Properly implement comparison of two objects with different type but semantically equivalent

Question

I've found a similar question

How to compare two distinctly different objects with similar properties

that may implicitly and/or in part reply to my question.

Suppose I want compare (without a lot of nested conditions) this object:

class ObjectA {
  public string PropertyX { get; set; }
  public char PropertyY { get; set; }
  public long PropertyZ { get; set; }
}

to a System.String. I'm interested only in equality or inequality (not a range of values about identity).

Implementing IEquatable<string> in ObjectA is a proper choice? I don't care of what simply works, I want to identify the proper pattern for such case.

As other information, please consider that ObjectA will often be supplied as sequence of IEnumerable<ObjectA>.

I don't need to know if "string" == or != objectA instance; sorting is not involved.

Edit to clarify (and help)

Sorry but writing a good question is sometime difficult...

Suppose I can't represent ObjectA as string for the purpose of comparison (violating encapsulation is not an option).

• In context-1 I've to match it against PropertyY.

• In context-2 I've to match it against an algorithm applied to PropertyY/PropertyZ.

@Oliver solution in the end of the question helps me again (and +1 again). I can simply define a custom interface:

interface IContextConverter {
  string ToEquatableStringForContext1();
  string ToEquatableStringForContext2();  
}

Since I've also an ObjectB with same logic but different properties, both will implement IContextConverter (or maybe I find a better name) avoiding to violate RAP.

Answer 1

I would strongly recommend to not implement IEquatable<string>, cause especially when working with collections, dictionaries, LINQ, etc. you don't really know when one of these methods will be called somewhere deep inside which leads maybe to subtle bugs.

Due to the fact that you like to compare two objects of different types a simple Comparer<T> wouldn't work also.

So either write a TypeConverter which converts your object into the desired type (in your case a string) or add a method to your object like .ToEquatableString() and use their output to compare your object with the other string.

Here is an example on you could get all elements, that match one of a string in another collection:

IEnumerable<String> otherElements = new[] {"abc", "def", "ghi" };
IEnumerable<ObjectA> myObjects = GetObjects();

var matchesFound = otherElements.Join( // Take the first collection.
              myObjects, // Take the second collection.
              s => s, // Use the elements in the first collection as key (the string).
              obj => obj.ToEquatableString(),  // Create a string from each object for comparison.
              (s, obj) => obj, // From the matching pairs take simply the objects found.
              StringComparer.OrdinalIgnoreCase); // Use a special string comparer if desired.

Answer 2

There are many possibilities.

If you feel an ObjectA is a kind of System.String, you could write a user-defined conversion (implicit or explicit) from ObjectA to System.String, or from System.String to ObjectA, or both directions.

You could also overload the == and != operators with a signature like operator ==(ObjectA oa, string s). Beware that there is difference between oa == s and s == oa.

Either of these two possibilities may lead to confusion. It would also be confusing to override the virtual Equals(object), or to introduce an overload Equals(string). Therefore I don't recommend implementing IEquatable<string>.

Why not simply write a method with an unused name, like public bool EqualsString(string s)? Then you will have to call this method explicitly, of course, but that will lead to less confusion. Another idea would be to use a constructor of signature public ObjectA(string s) and then implement "homogeneous" equality of ObjectA and ObjectA.

Answer 3

NOTE: I do not particulary recommend this solution for this particular case, but I have often used this framework to implement Value Equality for structs. Difficult trade-offs are common in our field, and answers that address those, accompanied by appropriate caveats, seem in order.

I take it that you wish to design Value Equality semamtics on your class in the same fashion as the .NET framework does this for the string class. At a minimum, the following is necessary:

public override bool Equals(object obj) { 
  return (obj is ObjectA) && this == (ObjectA)obj; 
}
bool IEquatable<ObjectA>.Equals(ObjectA rhs) { 
  return this == rhs; 
}
public static bool operator != (ObjectA lhs, ObjectA rhs) { 
  return ! (lhs == rhs); 
}
public static bool operator == (ObjectA lhs, ObjectA rhs) {
  return (lhs.PropertyX == rhs.PropertyX);
}
public override int GetHashCode() { 
  return PropertyX.GetHashCode() 
}

Expanding to allow Value comparisons beween ObjectA and string:

bool IEquatable<ObjectA>.Equals(string rhs) { 
  return this == rhs; 
}
public static bool operator != (ObjectA lhs, string rhs) { 
  return ! (lhs == rhs); 
}
public static bool operator != (string lhs, ObjectA rhs) { 
  return ! (lhs == rhs); 
}
public static bool operator == (ObjectA lhs, string rhs) {
  return (lhs.PropertyX == rhs);
}
public static bool operator == (string lhs, ObjectA rhs) {
  return (lhs == rhs.PropertyX);
}