c# Iterate array of parent type to invoke non-polymorphic method on derived type

Question

I am using very similar loops to iterate all public fields and properties of any passed object. I determine if the field/property is decorated with a particular custom attribute; if so, an action is performed on the value of the field or property. Two loops are necessary because the method to get a field value is different from the method to get a property value.

// Iterate all public fields using reflection
foreach (FieldInfo fi in obj.GetType().GetFields())
{
  // Determine if decorated with MyAttribute.
  var attribs = fi.GetCustomAttributes(typeof(MyAttribute), true);
  if (attribs.Length == 1)
  {
    // Get value of field.
    object value = fi.GetValue(obj);
    DoAction(value);
  }
}

// Iterate all public properties using reflection
foreach (PropertyInfo pi in obj.GetType().GetProperties())
{
  // Determine if decorated with MyAttribute.
  var attribs = pi.GetCustomAttributes(typeof(MyAttribute), true);
  if (attribs.Length == 1)
  {
    // Get value of property.
    object value = pi.GetValue(obj, null);
    DoAction(value);
  }
}

I would like to place the loop in a single, common method so that I can instead write, more simply:

DoEachMember(obj.GetType().GetFields());
DoEachMember(obj.GetType().GetProperties());

This requires DoEachMember() to accept the MemberInfo type (which is the parent type of both FieldInfo and PropertyInfo). The problem is there is no GetValue method in the MemberInfo class. Both FieldInfo and PropertyInfo use different methods to get the field/property value:

public void DoEachMember(MemberInfo mi, object obj)
{
  foreach (MemberInfo mi in obj.GetType().GetProperties())
  {
    object value mi.GetValue(obj); // NO SUCH METHOD!
  }
}

Thus, I declare a delegate to utilize inside the loop which takes a MemberInfo and returns the value of that member as an object:

// Delegate to get value from field or property.
delegate object GetValue(MemberInfo mi, object obj);

The Question

How can I detect the type of the objects in the members[] array, in order to define the delegate used inside the loop? Currently, I am using the first element of the array, members[0]. Is this a good design?

public void DoEachMember(MemberInfo[] members, object obj)
{
  // Protect against empty array.
  if (members.Length == 0) return;

  GetValue getValue; // define delegate

  // First element is FieldInfo
  if (members[0] as FieldInfo != null)
    getValue = (mi, obj) => ((FieldInfo)mi).GetValue(obj);

  // First element is PropertyInfo
  else if (members[0] as PropertyInfo != null)
    getValue = (mi, obj) => ((PropertyInfo)mi).GetValue(obj, null);

  // Anything else is unacceptable
  else
    throw new ArgumentException("Must be field or property.");

  foreach (MemberInfo mi in members)
  {
    // Determine if decorated with MyAttribute.
    var attribs = mi.GetCustomAttributes(typeof(MyAttribute), true);
    if (attribs.Length == 1)
    {
      object value = getValue(mi, obj);
      DoStuff(value);
    }
  }
}

Alternatively, I could detect the type upon each iteration, but there should be no reason individual array members will ever differ:

foreach (MemberInfo mi in members)
{
  // ...

  object value;

  if ((var fi = mi as FieldInfo) != null)
    value = fi.GetValue(obj);

  else if ((var pi = mi as PropertyInfo) != null)
    value = pi.GetValue(obj, null);

  else
    throw new ArgumentException("Must be field or property.");

  DoStuff(value);
}

Answer 1

You could project to the object values first and then work on those in your loop. Your whole code could be boiled down to this (plus your loop):

    /// <summary>
    /// Gets the value of all the fields or properties on an object that are decorated with the specified attribute
    /// </summary>
    private IEnumerable<object> GetValuesFromAttributedMembers<TAttribute>(object obj)
        where TAttribute : Attribute
    {
        var values1 = obj.GetType().GetFields()
                        .Where(fi => fi.GetCustomAttributes(typeof(TAttribute), true).Any())
                        .Select(fi => fi.GetValue(obj));
        var values2 = obj.GetType().GetProperties()
                        .Where(pi => pi.GetCustomAttributes(typeof(TAttribute), true).Any())
                        .Select(pi => pi.GetValue(obj, null));
        return values1.Concat(values2);
    }

Your current code mixes two concerns: finding the values and doing something with them. It would be cleaner to separate those concerns. The above LINQ could be placed in one method that fetches all values from a class that are in fields or properties that match a given attribute and another than is just a loop doing the work on whatever it is passed.

Not as clean but sticking with your original goal you could do this and pass in a delegate appropriate to the type of the MemberInfo you are retrieving:-

    public void DoEachMember<TAttribute, TMembertype>(IEnumerable<TMembertype> members,
                             Func<TMembertype, object> valueGetter)
        where TMembertype : MemberInfo
    {
        foreach (var mi in members)
        {
            if (mi.GetCustomAttributes(typeof(TAttribute), true).Any())
            {
                // Get value of field.
                object value = valueGetter(mi);
                DoAction(value);
            }
        }
    }