Hi I have a class with 6 string properties. A unique object will have different values for atleast one of these fields
To implement IEqualityComparer's GetHashCode function, I am concatenating all 6 properties and calling the GetHashCode on the resultant string.
I had the following doubts:
If your string fields are named a-f and known not to be null, this is ReSharper's proposal for your GetHashCode()
public override int GetHashCode() {
unchecked {
int result=a.GetHashCode();
result=(result*397)^b.GetHashCode();
result=(result*397)^c.GetHashCode();
result=(result*397)^d.GetHashCode();
result=(result*397)^e.GetHashCode();
result=(result*397)^f.GetHashCode();
return result;
}
}
GetHashCode does not need to return unequal values for "unequal" objects. It only needs to return equal values for equal objects (it also must return the same value for the lifetime of the object).
This means that:
Equals, then their GetHashCode must return the same value.GetHashCode implementation.If you cannot satisfy both points at the same time, you should re-evaluate your design because anything else will leave the door open for bugs.
Finally, you could probably make GetHashCode faster by calling GetHashCode on each of the 6 strings and then integrating all 6 results in one value using some bitwise operations.
readonly backing fields if you wanted to), which is sufficient. You 'll be OK.
GetHashCode() implementation, I see that it is a problem with conflicting requirements. In fact, you're only partly correct. The real requirements for GetHashCode() method are: 1. It should return same values for equal objects. 2. It should return the same value for the same object while it isn't modified. 3. The GetHashCode() should be fast. Agree, that there are some other recommendations for it. For example this one: For the best performance, a hash function should generate a random distribution for all input.return 42; is a "correct" implementation in the technical sense, but it will make your performance suffer if you put these objects into dictionaries.GetHashCode() should return the same hash code for all objects that return true if you call Equals() on those objects. This means, for example, that you can return zero as the hash code regardless of what the field values are. But that would make your object very inefficient when stored in data structures such as hash tables.
Combining the strings is one option, but note that you could for example combine just two of the stringsfor the hash code (while still comparing all the strings in equals!).
You can also combine the hashes of the six separate strings, rather than computing a single hash for a combined string. See for example Quick and Simple Hash Code Combinations
I'm not sure if this will be significantly faster than concatenating the string.
You can use the behavior from:
