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Q:How do I read an attribute on a class at runtime?

Q:如何在运行时读取类的属性?

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>();

我试图创建一个泛型方法,该方法将读取类的属性并在运行时返回该值。我该怎么做?

注意:域名属性是类domainnameattribute。

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

我试图产生的:

//This should return "MyTable"
String DomainNameValue = GetDomainName<MyClass>();
answer1: 回答1:
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);
public string GetDomainName<T>()
{
    var dnAttribute = typeof(T).GetCustomAttributes(
        typeof(DomainNameAttribute), true
    ).FirstOrDefault() as DomainNameAttribute;
    if (dnAttribute != null)
    {
        return dnAttribute.Name;
    }
    return null;
}

更新:

这种方法可以进一步推广到任何属性:

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);
    }
}

像这样使用:

string name = typeof(MyClass)
    .GetAttributeValue((DomainNameAttribute dna) => dna.Name);
answer2: 回答2:
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);
    }   
}
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);
    }   
}
answer3: 回答3:

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);

我用Darin Dimitrov的回答来创建一个通用的扩展到类中的任何成员的属性(而不是一个类的属性)。我张贴在这里,因为其他人可能会发现它有用:

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);
    }
}

使用示例:

//Read System.ComponentModel Description Attribute from method 'MyMethodName' in class 'MyClass'
var Attribute = typeof(MyClass).GetAttribute("MyMethodName", (DescriptionAttribute d) => d.Description);
answer4: 回答4:

Here's a good tutorial if you haven't seen it before http://msdn.microsoft.com/en-us/library/aa288454(VS.71).aspx

In particular of interest to you is the section here, called Accessing the Attribute http://msdn.microsoft.com/en-us/library/aa288454(VS.71).aspx#vcwlkattributestutorialanchor3

这里是一个很好的教程,如果你以前没见过它:HTTP / MSDN。微软。COM /我们/图书馆/ aa288454(:7100)。aspx

In particular of interest to you is the section here, called Accessing the Attribute http://msdn.microsoft.com/en-us/library/aa288454(VS.71).aspx#vcwlkattributestutorialanchor3

answer5: 回答5:

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;
}

Darin Dimitrov的第一个解决方案的简化版:

public string GetDomainName<T>()
{
    var dnAttribute = typeof(T).GetCustomAttribute<DomainNameAttribute>(true);
    if (dnAttribute != null)
    {
        return dnAttribute.Name;
    }
    return null;
}
answer6: 回答6:

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 : "";

已经有一个扩展来做这个。

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;
    }
}

所以:

var attr = typeof(MyClass).GetCustomAttribute<DomainNameAttribute>(false);
return attr != null ? attr.DomainName : "";
answer7: 回答7:
' Simplified Generic version. 
Shared Function GetAttribute(Of TAttribute)(info As MemberInfo) As TAttribute
    Return info.GetCustomAttributes(GetType(TAttribute), _
            
' Simplified Generic version. 
Shared Function GetAttribute(Of TAttribute)(info As MemberInfo) As TAttribute
    Return info.GetCustomAttributes(GetType(TAttribute), _
            
c#  generics  custom-attributes