Today at work we found a bug. My workmate, not used to C#, usually uses the & operator to compare boolean values. However, in C#, the & operator does not use lazy evaluation.

One curious thing about C# is that it can use two different operators to calculate an and expression: the & operator and the && operator. The difference between both is that the first one (&) can be used both with integer types and boolean types. When used with integer types it will perform a bitwise comparison between the two, and when used with boolean values it will use the logical and operation between the two boolean values, evaluating all the parts of the expression. This means that using a code like this one:

if (someObject != null & someObject.SomeProperty == someValue)

will throw a runtime error if someObject is null, because it will try to obtain the SomeProperty value.

However, the && operator is only available to boolean expressions, and it uses lazy evaluation, this is, if the first condition evaluated is false, it will calculate false without evaluating the rest of the expression, because an and is only true if all the expressions are true.

Conclusion, be sure to always use && when evaluating boolean values if you want to avoid run time surprises :).