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# Using enum to define a new enum

, 10 Aug 2012 CPOL
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This tip describes how to use an existing enum to define a new, for example a context specific enum.

## Introduction

As you know, `enum` is used to define a static set of values to use in different kinds of situations. Often enums define a single, self-explanatory set of values, but sometimes the same values may be needed in different contexts; Overlapping partially or none at all.

This tip describes how to use a master set of values to define context specific enums.

## A simple enum

Let's take animals as an example. You can buy a pet and you can go to zoo to see different kinds of animals. Depending on the logic, you may need a different set of animals in your code, but you need to ensure that they all are unique in the sense of enum definition. So regardless of the context, dog would always be 1 and cat always 2 and so on.

First, let's create a master set of animals:

```public enum MasterEnumAnimal {
Dog       = 1,
Cat       = 2,
Bird      = 3,
Alligator = 4,
Camel     = 5
} ```

Now depending, what you're going to do, buy a pet or go to a zoo to see an animal, you need a different set of possible enum values.

To define the subsets, simply use the original master definition in your subset definitions. Like the following:

```public enum Pet {
Dog  = MasterEnumAnimal.Dog,
Cat  = MasterEnumAnimal.Cat,
Bird = MasterEnumAnimal.Bird
}

public enum ZooAnimal {
Bird      = MasterEnumAnimal.Bird,
Alligator = MasterEnumAnimal.Alligator,
Camel     = MasterEnumAnimal.Camel
}```

As you can see both enums have different animals except both include a bird.

Now consider the following methods:

```public static void BuyAPet(Pet pet) {
System.Console.WriteLine(string.Format("You bought a {0}", pet));
}

public static void SeeAnAnimal(ZooAnimal animal) {
System.Console.WriteLine(string.Format("You saw {0}", animal));
}```

Both of the methods accept a certain set of animals, but the possible animals are different based on the context. You see different animals in the zoo compared to what you buy as a pet. However, you can always tell which kind of animal is in question.

An example of using these context specific enums:

```BuyAPet(Pet.Dog);
SeeAnAnimal(ZooAnimal.Bird); ```

Running the above example would give an output like:

```You bought a Dog
You saw Bird```

## Using master enum with flags

Using a flagged enum is no different. Let's take another kind of example with different kinds of materials. You use partly different materials when building a house or making a furniture, but some of the material types are overlapping.

To define the master set of materials, consider the following:

```[System.Flags()]
public enum MasterEnumMaterial {
Wood           = 1 << 0,
Metal          = 1 << 1,
Glass          = 1 << 2,
Leather        = 1 << 3,
SyntheticFiber = 1 << 4,
Concrete       = 1 << 5,
Tile           = 1 << 6,
Stone          = 1 << 7
}```

To define proper subsets of materials for each purpose, you can use the following definitions:

```public enum FurnitureMaterial {
Wood           = MasterEnumMaterial.Wood,
Metal          = MasterEnumMaterial.Metal,
Glass          = MasterEnumMaterial.Glass,
Leather        = MasterEnumMaterial.Leather,
SyntheticFiber = MasterEnumMaterial.SyntheticFiber
}

public enum BuildingMaterial {
Wood     = MasterEnumMaterial.Wood,
Metal    = MasterEnumMaterial.Metal,
Glass    = MasterEnumMaterial.Glass,
Concrete = MasterEnumMaterial.Concrete,
Tile     = MasterEnumMaterial.Tile,
Stone    = MasterEnumMaterial.Stone
}```

As you can see, several materials are common, but some are not. For example, who wants a sofa made of concrete and stone .

Now, to use these materials in different contexts, consider following methods:

```public static MasterEnumMaterial BuildAHouse(BuildingMaterial usedMaterials) {
System.Console.WriteLine("You built a house made of:");
foreach (BuildingMaterial material in System.Enum.GetValues(typeof(BuildingMaterial))) {
if (usedMaterials.HasFlag(material)) {
System.Console.WriteLine(string.Format("  - {0}", material));
}
}
return (MasterEnumMaterial)usedMaterials;
}

public static MasterEnumMaterial BuyAFurniture(FurnitureMaterial materials) {
System.Console.WriteLine("You bought a furniture made of:");
foreach (FurnitureMaterial material in System.Enum.GetValues(typeof(FurnitureMaterial))) {
if (materials.HasFlag(material)) {
System.Console.WriteLine(string.Format("  - {0}", material));
}
}
return (MasterEnumMaterial)materials;
}```

Now, to test these methods, let's build a house and buy a furniture:

```MasterEnumMaterial houseMaterials;
MasterEnumMaterial furnitureMaterials;

houseMaterials = BuildAHouse(BuildingMaterial.Concrete | BuildingMaterial.Glass | BuildingMaterial.Wood);
```

Executing the previous methods would give an output like the following:

```You built a house made of:
- Wood
- Glass
- Concrete

You bought a furniture made of:
- Wood
- Leather```

But more importantly, if you want to know the common materials in the house you built and the furniture you bought, you could resolve this with

```foreach (MasterEnumMaterial material in System.Enum.GetValues(typeof(MasterEnumMaterial))) {
if (houseMaterials.HasFlag(material) && furnitureMaterials.HasFlag(material)) {
System.Console.WriteLine(string.Format("  - {0}", material));
}
}```

This would result in following output:

```You have following common materials in your house and furniture:
- Wood ```

## Conclusions

As you can see, using a single, predefined set of enum values can help to keep a single, uniquelly defined set of values and using subsets of this master enum helps to define context specific sets from the master list of values.

The download contains a small test project for the examples included in this tip.

While you can define a subset from a single enum as seen in this example, you can also define an enum combining several different enums. But that's another story

## History

• July 21, 2012: Tip created.
• August 11, 2012: Small corrections.

## Share

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Finland
I've been a programmer since mid 80's using languages like assembler, C/C++, PL/I (mainframe environment), pascal, VB (I know, I know, no comments please) and C# and utilizing different techniques and tools.

However I'm specialized in databases and database modeling. Mostly I have used products like Oracle (from version 6), SQL Server (from version 4.2), DB2 and Solid Server (nowadays an IBM product).

For the past 10+ years my main concerns have been dealing with different business processes and how to create software to implement and improve them. At my spare time (what ever that actually means) I'm also teaching and consulting on different areas of database management, development and database oriented software design.

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