Execution order of conditions in C# If statement

Question

There are two if statements below that have multiple conditions using logical operators. Logically both are same but the order of check differs. The first one works and the

Answer 1

See, for example, this MSDN page for && which describes the short-circuit evaluation.

You can check or prove execution sequence like this:

int i;
bool b;
b=((i=3)==0 && (i=4)!=0);
Console.WriteLine(i);
b=((i=3)!=0 || (i=4)!=0);
Console.WriteLine(i);

You get 3 in both cases - which shows, that in both cases the short-circuit behaviour takes place. On the other hand, you could use the '&' or '|', respectively, logical operator, to prevent that. Such you will get a result of 4, because both conditions have been evaluated.

Answer 2

The conditional-AND operator (&&) performs a logical-AND of its bool operands, but only evaluates its second operand if necessary.

Answer 3

The && and || operators are often used to check for object conditions.

1) The "&&" condition evaluates its first operand as false, it does not evaluate its second operand. If it returns true, the second condition evaluates. If second condition is true, then only it will return true. So && can be used to make sure that all conditions are satisfied as valid.

2) The "||" condition evaluates its first operand as true, it does not evaluate its second operand. If first condition evaluates as false, then only it will evaluate to second condition. If it is satisfied, then it will return true. Otherwise, false.

Answer 4

You should use:

  if (employees != null && employees.Count > 0)
  {
        string theEmployee = employees[0];
  }

&& will shortcircuit and employees.Count will not be execucted if employees is null.

In your second example, the application will throw an exception if employees is null when you attempt to Count the elements in the collection.

http://msdn.microsoft.com/en-us/library/2a723cdk(v=vs.71).aspx

Answer 5

left to right while expression is still questionable.

Answer 6

Is it guaranteed that it will always check the first condition and if that is not satisfied the second condition will not be checked?

Short answer is yes.