C# AND operator with null checking

Question

I read that the AND operator && in .NET conditionally evaluates its second argument.

Is it safe to check for null and then check some field within a single statement?

For example, imagine we have a class SomeClass, which has an integer field Id:

class SomeClass
{
   public Int32 Id { get; set; }
}

Then, we receive an object of this type somewhere and try to perform some actions on it:

public void SomeMethod(SomeClass obj)
{
   if (obj != null)
      if (obj.Id == 1)
      {
         // do some actions
      }
}

Can we rewrite this as follows to reduce the amount of code? Will the null-check be safe?

public void SomeMethod(SomeClass obj)
{
   if (obj != null && obj.Id == 1)
      // do some actions
}

Answer 1

Yes it will be safe, && conditions are treated from "left to right", if the first doesn't match in your case, the second won't be evaluated

msdn

The operation

x && y

corresponds to the operation

x & y

except that if x is false, y is not evaluated, because the result of the AND operation is false no matter what the value of y is. This is known as "short-circuit" evaluation.

Answer 2

Yes, that is absolutely safe to do.

As you state in your question, if the object is null, only the first condition is evaluated, so the second condition (which would raise a NullReferenceException) is never executed. This is known also as "short-circuit" evalation.