Effective C#

Item 10 and 11

Understand the Pitfalls of GetHashCode

Item 10

Where to use?

• Define the hash value for keys in a hash-based collection– Hashtable– Dictionary

Three rules

• If two objects are equal (as defined by operator==), they must generate the same hash value

• For any object A, A.GetHashCode() must be an instance invariant

• The hash function should generate a random distribution among all integers for all inputs

Issues with Object.GetHashCode()

• Rule 1: Pass• Rule 2: Pass• Rule 3: Failed

ValueType.operator==() compares the first field in the struct

The following code snippet always returns true

Check 3 rules again

• Rule 1: Pass *• Rule 2: Pass *• Rule 3: Pass *

Modify previous code

Review Rule 1

• if two objects are equal, as defined by operator==(), they must return the same hash value

• => The same data elements should participate in both computations

Review Rule 2

• the return value of GetHashCode() must be an instance invariant

Hash is changed by data member

Improved by immutability of the properties

Review Rule 3

• GetHashCode() should generate a random distribution among all integers for all inputs– A common and successful algorithm is to XOR all

the return values from GetHashCode() on all fields in a type

– exclude mutable fields from the calculations.

Prefer foreach Loops

Item 11

Loops

BEST

WORST

Loop3: one bounds check for the price of two

foreach always generates the best code (1/2)

foreach always generates the best code (2/2)

Automatically casts each operand

Any hand-coded for loops are broken

Example

Resource management

Example

Benefits

• generates the right code for upper and lower bounds in arrays

• iterates multidimensional arrays• coerces the operands into the proper type• generates the most efficient looping

constructs

Weak points

• Can’t modify enumeration member

• Can’t delete member in the conllection