C＃-在接口的GetCustomAttribute中找不到自定义属性

[AttributeUsageAttribute(AttributeTargets.Method)]
public sealed class AuthorizationAttribute : Attribute
{
    public AuthorizationAttribute(bool required)
    {
        Required = required;
    }

    public bool Required;
}

[OperationContract]
[Authorization(true)]
[WebGet(ResponseFormat = WebMessageFormat.Json, UriTemplate = \"randommethod\")]
ReturnObject RandomMethod();

foreach(object attribute in methodInfo.GetCustomAttributes(true)) // Returns all 3 of my attributes.

if (attribute is AuthorizationAttribute) //Does not pass

Attribute.IsDefined(methodInfo, typeof(AuthorizationAttribute));
attribute.GetType().IsAssignableFrom(typeof(AuthorizationAttribute));

AuthorizationAttribute authAttribute = attribute as AuthorizationAttribute;
Attribute attribute = Attribute.GetCustomAttribute(methodInfo, typeof(AuthorizationAttribute));

[A]Lib.OAuth.AuthorizationAttribute cannot be cast to [B]Lib.OAuth.AuthorizationAttribute. 
Type A originates from \'Lib, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null\' in the
context \'LoadNeither\' at location \'F:\\RestServices\\bin\\Lib.dll\'. Type B originates from \'Lib,
Version=1.0.0.0, Culture=neutral, PublicKeyToken=null\' in the context \'Default\' at location 
\'C:\\Windows\\Microsoft.NET\\Framework64\\v4.0.30319\\Temporary ASP.NET Files\\oauth_rest\\951069b9
\\9f7b77fe\\assembly\\dl3\\54c48906\\f928a6ad_01facb01\\Lib.dll\'.