如何在运行时读取类的属性?


106

我正在尝试创建一个通用方法,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:DomainName属性属于DomainNameAttribute类。

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

我想产生的是:

//This should return "MyTable"
String DomainNameValue = GetDomainName<MyClass>();


2
重要的必然问题是如何使用自定义属性stackoverflow.com/questions/2656189/…
Chris Marisic 2015年

Answers:


235
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:

可以进一步推广此方法以使用任何属性:

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);
    }
}

并像这样使用:

string name = typeof(MyClass)
    .GetAttributeValue((DomainNameAttribute dna) => dna.Name);

6
感谢您在回答问题时的努力!
Zaffiro

1
通过扩展MemberInfo,Type的基类以及Type的所有(或至少大多数)成员,可以进一步推广此扩展方法。这样做将打开此窗口,以允许从“属性”,“字段”甚至“事件”中读取属性。
M.Babcock 2014年

4
太复杂了。无需使用lambda选择属性值。如果您足够写lambda,那么您就足够了解该字段了。
Darrel Lee

我该如何扩展这种方法以进入const Filed静态类?
阿米尔(Amir)

51

已经存在执行此操作的扩展。

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 : "";

1
真正。但是只有.NET 4.5和更高版本。我仍在开发无法使用此方法的库代码:(
andreas

15
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);
    }   
}

5
+1表示不使用“ var”,因此很容易理解它的工作原理。
RenniePet 2014年

它不会编译。但是“ System.Reflection.MemberInfo info = typeof(MyClass).GetTypeInfo();” 做
马塞尔·詹姆斯

4

我使用Darin Dimitrov的答案来创建泛型扩展,以获取类中任何成员的成员属性(而不是类的属性)。我将其发布在这里是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);
    }
}

用法示例:

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

继承对派生属性不起作用-为此,您需要调用一个单独的静态方法(System.Attribute.GetCustomAttributes)stackoverflow.com/a/7175762/184910
murraybiscuit

3

Darin Dimitrov的第一个解决方案的简化版本:

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


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

内联泛型函数的主体可能就很容易使用。对我来说,使函数在MyClass类型上通用没有任何意义。

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

0

如果有人需要可为空的结果,并且该结果可在Enums,PropertyInfo和类中使用,则这是我解决的方法。这是Darin Dimitrov的更新解决方案的修改。

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;
    }
}

用法示例:

// 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();
}

0

而是编写大量代码,只需执行以下操作:

{         
   dynamic tableNameAttribute = typeof(T).CustomAttributes.FirstOrDefault().ToString();
   dynamic tableName = tableNameAttribute.Substring(tableNameAttribute.LastIndexOf('.'), tableNameAttribute.LastIndexOf('\\'));    
}

0

当您具有同名的重写方法时,请使用下面的帮助器

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);
        }

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

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