Jul2012
4

Enum types, FlagsAttribute & Zero value

by nmgomes

We all know about Enums types and use them every single day. What is not that often used is to decorate the Enum type with the FlagsAttribute.

When an Enum type has the FlagsAttribute we can assign multiple values to it and thus combine multiple information into a single enum.

The enum values should be a power of two so that a bit set is achieved.

Here is a typical Enum type:

public enum OperationMode
{
    /// <summary>
    /// No operation mode
    /// </summary>
    None = 0,
    /// <summary>
    /// Standard operation mode
    /// </summary>
    Standard = 1,
    /// <summary>
    /// Accept bubble requests mode
    /// </summary>
    Parent = 2
}

In such scenario no values combination are possible. In the following scenario a default operation mode exists and combination is used:

[Flags]
public enum OperationMode
{
    /// <summary>
    /// Asynchronous operation mode
    /// </summary>
    Async = 0,
    /// <summary>
    /// Synchronous operation mode
    /// </summary>
    Sync = 1,
    /// <summary>
    /// Accept bubble requests mode
    /// </summary>
    Parent = 2
}

Now, it’s possible to do statements like:

[DefaultValue(OperationMode.Async)] [TypeConverter(typeof(EnumConverter))] public OperationMode Mode { get; set; } /// <summary> /// Gets a value indicating whether this instance supports request from childrens. /// </summary> public bool IsParent { get { return (this.Mode & OperationMode.Parent) == OperationMode.Parent; } }

or

switch (this.Mode) { case OperationMode.Sync | OperationMode.Parent: Console.WriteLine("Sync,Parent"); break;

[…]

 

But there is something that you should never forget: Zero is the absorber element for the bitwise AND operation.

So, checking for OperationMode.Async (the Zero value) mode just like the OperationMode.Parent mode makes no sense since it will always be true:

(this.Mode & 0x0) == 0x0

Instead, inverse logic should be used: OperationMode.Async = !OperationMode.Sync

public bool IsAsync
{
    get { return (this.Mode & ContentManagerOperationMode.Sync) != ContentManagerOperationMode.Sync; }
}

or

public bool IsAsync
{
    get { return (int)this.Mode == 0; }
}

[Begin: Edit]

Some readers suggested to use the Enum.HasFlag method. Although it’s another valid approach it’s also widely accepted that this method has some performance issues related an internal box/unboxing.

Use it when performance is not a problem (I see this method mostly used by rookie developers).

[End: Edit]

Final Note:

Benefits

Allow multiple values combination

The above samples snippets were taken from an ASP.NET control and enabled the following markup usage:

<my:Control runat="server" Mode="Sync,Parent">

Drawback

Zero value is the absorber element for the bitwise AND operation

Be very carefully when evaluating the Zero value, either evaluate the enum value as an integer or use inverse logic.

Filed in: ASP.NET | .NET

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