Question

Could anybody help me to understand, how the GetHashCode() method works for string values?

From MSDN I found:

If two string objects are equal, the GetHashCode method returns identical values. However, there is not a unique hash code value for each unique string value. Different strings can return the same hash code.

So, different strings can return the same hash code, hashcode is not unique for strings. Can it lead to bugs in the core of a program?

Answer 1

Well, this can lead to bugs, if your algorithms rely on each string to have a unique hash-value.

For example a hash map (Dictionary in .NET) might fail on collisions (i.e. two objects with the same hash that are not equal), or it doesn’t if it handles collisions, that depends on the exact implementation. Fail in that case means: If you add a new object to the map and there is already an object in the map that has the same hash value as the new object, then the new object will override the old one instead of just being added. As far as I know the Dictionary class in .NET can handle collisions though.

If you need more concrete advice you’ll need to ask a more concrete question: what are you trying to archive, how are you planning to do it etc.

As a side-note: hash values for strings are usually not unique as the size of a hash value is limited, whereas the length of a string is not. Think of it like this: Say the hash function is MD5 (it’s not the default in .Net) and you have a string both consisting of hex-dec chars (0-9A-Z) and the string is 200 characters long: there are 200^16 possible values for the string, but only 32^16 possible values for its hash-value.