Comparing two instances of a class

Question

I have a class like this

public class TestData
{
   public string Name {get;set;}
   public string type {get;set;}

   public List<string> Members = new List<string>();

   public void AddMembers(string[] members)
   {
      Members.AddRange(members);
   }   
}

I want to know if it is possible to directly compare to instances of this class to eachother and find out they are exactly the same? what is the mechanism? I am looking gor something like if(testData1 == testData2) //Do Something And if not, how to do so?

Answer 1

Implement the IEquatable<T> interface. This defines a generalized method that a value type or class implements to create a type-specific method for determining equality of instances. More information here:

http://msdn.microsoft.com/en-us/library/ms131187.aspx

Answer 2

First of all equality is difficult to define and only you can define as to what equality means for you

1. Does it means members have same value
2. Or they are pointing to same location.

Here is a discussion and an answer here

What is "Best Practice" For Comparing Two Instances of a Reference Type?

Answer 3

You will need to define the rules that make object A equal to object B and then override the Equals operator for this type.

http://msdn.microsoft.com/en-us/library/ms173147(v=vs.80).aspx

Answer 4

I see many good answers here but just in case you want the comparison to work like

if(testData1 == testData2) // DoSomething

instead of using Equals function you can override == and != operators:

public static bool operator == (TestData left, TestData right)
{
    bool comparison = true; //Make the desired comparison
    return comparison;
}

public static bool operator != (TestData left, TestData right)
{
    return !(left == right);
}

Answer 5

There are three ways objects of some reference type T can be compared to each other:

1. With the object.Equals method
2. With an implementation of IEquatable<T>.Equals (only for types that implement IEquatable<T>)
3. With the comparison operator ==

Furthermore, there are two possibilities for each of these cases:

1. The static type of the objects being compared is T (or some other base of T)
2. The static type of the objects being compared is object

The rules you absolutely need to know are:

• The default for both Equals and operator== is to test for reference equality
• Implementations of Equals will work correctly no matter what the static type of the objects being compared is
• IEquatable<T>.Equals should always behave the same as object.Equals, but if the static type of the objects is T it will offer slightly better performance

So what does all of this mean in practice?

As a rule of thumb you should use Equals to check for equality (overriding object.Equals as necessary) and implement IEquatable<T> as well to provide slightly better performance. In this case object.Equals should be implemented in terms of IEquatable<T>.Equals.

For some specific types (such as System.String) it's also acceptable to use operator==, although you have to be careful not to make "polymorphic comparisons". The Equals methods, on the other hand, will work correctly even if you do make such comparisons.

You can see an example of polymorphic comparison and why it can be a problem here.

Finally, never forget that if you override object.Equals you must also override object.GetHashCode accordingly.

Answer 6

You can override the equals method and inside it manually compare the objects

Also take a look at Guidelines for Overloading Equals() and Operator ==