn't recognised. Funny thing though, the text I get when I try to use the method is:
"Error 1 'System.Data.Objects.ObjectSet<entityresearch.purchase>' does not contain a definition for 'find' EntityResearch...."
I thought the find method would be in System.Data.Entity?
I've gone so far as repairing my .NET 4.5 but that didn't work either.
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Repairing anything isn't going to do anything for you. Nothing is broken except your code.
There are multiple Find methods. The one that works on DbSets is in System.Data.Entity. If it's saying the you're using the System.Data.Objects namespace version, you're not using Entity Framework. Your entity sets have been defined using ObjectSet instead of DbSet.
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How can I define my entity sets using DbSet then? To all intents and purposes I am using Entity framework, it's calling it up in app.config, and in the references, so what can be going wrong?
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Using CodeFirst, in your DbContext derived class:
public DbSet<myclasstype> MyTableName { get; set; }
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Thanks Dave, But what If I'm using DB first?
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Wait, you mentioned EF 4.0 .DLL's, correct?
Is this an old EF 4.0 project you're upgrading?? If so, see Upgrading to EF6[^].
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I started a new project to test the find method, and I'm still getting the problem.
My app.config looks like this:
<configuration>
<configSections>
<!-- For more information on Entity Framework configuration, visit http://go.microsoft.com/fwlink/?LinkID=237468 -->
<section name="entityFramework" type="System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
</configSections>
<startup>
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
</startup>
<entityFramework>
<defaultConnectionFactory type="System.Data.Entity.Infrastructure.SqlConnectionFactory, EntityFramework" />
</entityFramework>
</configuration>
The System.Data.Entity DLL reports as being version 4.0. I downloaded a version 5.0 of this DLL, but even that reports as version 4.0
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I've found what I've been doing wrong Dave. When I've been adding the Entity Data model when I started the project, I've been deleting the two .tt template files and setting the code generation strategy in the edmx properties to 'default'. This hasn't been generating the (correct) code to use dBSet. I've still got a lot to learn yet about this, as it's opened a few more cans of worms for me, (For example, what happened to my 'AddObject' methods?)
This highlights my original point. I'm a freelance developer, and have been for about 10 years now. I work by myself, and I have no-one to bounce ideas off, or show me my mistakes. I learnt EF by buying Julia Lerman's book and studying enough of it to get my current project up and running, but not going any further. When I worked for a software house in England, I used to dread code review day, but I can see now how useful it was.
I know I'm still missing a lot, and I would be grateful if you could point me to a good tutorial about the correct way to deal with Entity Framework, and .tt files and all.
Thanks for all your patience Dave, it's been much appreciated.
George
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The best out there are Julia's books. I have them all and am going the CodeFirst route on my current project. I've been working on this one for the last year and will probably be doing it for another.
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Hi there.
I'm sorry, but this is probably not the answer you would wish for, but to be honest, the best practice of MS Entity Framework is to not use it at all...
First of all, you should REALLY consider the actual need for using such technology on a project by project basis, don't just use it because it's an easy way to stay away from writing SELECT statements and Plain SQL.
EF solutions adds A LOT of complexity to a project behind the scenes. It doesn't just map one table to a class and gets done with it, but it seriously does some crazy s*** behind the curtains that you don't want to see. (probably why you don't see it in the first place).
Why do I say this? Consider the following:
For each first-time call to a EF mapped database the machinery has to go through (among other things) these steps:
1. Mapping of the views:
Mapping Views are executable representations of the transformations specified in the mapping for each entity set and association. Internally, these mapping views take the shape of "canonical query trees".
There are two types of mapping views:
Query views:
These represent the transformation necessary to go from the database schema to the conceptual schema.
Update views:
These represent the transformation necessary to go from the conceptual model to the database schema.
The process of computing these views based on the specification of the mapping is what's called "view generation".
View generation can either take place dynamically when a model is loaded, or at build time, by using "pre-generated views".
The latter are serialized in the form of Entity SQL statements to a C# file.
2. Validation of the Schema:
When views are generated, they are also validated. From a performance standpoint, the vast majority of the cost of view generation is actually the validation of the views which ensures that the connections between the entities make sense and have the correct cardinality for all the supported operations.
When a query is executed, the query is combined with the corresponding query view, and the result is run through the compiler to create a representation of the query that the backing store can understand.
For MS SQL Server it will be T-SQL SELECT statements.
The first time an update over an entity set is performed, the update view is run through a similar process to transform it into DML statements for the target DB.
If two Entities are connected via an inheritance chain or an Association, they are said to be connected. Similarly if two tables are connected via a foreign key, they are connected, and if this is not the case in your project, drop EF all together as it is overkill.
As the number of connected Entities and tables in your schemas increase, the view generation cost increases.
So, is all hope lost?
Well, you can speed things up by pre-configuring the views needed. This gives a formidable boost in performance in most cases.
If your model is in EDMX format, you should read up on T4 Template creation and use that.
Using pre-generated views moves the cost of view generation from model loading (run time) to compile time. So basically your users will rejoice and you will bang your head in the desk waiting for compiling to get done with it.
Some tips and tricks to consider:
1. EDMX models are validated at compile time, even if the model is unchanged. If your model has already been validated, you can suppress validation at compile time by setting the "Validate on Build" property to false in the properties window.
If your model isn't an edmx but is created using EDMGen, one of the outputs will be a "Views" file. This is a code file containing Entity SQL snippets for each entity set. To enable pre-generated views, you simply include the file in your project.
If you manually make edits to the schema files for the model, you will need to re-generate the views file. You can do this by running EDMGen with the /mode:ViewGeneration flag.
For further reference, see "Pre-Generate Views to Improve Query Performance"http://msdn.microsoft.com/en-us/library/bb896240.aspx.
2. If your application is only used for query scenarios, you can mark the model as read-only by adding a GenerateUpdateViews attribute on the EntityContainerMapping element in your XML mapping and setting it to false.
3. Using statically compiled query instances (CompiledQuery)
The Customer Advisory Team discusses this in their "Potential Performance Issues with Compiled LINQ Query Re-Compiles" blog post: http://blogs.msdn.com/b/appfabriccat/archive/2010/08/06/potential-performance-issues-with-compiled-linq-query-re-compiles.aspx.
4. Entity Framework 5 introduces automatic caching for LINQ to Entities. In past editions of Entity Framework creating a CompiledQuery to speed your performance was a common practice, as this would make your LINQ to Entities query cacheable. Since caching is now done automatically without the use of a CompiledQuery, we call this feature “autocompiled queries”. For more information, see:
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here is my code:
public partial class Form1 : Form
{
string cs = @"Data Source=NADEEM-PC\SQLEXPRESS;Initial Catalog=School Managment System;Integrated Security=True";
static SqlConnection con = null;
static SqlCommand cmd = null;
public Form1()
{
InitializeComponent();
}
private void Form1_Load(object sender, EventArgs e)
{
}
void Summary()
{
int Male_students;
int Female_students;
int Male_teachers;
int Female_teachers;
con = new SqlConnection(cs);
con.Open();
try
{
cmd = new SqlCommand("SELECT COUNT(*) FROM [Students Records] WHERE Gender='Male' ", con);
Male_students = int.Parse(cmd.ExecuteScalar().ToString());
cmd = new SqlCommand("SELECT COUNT(*) FROM [Students Records] WHERE Gender='Female' ", con);
Female_students = int.Parse(cmd.ExecuteScalar().ToString());
cmd = new SqlCommand("SELECT COUNT(*) FROM [Teachers Records] WHERE Gender='Male' ", con);
Male_teachers = int.Parse(cmd.ExecuteScalar().ToString());
cmd = new SqlCommand("SELECT COUNT(*) FROM [Teachers Records] WHERE Gender='Female' ", con);
Female_teachers = int.Parse(cmd.ExecuteScalar().ToString());
this.SM.Text = "" + Male_teachers.ToString() + "";
this.SF.Text = "" + Female_teachers.ToString() + "";
this.ST.Text = "" + (Male_teachers + Female_teachers).ToString() + "";
this.TM.Text = "" + Male_students.ToString() + "";
this.TF.Text = "" + Female_students.ToString() + "";
this.TT.Text = "" + (Male_students + Female_students).ToString() + "";
con.Close();
}
catch (System.Exception ex)
{
MessageBox.Show(ex.Message);
}
finally
{
if (con.State == ConnectionState.Open)
{
con.Close();
}
}
}
}
i don't get the desire result on labels named SM,SF,ST,TM,TF,TT
please help me..  .
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Well, you don't tell us what results you expect, or actually get, so we can't really say "do this and all will be well" but frankly there is a lot of redundancy in that code.
For example:
Male_students = int.Parse(cmd.ExecuteScalar().ToString());
Male_students is an integer.
ExecuteScalar returns an integer.
So why are you converting an integer to a string, in order to parse it and convert it back to an integer again?
And:
this.SM.Text = "" + Male_teachers.ToString() + "";
You don't need this at all, and what do you think adding a empty string at the start and end of a string is actually going to do? Anything?
Those who fail to learn history are doomed to repeat it. --- George Santayana (December 16, 1863 – September 26, 1952)
Those who fail to clear history are doomed to explain it. --- OriginalGriff (February 24, 1959 – ∞)
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I want to show the total number of male students in label names Sm and i don't get it!!
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Yeah, I kinda guessed that...
Don't you think what you do get, and the number you know are in the DB might be relevant? Just a little? 
Those who fail to learn history are doomed to repeat it. --- George Santayana (December 16, 1863 – September 26, 1952)
Those who fail to clear history are doomed to explain it. --- OriginalGriff (February 24, 1959 – ∞)
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can't understand what r u saying?? 
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Well, you run your code, yes?
It does something, yes?
What exactly it does do only you can see. (Remember that we can't see your screen, access your HDD, or read your mind, so we only get to work with what you tell us.)
So without knowing what it should do: "There are 15 male students, 12 female" and what it actually does do: "It shows 6 Male and 140 Female students" we don't have much to go on in the way of information.
We can't run your code and get the same results you do, because we don't have access to your database. So you need to tell us the information we can't find out any other way.
If you rang the garage and said "my car broke" you would expect them to want information like "what broke?", "where is it?", "was it the engine?", "have you run out of fuel?", and so forth, so why would you expect us to be any different?
Those who fail to learn history are doomed to repeat it. --- George Santayana (December 16, 1863 – September 26, 1952)
Those who fail to clear history are doomed to explain it. --- OriginalGriff (February 24, 1959 – ∞)
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It doesn't show any number there is nothind no text on the label
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No text at all? Ok.
You also said: "I want to show the total number of male students in label names Sm and i don't get it!!"
Which is possibly wrong because your code puts Male Teachers in there:
this.SM.Text = "" + Male_teachers.ToString() + "";
That means the code above isn't executed at all - because int.ToString cannot return an empty string - it will always return a number as a string, even if that number is zero.
So, if that code isn't being executed, and since you don't say that you get a MessageBox or a fatal exception, the most likely is that you aren't calling the Summary method at all. And a quick look at your code says you aren't calling it anywhere in the code fragment you showed us. If you don't call it, you don;t get any displayed values.
Did you mean to call it from your Form1_Load event handler perhaps?
Those who fail to learn history are doomed to repeat it. --- George Santayana (December 16, 1863 – September 26, 1952)
Those who fail to clear history are doomed to explain it. --- OriginalGriff (February 24, 1959 – ∞)
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