Asked to declare in C #

Trung Do Thanh

14y3.1kVisual Basic .NET

Hi everyone,

New I would like to ask the types of programming such declaration is this:

Please people just help

1. Function declaration

IEnumerable LoadMyEntities (Expression > predicate)

{

/ / Coding here ...

}

===================

2. this.Load + = new System.EventHandler (this.Page_Load);

===================

3. public enum e_IsReimbursement

{

NO = 0,
YES = 1

}

===================

4.public enum e_DaysOfWeek

{

Monday,
TUESDAY,
WEDNESDAY,
Thursday,
FRIDAY,
SATURDAY,
SUNDAY,
HALF_SATURDAY

}

===================

5. Declaration type enum is nothing better than a const variable

public const string DEFAULT_SELECTED_DEPARTMENT = "<---- Select Department ---->";

===================

6. Text ie what

private T GetObjectFromLmsCache (QueryParser queries, out string sqlString, and out bool keyExist)

{

/ / Coding here ...

}

===================

7. Explained in this function declaration

public override IList getObjectListFromDb (QueryParser query)

{

string sqlString;
bool keyExist;

IList returnValue = GetObjectFromLmsCache > (queries, out sqlString, and out keyExist);
if (keyExist)
{
return returnValue;
}

returnValue = base.getObjectListFromDb (query);

SetObjectToLmsCache (sqlString, returnValue);
return returnValue;

}

Happy if people interested tohelp.

Sincerely thank you.

6 Replies

Know the answer? Post it — somebody with the same question will find it here.

Vulpes14y ago
Accepted answer
Well, it's more interesting than mine :)

I see you've realized that you often need to cast to object first before you can cast to T in order for generic code to compile.

Here's a simple example of using an expression tree. In the Person.IsTeenager() method, the lambda expression which determines if that's the case is automatically converted to an expression tree of the appropriate type.

This expression tree is then used in the query to determine which Persons in an array of Persons are teenagers. It needs to be compiled first to convert it into a delegate so that it can be used in the Where clause of that query:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;

class Person
{
public string Name {get; set;}
public int Age {get; set;}

public Person(string name, int age)
{
Name = name;
Age = age;
}

public static Expression> IsTeenager()
{
return p => p.Age >= 13 && p.Age <= 19;
}
}

class Program
{
static void Main()
{
Person[] persons =
{
new Person("John", 15),
new Person("Dave", 45),
new Person("Fred", 18),
new Person("Anne", 35)
};

IEnumerable teenagers = persons.Where(Person.IsTeenager().Compile());
foreach(Person t in teenagers) Console.WriteLine("Name {0}, Age {1}", t.Name, t.Age);
Console.ReadKey();
}
}

The output should be:

Name John, Age 15
Name Fred, Age 18
Trung Do Thanh14y ago
Ok.
I understand your current reply, I'll try to make example to clearer about this.

Thank you so much.
Trung Do Thanh14y ago
Thank you very much Vulpes.
I understand about object and how declare in method.
I changed your code that is: How do you thinhk about my new code. :)

static void Main(string[] args)
{
int i = GetDefault();
Console.WriteLine(i);
DateTime dt = GetDefault();
Console.WriteLine(dt);
string str = GetDefault();
Console.WriteLine(str);
Console.ReadKey();
}
static T GetDefault()
{
switch (typeof(T).Name.ToLower())
{
case ("int32"): return (T)(object)123;
case ("datetime"): return (T)(object)DateTime.Now;
case ("string"): return (T)(object)"niceday";
default : return default(T);
}
}

I want to know clearer about question 1 that is "expression tree", can you write a simple example to help me clearer.
Vulpes14y ago
It sounds like you may not be familiar with generic methods in C#.

Consider this simple program:

using System;

class Test
{
static void Main()
{
int i = GetDefault();
Console.WriteLine(i);
DateTime dt = GetDefault();
Console.WriteLine(dt);
Console.ReadKey();
}

static T GetDefault()
{
return default(T);
}
}

Here GetDefault is a generic method with a type parameter T (enclosed in angle brackets). This method simply returns the default value of whatever type parameter is used. The type of this default value will, of course, be T itself.

So, you can call it with a type parameter of int, DateTime or anything else you like. It will always work.

GetObjectFromLmsCache is similar, though more complicated as it takes a number of 'ordinary' parameters as well. So whatever type parameter is passed, a value of that type will be returned by the method.

The type parameter can itself be a generic type such as IList.

Any clearer?
Trung Do Thanh14y ago
Thank you very much Vulpes. But I want to clealy know about difference of two declare:
+ private T GetObjectFromLmsCache (param...)
+ private T GetObjectFromLmsCache (param...)

What is before (param...)
Vulpes14y ago
All I can do is to give you some explanation of what those code items are, as it's virtually impossible to tell what the code as a whole is doing:

1. Expression > is what's called an 'expression tree', a tree-like data structure which represents code and enables it to be 'packaged up' and interpreted at runtime.

Expression trees are commonly used in the 'plumbing' of frameworks such as LINQ to SQL or the Entity Framework and so the LoadMyEntities method could be something to do with these though, frankly, it might be something entirely different.

The LoadMyEntities method accepts an expression tree as a parameter and returns a sequence of MyEntity objects which you could iterate with foreach, convert to an array or list, or use in a LINQ query.

2. This line is wiring up the Page's Load event to the Page_Load method. So, when the Page is loaded, this method will execute automatically.

3. e_IsReimbursement is an enum (enunerated type) which has two possible values. NO or YES.

4. e_DaysOfWeek is an enum which has 8 possible values. By default, MONDAY's underlying value will be 0 and HALF_SATURDAY's value will be 7.

5. DEFAULT_SELECTED_DEPARTMENT is a string constant whose value, once assigned, will not change during the running of the application.

6. GetObjectFromLmsCache is a generic method which takes a type parameter T (T could be any type). It takes an 'ordinary' parameter of type QueryParser and two 'output' parameters of types string and bool (notice that the word 'and' before the second 'out' should be removed here). It also returns a value of type T. Code calling this method is guaranteed that the 'output' parameters will have been assigned a value when the method returns.

7. getObjectListFromDb is also a generic method which takes a type parameter T. It takes a parameter of type QueryParser and returns a value of IList. It also overrides a virtual method of the same name in its base class.

This method calls the GetObjectFromLmsCache method internally using a type parameter of List where T is the type parameter passed to GetObjectFromLmsCache itself. Consequently, it returns a value of IList, not T.

This value is returned by getObjectListFromDb if 'keyExists' is true. Otherwise the base class's version of getObjectListFromDb is called with a type parameter of T and will therefore still return a value of type IList, which is assigned to the value returnValue.

Another method called SetObjectToLmsCache is then called and passed two parameters, including returnValue, before returnValue is returned by the getObjectListFromDb method itself.