Retrieve enum value based on XmlEnumAttribute name

问题:

I need a Generic function to retrieve the name or value of an enum based on the XmlEnumAttribute "Name" property of the enum. For example I have the following enum defined:

Public Enum Currency
   <XmlEnum("00")> CDN = 1
   <XmlEnum("01")> USA= 2
   <XmlEnum("02")> EUR= 3
   <XmlEnum("03")> JPN= 4
End Enum

The first Currency enum value is 1; the enum name is "CDN"; and the XMLEnumAttribute Name property value is "00".

If I have the enum value, I can retrieve the XmlEnumAttribute "Name" value using the following generic function:

Public Function GetXmlAttrNameFromEnumValue(Of T)(ByVal pEnumVal As T) As String

        Dim type As Type = pEnumVal.GetType
        Dim info As FieldInfo = type.GetField([Enum].GetName(GetType(T), pEnumVal))
        Dim att As XmlEnumAttribute = CType(info.GetCustomAttributes(GetType(XmlEnumAttribute), False)(0), XmlEnumAttribute) 'If there is an xmlattribute defined, return the name

        Return att.Name
    End Function

So using the above function, I can specify the Currency enum type, pass a value of 1, and the return value will be "00".

What I need is a function to perform if the opposite. If I have the XmlEnumAttribute Name value "00", I need a function to return a Currency enum with a value of 1. Just as useful would be a function that would return the enum name "CDN". I could then simply parse this to get the enum value.

Any assistance would be appreciated.

回答1:

A requirement to solve this exact same problem led me to this question and answer. As I develop in VB.NET, I rewrote CkH's solution into VB and modified it to use your GetXmlAttrNameFromEnumValue function.

Public Shared Function GetCode(Of T)(ByVal value As String) As T
    For Each o As Object In System.Enum.GetValues(GetType(T))
        Dim enumValue As T = CType(o, T)
        If GetXmlAttrNameFromEnumValue(Of T)(enumValue).Equals(value, StringComparison.OrdinalIgnoreCase) Then
            Return CType(o, T)
        End If
    Next

    Throw New ArgumentException("No code exists for type " + GetType(T).ToString() + " corresponding to value of " + value)
End Function

C# Version:

public static string GetXmlAttrNameFromEnumValue<T>(T pEnumVal)
{
    // http://stackoverflow.com/q/3047125/194717
    Type type = pEnumVal.GetType();
    FieldInfo info = type.GetField(Enum.GetName(typeof(T), pEnumVal));
    XmlEnumAttribute att = (XmlEnumAttribute)info.GetCustomAttributes(typeof(XmlEnumAttribute), false)[0];
    //If there is an xmlattribute defined, return the name

    return att.Name;
}
public static T GetCode<T>(string value)
{
    // http://stackoverflow.com/a/3073272/194717
    foreach (object o in System.Enum.GetValues(typeof(T)))
    {
        T enumValue = (T)o;
        if (GetXmlAttrNameFromEnumValue<T>(enumValue).Equals(value, StringComparison.OrdinalIgnoreCase))
        {
            return (T)o;
        }
    }

    throw new ArgumentException("No XmlEnumAttribute code exists for type " + typeof(T).ToString() + " corresponding to value of " + value);
}

回答2:

I do something similar with custom attributes and I use this method to get the EnumValue based on the Attribute Value. GetStringValue is my custom method, similar to your example above.

public static class Enums 
{ 
    public static T GetCode<T>(string value) 
    { 
        foreach (object o in System.Enum.GetValues(typeof(T))) 
        { 
            if (((Enum)o).GetStringValue().Equals(value, StringComparison.OrdinalIgnoreCase))
                return (T)o; 
        } 
        throw new ArgumentException("No code exists for type " + typeof(T).ToString() + " corresponding to value of " + value); 
    } 
}  

For the whole process I use check this post and the answers: Extending Enums, Overkill?

Sorry this is in C#, just realized you were using VB.NET above.

回答3:

Slightly modified from: http://www.wackylabs.net/2006/06/getting-the-xmlenumattribute-value-for-an-enum-field/

public static string ToString2 (this Enum e) {
    // Get the Type of the enum
    Type t = e.GetType ();

    // Get the FieldInfo for the member field with the enums name
    FieldInfo info = t.GetField (e.ToString ("G"));

    // Check to see if the XmlEnumAttribute is defined on this field
    if (!info.IsDefined (typeof (XmlEnumAttribute), false)) {
        // If no XmlEnumAttribute then return the string version of the enum.
        return e.ToString ("G");
    }

    // Get the XmlEnumAttribute
    object[] o = info.GetCustomAttributes (typeof (XmlEnumAttribute), false);
    XmlEnumAttribute att = (XmlEnumAttribute)o[0];
    return att.Name;
}

回答4:

Here's a variation that generates a dictionary from the enum, allowing you to potentially cache the reflection part of it should you need to use it a lot.

/// <summary>
/// Generates a dictionary allowing you to get the csharp enum value
/// from the string value in the matching XmlEnumAttribute.
/// You need this to be able to dynamically set entries from a xsd:enumeration
/// when you've used xsd.exe to generate a .cs from the xsd.
/// https://msdn.microsoft.com/en-us/library/x6c1kb0s(v=vs.110).aspx
/// </summary>
/// <typeparam name="T">The xml enum type you want the mapping for</typeparam>
/// <returns>Mapping dictionary from attribute values (key) to the actual enum values</returns>
/// <exception cref="System.ArgumentException">T must be an enum</exception>
private static Dictionary<string, T> GetEnumMap<T>() where T : struct, IConvertible
{
        if (!typeof(T).IsEnum)
        {
                throw new ArgumentException("T must be an enum");
        }
        var members = typeof(T).GetMembers();
        var map = new Dictionary<string, T>();
        foreach (var member in members)
        {
                var enumAttrib = member.GetCustomAttributes(typeof(XmlEnumAttribute), false).FirstOrDefault() as XmlEnumAttribute;
                if (enumAttrib == null)
                {
                        continue;
                }
                var xmlEnumValue = enumAttrib.Name;
                var enumVal = ((FieldInfo)member).GetRawConstantValue();
                map.Add(xmlEnumValue, (T)enumVal);
        }
        return map;
}

usage:

var map = GetEnumMap<Currency>();
return map["02"]; // returns Currency.EUR

回答5:

@Dean, @Jason and @Camron, thank you for your solutions. Your solutions helped me in solving my problem, where, given an XmlEnumAttribute name, the actual enum value was needed.

My variant is mentioned here.

I am also including it here as as asked by one of our moderators:

The actual problem was how to get Item10, when given a value of 10. Taking a cue from the solution cited by our above mentioned friends, I came up with the following method, which when passed a value contained in XmlEnumAttribute, would return the enum value:

private static T GetEnumValueFromXmlAttrName<T>(string attribVal)
{
    T val = default(T);

    if (typeof(T).BaseType.FullName.Equals("System.Enum"))
    {
        FieldInfo[] fields = typeof(T).GetFields();

        foreach (FieldInfo field in fields)
        {
            object[] attribs = field.GetCustomAttributes(typeof(XmlEnumAttribute), false);

            foreach (object attr in attribs)
            {
                if ((attr as XmlEnumAttribute).Name.Equals(attribVal))
                {
                    val = (T)field.GetValue(null);
                    return val;
                }
            }
        }
    }
    else
        throw new Exception("The supplied type is not an Enum.");

    return val;
}