Is it possible to make an enum,with the contents which are read from database?

Question

Hi everyone,i know how to make a fixed enum like

public enum test
        {
           
            qNone = 0,
            wFixedSingle = 1,
            eFixed3D = 2,
        }

but i would like to ask is it possible that the enumerators are read from the database,that is to say,is it possible to make a unfixed enum?the enumerator in the enum can be decided according to the data in teh database? could anyone do me a kindness favor to help me? thank u very much in advance

best regards

martin

Answer 1

Check this link. Dynamic Enum Creation

Thanks,

---

Parth

Answer 2

That's not possible, no.  The values of an Enumerator must be fixed at compile time.

You might be thinking of a Dictionary, or a HashSet, if you have a set of valid values. (Dictionary if they map to ints or something else, HashSet if they don't.)

Answer 3

Enumerations were conceived as a nicer, language-level way to stop using #define macros.  You can possibly write something to generate an enumeration from a table in the database to help ease these things during development, but you have to have constants in the definition of an enum.  Now, if you wanted to make a static class with static public properties whose values are loaded at run time, this is doable in a fushion like this:

public static class CategoryThing
{
  private static int qNone;
  private static int qFixedSingle;
  private static int eFixed3D;
  static CategoryThing()
  {
    //Database read into a table
    qNone = (int)dt.FindRow("None");
    /etc
  }
  public static int None
  {
    get
    {
       return qNone;
    }
  }
  //And so on.
}

Now, you won't be able to use this as an enum in some things like a foreach loop, but this will get you some of the benefits like type checking and named constants.

Answer 4

hello.

maybe the DLR match you need.

you can create a class inherit from DynamicObject.

take a look at DynamicObject Class at msdn .hope can help you.

---

DON'T TRY SO HARD,THE BEST THINGS COME WHEN YOU LEAST EXPECT THEM TO.

Answer 5

Hi, servy42,

thank u very much for ur response.

now i am create an object and hope to bind to a propertygrid.

i need that there could be sth like that

public class myObject
        {

  public virtual status myStatus { get; set; }
}

and

then

public enum status
        {           
            None = 0,FixedSingle = 1,Fixed3D = 2,
        }

but now i need that the combox can have more value from the database,i tried dictionary but that does not work

best regards

martin

Answer 6

The whole purpose of an enum is to allow you to give literals nice (named constant) names to make code easier to read:

int someType = 1;  //Poor
DataType someType = DataType.Int;  //Better

At runtime the behavior is the same and, furthermore, enums don't actually provide much more compile time validation than ints (you can still assign any int you want to an enum value).  There is a false sense of strong type checking with enums that doesn't actually exist.  C# is pretty good about warning you when you are misusing an enum but warnings can be ignored.

If you have a list of values that are only available at runtime then an enum is not the best structure because you can't reference the values at compile time anyway.  A dictionary might work depending upon how you represent values.  If the names are compile time constants but the values are dynamic then a simple wrapper type that is exposed through a singleton or static class works just as well at runtime and gives you a semblance of enum functionality.

Michael Taylor - 4/13/2012
http://msmvps.com/blogs/p3net

Answer 7

Hi, CoolDadTx

thank u very much for ur response.

now i am create an object and hope to bind to a propertygrid.

i need that there could be sth like that

public class myObject
        {

  public virtual status myStatus { get; set; }
}

and

then

public enum status
        {           
            None = 0,FixedSingle = 1,Fixed3D = 2,
        }

but now i need that the combox can have more value from the database,i tried dictionary but that does not work

best regards

martin

Answer 8

Using enums with unvalidated data that comes from the user, a file store, or the network is always a recipe for disaster.  Enums are great once you've thoroughly vetted the source value.  Some code somewhere must perform this validation.

Answer 9

If you wanted to have a static set of enumerable-like values at compile time but want the actual numeric values to come at runtime (from a database) then something like this works:

public static class test
{
   public static int qNone { get; private set; }
   public static int wFixedSize { get; private set; }
   public static int eFixed3D { get; private set; }
}

Now you have an enumerator-like syntax:

if (myType == test.qNone) ...

To initialize the values you could either use a static constructor or an initialize method that queries the values from wherever.  This becomes part of your app init code.

Michael Taylor - 4/13/2012
http://msmvps.com/blogs/p3net

Answer 10

Hi,

IMO if the values are static then you should use an enum but if you tell that values are coming from a database then it is likely that those values should be just thought as reference table inside your db. It doesn't prevent from gathering some known states if they are playing a role as part of some rules.

---

Please always mark whatever response solved your issue so that the thread is properly marked as "Answered".

Answer 11

hi , martin

for your purpose, you can create a class for combox item databinding.

    public class ItemData
    {

        public ItemData()
        {

        }

        public ItemData(int pId, string pName)
        {
            this.ID = pId;
            this.Name = pName;
        }

        public int ID { get; set; }
        public string Name { get; set; }

        public override string ToString()
        {
            return this.Name;
        }
    }

and then .using List<ItemData> query from database binding to combox control

finnally ,you Cast Combox.SelectedItem as ItemData to get the seleted ItemData instance.

hope may help you something

---

DON'T TRY SO HARD,THE BEST THINGS COME WHEN YOU LEAST EXPECT THEM TO.

Answer 12

I believe that although it is quite likely you can create an enum via reflection, the cost of doing so would not be worth it. 

 

What is the scenario where an enum is more efficient than the datasource itself?

--
Mike

Answer 13

I want to illustrate what an enumeration is in IL.  Enumerations are constants.  Enumerations are not static variables, fields or values.  They are fixed constant values.  Enumerations cannot be dynamically defined and used as such.  A disassembly of the IL demonstrates this.

    class MyEnumTest
    {
        public int Field = 0;
        public TestEnum UseTest(TestEnum test)
        {
            TestEnum te = TestEnum.eFixed3D;
            if ( test==TestEnum.qNone )
            {
                int j = (int)test;
            }
            if ( Field == 21 )
            {
                int j = Field;
            }
            return te;
        }
    }
    public enum TestEnum
    {
        qNone = 0,
        wFixedSingle = 1,
        eFixed3D = 2,
    }

This produces the following IL..... first the enum definition, and then the method definition.  Notice the  word "literal".

.class public auto ansi sealed TestEnum
    extends [mscorlib]System.Enum
{
    .field public static literal valuetype Namespace.TestEnum eFixed3D = int32(2)

    .field public static literal valuetype Namespace.TestEnum qNone = int32(0)

    .field public specialname rtspecialname int32 value__

    .field public static literal valuetype Namespace.TestEnum wFixedSingle = int32(1)

}

.method public hidebysig instance valuetype NamespaceName.TestEnum 
        UseTest(valuetype TestEnum test) cil managed
{
  // Code size       50 (0x32)
  .maxstack  2
  .locals init ([0] valuetype Namespace.TestEnum te,
           [1] int32 j,
           [2] valuetype TestEnum CS$1$0000,
           [3] bool CS$4$0001)

//000012:         {
  IL_0000:  nop
//000013:             TestEnum te = TestEnum.eFixed3D;
  IL_0001:  ldc.i4.2
  IL_0002:  stloc.0
//000014:             if ( test==TestEnum.qNone )
  IL_0003:  ldarg.1
  IL_0004:  ldc.i4.0
  IL_0005:  ceq
  IL_0007:  ldc.i4.0
  IL_0008:  ceq
  IL_000a:  stloc.3
  IL_000b:  ldloc.3
  IL_000c:  brtrue.s   IL_0012
//000015:             {
  IL_000e:  nop
//000016:                 int j = (int)test;
  IL_000f:  ldarg.1
  IL_0010:  stloc.1
//000017:             }
  IL_0011:  nop
//000018:             if ( Field == 21 )
  IL_0012:  ldarg.0
  IL_0013:  ldfld      int32 MyEnumTest::Field
  IL_0018:  ldc.i4.s   21
  IL_001a:  ceq
  IL_001c:  ldc.i4.0
  IL_001d:  ceq
  IL_001f:  stloc.3
  IL_0020:  ldloc.3
  IL_0021:  brtrue.s   IL_002c
//000019:             {
  IL_0023:  nop
//000020:                 int j = Field;
  IL_0024:  ldarg.0
  IL_0025:  ldfld      int32 MyEnumTest::Field
  IL_002a:  stloc.1
//000021:             }
  IL_002b:  nop
//000022:             return te;
  IL_002c:  ldloc.0
  IL_002d:  stloc.2
  IL_002e:  br.s       IL_0030
//000023:         }
  IL_0030:  ldloc.2
  IL_0031:  ret
} // end of method MyEnumTest::UseTest

Notice the lines of code where the If condition is evaluated in the IL, lines 0004 and 0013.  When the variable Field is loaded, a value is loaded from memory.  When the MyEnum value is loaded, a hard coded constant is used.  The Compiler knows the values are constant and will not change over the lifetime of the application, so it uses the value directly instead of fetching a value from somewhere in memory.  The values are stored as metadata, which other assemblies can read.

Constants (C# Programming Guide) - MSDN – Explore Windows, Web ...

"The enum type enables you to define named constants for integral built-in types (for example int, uint, long, and so on). For more information, see enum (C# Reference)."

The point is that enumerations are not used as values stored in memory.  Enumeration values are used as hard coded constants within the assembly.  Can you define and create an enumeration dynamically?  Sure, but you would have to compile an assembly in order to use it.  I would use the above suggestion to create a static class with static variables.  Your code that consumed it would like identical to an enumeration.

Hope this helps.

Rudy   =8^D

---

Mark the best replies as answers. "Fooling computers since 1971."

http://thesharpercoder.blogspot.com/

Answer 14

hi.thank u for all of u ur kindess help.i will now read them and try.thank u very much

best regards

martin

Answer 15

Hi Matthew,

thank u very much for ur kindness help.

but now i am required to use propertygridcontrol (in fact propertygridcontrol  of devexpress,it is more or less the same as ms propertygridcontrol).

so there is a big object including some information and this object is binded to the propertygridcontrol.

it is something like that:

 public class personinfo
        {           
            [Category("Name")]
            [DisplayName("Name")]
            public float length { get; set; }

             [Category("Name")]
            [DisplayName("Surname")]
            public string name { get; set; }        


            [Category("Name")]
            [DisplayName("Surname")]
            public string surname{ get; set; }

            [Category("Chimiche")]
            [DisplayName("Material")]
            public string material { get; set; }

            [Category("Nation")]
            [DisplayName("Nation")]
            public virtual nationlist nation { get; set; }
        }

the nationlist,now i design it as an enum,i hope that the content of this "nationlist" can be read from a database.

then i will bind this personinfo to a propertygirdcontrol,so that in this propertygirdcontrol ,in item "nation"people can in choose from many countries read from db in the combo box

now i am not sure if it is possible.

anyway i appreciate ur help very much

best regards

martin

     

Answer 16

hi Matthew:

the List u mean this:

http://msdn.microsoft.com/en-us/library/3wcytfd1.aspx

?thank u very much

best regards

martin

Answer 17

Hi martinwang.

for your purpose that show data in propertygrid control.

you would assign a custom UTTypeEditor to you nationlist property .

the custom UITypeEditor should do Dropdown action to show data list from database.

you can reference below url

http://msdn.microsoft.com/en-us/library/system.drawing.design.uitypeeditor.aspx

good luck.

---

DON'T TRY SO HARD,THE BEST THINGS COME WHEN YOU LEAST EXPECT THEM TO.

Answer 18

hi Matthew:

 thank u very much ur kindness help.i've resolved my problem

best regards

martin