Check if property has attribute

Question

Given a property in a class  with attributes - what is the fastest way to determine if it contains a given attribute  For example        IsNotNullable       IsPK       IsIdentity       SequenceNameAttribute  Id        public Int32 Id               get                       return  Id                    set                        Id   value                    What is the fastest method to determine that for example it has the  IsIdentity  attribute

User · Answer

If you are trying to do that in a Portable Class Library PCL (like me), then here is how you can do it :)

public class Foo
{
   public string A {get;set;}

   [Special]
   public string B {get;set;}   
}

var type = typeof(Foo);

var specialProperties = type.GetRuntimeProperties()
     .Where(pi => pi.PropertyType == typeof (string) 
      && pi.GetCustomAttributes<Special>(true).Any());

You can then check on the number of properties that have this special property if you need to.

User · Answer

You can use a common  generic  method to read attribute over a given MemberInfo  public static bool TryGetAttribute lt T gt  MemberInfo memberInfo  out T customAttribute  where T  Attribute                   var attributes   memberInfo GetCustomAttributes typeof T   false  FirstOrDefault                    if  attributes    null                        customAttribute   null                      return false                                    customAttribute    T attributes                  return true

User · Answer

This can now be done without expression trees and extension methods in a type safe manner with the new C  feature nameof   like this   Attribute IsDefined typeof YourClass  GetProperty nameof YourClass Id    typeof IsIdentity      nameof   was introduced in C  6

User · Answer

If you are using  NET 3 5 you might try with Expression trees  It is safer than reflection   class CustomAttribute   Attribute      class Program        Custom      public int Id   get  set         static void Main                 Expression lt Func lt Program  int gt  gt  expression   p   gt  p Id          var memberExpression    MemberExpression expression Body          bool hasCustomAttribute   memberExpression              Member              GetCustomAttributes typeof CustomAttribute   false  Length  gt  0

User · Answer

To update and or enhance the answer by  Hans Passant I would separate the retrieval of the property into an extension method   This has the added benefit of removing the nasty magic string in the method GetProperty    public static class PropertyHelper lt T gt        public static PropertyInfo GetProperty lt TValue gt           Expression lt Func lt T  TValue gt  gt  selector                Expression body   selector          if  body is LambdaExpression                        body     LambdaExpression body  Body                    switch  body NodeType                        case ExpressionType MemberAccess                  return  PropertyInfo   MemberExpression body  Member              default                  throw new InvalidOperationException                        Your test is then reduced to two lines  var property   PropertyHelper lt MyClass gt  GetProperty x   gt  x MyProperty   Attribute IsDefined property  typeof MyPropertyAttribute

User · Answer

This is a pretty old question but I used  My method has this parameter but it could be built   Expression lt Func lt TModel  TValue gt  gt  expression   Then in the method this   System Linq Expressions MemberExpression memberExpression           expression Body as System Linq Expressions MemberExpression  Boolean hasIdentityAttr   System Attribute         IsDefined memberExpression Member  typeof IsIdentity

User · Answer

You can use the Attribute IsDefined method   https   msdn microsoft com en-us library system attribute isdefined v vs 110  aspx   if Attribute IsDefined YourProperty typeof YourAttribute            Conditional execution        You could provide the property you re specifically looking for or you could iterate through all of them using reflection  something like   PropertyInfo   props   typeof YourClass  GetProperties

User · Answer

There s no fast way to retrieve attributes   But code ought to look like this  credit to Aaronaught    var t   typeof YourClass   var pi   t GetProperty  Id    var hasIsIdentity   Attribute IsDefined pi  typeof IsIdentity      If you need to retrieve attribute properties then  var t   typeof YourClass   var pi   t GetProperty  Id    var attr    IsIdentity   pi GetCustomAttributes typeof IsIdentity   false   if  attr Length  gt  0           Use attr 0   you ll need foreach on attr if MultiUse is true

[c#] Check if property has attribute

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