131

I am trying to create a generic method that will read an attribute on a class and return that value at runtime. How do would I do this?

Note: DomainName attribute is of class DomainNameAttribute.

[DomainName("MyTable")]
Public class MyClass : DomainBase
{}

What I am trying to generate:

//This should return "MyTable"
String DomainNameValue = GetDomainName<MyClass>();
MrLore
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Zaffiro
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    Official microsoft link : http://msdn.microsoft.com/en-us/library/71s1zwct.aspx – Mahesh Sep 09 '13 at 23:52
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    Important corollary question how to get all types in assembly with custom attribute http://stackoverflow.com/questions/2656189/how-do-i-read-an-attribute-on-a-class-at-runtime – Chris Marisic Nov 02 '15 at 17:21

9 Answers9

281
public string GetDomainName<T>()
{
    var dnAttribute = typeof(T).GetCustomAttributes(
        typeof(DomainNameAttribute), true
    ).FirstOrDefault() as DomainNameAttribute;
    if (dnAttribute != null)
    {
        return dnAttribute.Name;
    }
    return null;
}

UPDATE:

This method could be further generalized to work with any attribute:

public static class AttributeExtensions
{
    public static TValue GetAttributeValue<TAttribute, TValue>(
        this Type type, 
        Func<TAttribute, TValue> valueSelector) 
        where TAttribute : Attribute
    {
        var att = type.GetCustomAttributes(
            typeof(TAttribute), true
        ).FirstOrDefault() as TAttribute;
        if (att != null)
        {
            return valueSelector(att);
        }
        return default(TValue);
    }
}

and use like this:

string name = typeof(MyClass)
    .GetAttributeValue((DomainNameAttribute dna) => dna.Name);
Darin Dimitrov
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    Thanks for your diligence in answering the question! – Zaffiro Apr 18 '10 at 02:07
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    This extension method could be further generalized by extending MemberInfo, a base class of Type and all - or at least *most* - of a Type's members. Doing so would open this up to allow reading attributes from Properties, Fields, and even Events. – M.Babcock Aug 12 '14 at 01:54
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    Overly complicated. There's no need to use lambda to select the attribute value. If you enough to write the lambda, you know enough to just access the field. – Darrel Lee Apr 14 '15 at 22:01
  • How can i extend this approach this to get `const Filed` in static class? – Amir Nov 02 '15 at 19:18
67

There is already an extension to do this.

namespace System.Reflection
{
    // Summary:
    //     Contains static methods for retrieving custom attributes.
    public static class CustomAttributeExtensions
    {
        public static T GetCustomAttribute<T>(this MemberInfo element, bool inherit) where T : Attribute;
    }
}

So:

var attr = typeof(MyClass).GetCustomAttribute<DomainNameAttribute>(false);
return attr != null ? attr.DomainName : "";
Darrel Lee
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    True. But only .NET 4.5 and newer. I'm still developing library code where I can't use this method :( – andreas Sep 21 '18 at 17:45
16
System.Reflection.MemberInfo info = typeof(MyClass);
object[] attributes = info.GetCustomAttributes(true);

for (int i = 0; i < attributes.Length; i++)
{
    if (attributes[i] is DomainNameAttribute)
    {
        System.Console.WriteLine(((DomainNameAttribute) attributes[i]).Name);
    }   
}
Merritt
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7

I used Darin Dimitrov's answer to create a generic extension to get member attributes for any member in a class (instead of attributes for a class). I'm posting it here because others may find it useful:

public static class AttributeExtensions
{
    /// <summary>
    /// Returns the value of a member attribute for any member in a class.
    ///     (a member is a Field, Property, Method, etc...)    
    /// <remarks>
    /// If there is more than one member of the same name in the class, it will return the first one (this applies to overloaded methods)
    /// </remarks>
    /// <example>
    /// Read System.ComponentModel Description Attribute from method 'MyMethodName' in class 'MyClass': 
    ///     var Attribute = typeof(MyClass).GetAttribute("MyMethodName", (DescriptionAttribute d) => d.Description);
    /// </example>
    /// <param name="type">The class that contains the member as a type</param>
    /// <param name="MemberName">Name of the member in the class</param>
    /// <param name="valueSelector">Attribute type and property to get (will return first instance if there are multiple attributes of the same type)</param>
    /// <param name="inherit">true to search this member's inheritance chain to find the attributes; otherwise, false. This parameter is ignored for properties and events</param>
    /// </summary>    
    public static TValue GetAttribute<TAttribute, TValue>(this Type type, string MemberName, Func<TAttribute, TValue> valueSelector, bool inherit = false) where TAttribute : Attribute
    {
        var att = type.GetMember(MemberName).FirstOrDefault().GetCustomAttributes(typeof(TAttribute), inherit).FirstOrDefault() as TAttribute;
        if (att != null)
        {
            return valueSelector(att);
        }
        return default(TValue);
    }
}

Usage example:

//Read System.ComponentModel Description Attribute from method 'MyMethodName' in class 'MyClass'
var Attribute = typeof(MyClass).GetAttribute("MyMethodName", (DescriptionAttribute d) => d.Description);
Sevin7
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  • Inheritance doesn't work on derived properties - for that, you'll need to call a separate static method (System.Attribute.GetCustomAttributes) http://stackoverflow.com/a/7175762/184910 – murraybiscuit May 13 '17 at 06:19
3

A simplified version of Darin Dimitrov's first solution:

public string GetDomainName<T>()
{
    var dnAttribute = typeof(T).GetCustomAttribute<DomainNameAttribute>(true);
    if (dnAttribute != null)
    {
        return dnAttribute.Name;
    }
    return null;
}
jk7
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0
' Simplified Generic version. 
Shared Function GetAttribute(Of TAttribute)(info As MemberInfo) As TAttribute
    Return info.GetCustomAttributes(GetType(TAttribute), _
                                    False).FirstOrDefault()
End Function

' Example usage over PropertyInfo
Dim fieldAttr = GetAttribute(Of DataObjectFieldAttribute)(pInfo)
If fieldAttr IsNot Nothing AndAlso fieldAttr.PrimaryKey Then
    keys.Add(pInfo.Name)
End If

Probably just as easy to use the body of generic function inline. It doesn't make any sense to me to make the function generic over the type MyClass.

string DomainName = GetAttribute<DomainNameAttribute>(typeof(MyClass)).Name
// null reference exception if MyClass doesn't have the attribute.
Darrel Lee
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0

In case anyone needs a nullable result and for this to work across Enums, PropertyInfo and classes, here's how I solved it. This is a modification of Darin Dimitrov's updated solution.

public static object GetAttributeValue<TAttribute, TValue>(this object val, Func<TAttribute, TValue> valueSelector) where TAttribute : Attribute
{
    try
    {
        Type t = val.GetType();
        TAttribute attr;
        if (t.IsEnum && t.GetField(val.ToString()).GetCustomAttributes(typeof(TAttribute), true).FirstOrDefault() is TAttribute att)
        {
            // Applies to Enum values
            attr = att;
        }
        else if (val is PropertyInfo pi && pi.GetCustomAttributes(typeof(TAttribute), true).FirstOrDefault() is TAttribute piAtt)
        {
            // Applies to Properties in a Class
            attr = piAtt;
        }
        else
        {
            // Applies to classes
            attr = (TAttribute)t.GetCustomAttributes(typeof(TAttribute), false).FirstOrDefault();
        }
        return valueSelector(attr);
    }
    catch
    {
        return null;
    }
}

Usage examples:

// Class
SettingsEnum.SettingGroup settingGroup = (SettingsEnum.SettingGroup)(this.GetAttributeValue((SettingGroupAttribute attr) => attr.Value) as SettingsEnum.SettingGroup?);

// Enum
DescriptionAttribute desc = settingGroup.GetAttributeValue((DescriptionAttribute attr) => attr) as DescriptionAttribute;

// PropertyInfo       
foreach (PropertyInfo pi in this.GetType().GetProperties())
{
    string setting = ((SettingsEnum.SettingName)(pi.GetAttributeValue((SettingNameAttribute attr) => attr.Value) as SettingsEnum.SettingName?)).ToString();
}
Mideus
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0

When you have Overridden Methods with same Name Use the helper below

public static TValue GetControllerMethodAttributeValue<T, TT, TAttribute, TValue>(this T type, Expression<Func<T, TT>> exp, Func<TAttribute, TValue> valueSelector) where TAttribute : Attribute
        {
            var memberExpression = exp?.Body as MethodCallExpression;

            if (memberExpression.Method.GetCustomAttributes(typeof(TAttribute), false).FirstOrDefault() is TAttribute attr && valueSelector != null)
            {
                return valueSelector(attr);
            }

            return default(TValue);
        }

Usage: var someController = new SomeController(Some params); var str = typeof(SomeController).GetControllerMethodAttributeValue(x => someController.SomeMethod(It.IsAny()), (RouteAttribute routeAttribute) => routeAttribute.Template);

Vamsi J
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-1

Rather then write a lot of code, just do this:

{         
   dynamic tableNameAttribute = typeof(T).CustomAttributes.FirstOrDefault().ToString();
   dynamic tableName = tableNameAttribute.Substring(tableNameAttribute.LastIndexOf('.'), tableNameAttribute.LastIndexOf('\\'));    
}
Brian Tompsett - 汤莱恩
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Naeem Ahmed
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