c# - while - order of operations if statement




Execution order of conditions in C# If statement (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.

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

I referred MSDN for checking whether the order of execution of the conditions defined; but I could not find.

Consider a multiple check condition that has && as the logical operator. Is it guaranteed that it will always check the first condition and if that is not satisfied the second condition will not be checked?

I used to use approach 1 and it works well. Looking for an MSDN reference substantiaing the use.

UPDATE

Refer "short-circuit" evaluation

CODE

  List<string> employees = null;  

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

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

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.


The && and || operators short-circuit. That is:

1) If && evaluates its first operand as false, it does not evaluate its second operand.

2) If || evaluates its first operand as true, it does not evaluate its second operand.

This lets you do null check && do something with object, as if it is not null the second operand is not evaluated.


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


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


left to right while expression is still questionable.





short-circuiting