Or versus OrElse

Question

What\'s the difference between or and OrElse?

if temp is dbnull.value or temp = 0

produces the error:

Answer 1

Unless your code logic requires the short-circuiting behavior OrElse provides, I would lean toward using the Or operator because:

• Using "Or" is simple and requires less typing.
• The computational time savings of using OrElse is negligible in most cases.
• Most importantly, using OrElse can hide errors in later clauses that may not be initially revealed until those conditions would eventually be met by the program logic.

Answer 2

The Bert' s answer is not very accurate. The '|' or '&' is logical operator, in C #, it always treat as bit operator, please see the following code as example

        static void Main()
        {
            object a = null;
            int b = 3;
            if (a == null | a.ToString() == "sdffd")
            {
                Console.WriteLine("ffffdd");
            }
            Console.WriteLine(b | b);
            Console.Read();
        }

The following is IL

    .method private hidebysig static void  Main() cil managed
{
  .entrypoint
  // Code size       62 (0x3e)
  .maxstack  3
  .locals init ([0] object a,
           [1] int32 b,
           [2] bool CS$4$0000)
   IL_0000:  nop
   IL_0001:  ldnull
   IL_0002:  stloc.0
   IL_0003:  ldc.i4.3
   IL_0004:  stloc.1
   IL_0005:  ldloc.0
   IL_0006:  ldnull
   IL_0007:  ceq
   IL_0009:  ldloc.0
   IL_000a:  callvirt   instance string [mscorlib]System.Object::ToString()
   IL_000f:  ldstr      "sdffd"
   IL_0014:  call       bool [mscorlib]System.String::op_Equality(string,
                                                                 string)
   IL_0019:  or
   IL_001a:  ldc.i4.0
   IL_001b:  ceq
   IL_001d:  stloc.2
   IL_001e:  ldloc.2
   IL_001f:  brtrue.s   IL_002e
   IL_0021:  nop
   IL_0022:  ldstr      "ffffdd"
   IL_0027:  call       void [mscorlib]System.Console::WriteLine(string)
   IL_002c:  nop
   IL_002d:  nop
   IL_002e:  ldloc.1
   IL_002f:  ldloc.1
   IL_0030:  or
   IL_0031:  call       void [mscorlib]System.Console::WriteLine(int32)
   IL_0036:  nop
   IL_0037:  call       int32 [mscorlib]System.Console::Read()
   IL_003c:  pop
   IL_003d:  ret
    } // end of method Program::Main

when you use || to test "a == null" and "a.ToString() == "sdffd", the IL will be

 .method private hidebysig static void  Main() cil managed
{
  .entrypoint
  // Code size       63 (0x3f)
  .maxstack  2
  .locals init ([0] object a,
           [1] int32 b,
           [2] bool CS$4$0000)
  IL_0000:  nop
  IL_0001:  ldnull
  IL_0002:  stloc.0
  IL_0003:  ldc.i4.3
  IL_0004:  stloc.1
  IL_0005:  ldloc.0
  IL_0006:  brfalse.s  IL_001d
  IL_0008:  ldloc.0
  IL_0009:  callvirt   instance string [mscorlib]System.Object::ToString()
  IL_000e:  ldstr      "sdffd"
  IL_0013:  call       bool [mscorlib]System.String::op_Equality(string,
                                                                 string)
  IL_0018:  ldc.i4.0
  IL_0019:  ceq
  IL_001b:  br.s       IL_001e
  IL_001d:  ldc.i4.0
  IL_001e:  stloc.2
  IL_001f:  ldloc.2
  IL_0020:  brtrue.s   IL_002f
  IL_0022:  nop
  IL_0023:  ldstr      "ffffdd"
  IL_0028:  call       void [mscorlib]System.Console::WriteLine(string)
  IL_002d:  nop
  IL_002e:  nop
  IL_002f:  ldloc.1
  IL_0030:  ldloc.1
  IL_0031:  or
  IL_0032:  call       void [mscorlib]System.Console::WriteLine(int32)
  IL_0037:  nop
  IL_0038:  call       int32 [mscorlib]System.Console::Read()
  IL_003d:  pop
  IL_003e:  ret
} // end of method Program::Main

Now you can see the difference, please don't think the '|' or 'and' as conditional operator, it just a logical operator, I don't think there is necessary to use it to judge condition

Answer 3

OrElse is a short-circuiting operator, Or is not.

By the definition of the boolean 'or' operator, if the first term is True then the whole is definitely true - so we don't need to evaluate the second term.

OrElse knows this, so doesn't try and evaluate temp = 0 once it's established that temp Is DBNull.Value

Or doesn't know this, and will always attempt to evaluate both terms. When temp Is DBNull.Value, it can't be compared to zero, so it falls over.

You should use... well, whichever one makes sense.

Answer 4

(I've looked at other answers and realized I was terribly wrong)

The OrElse operator "performs short-circuiting logical disjunction on two expressions", that is to say: if the left operand is true and so the entire expression is guaranteed to be true the right operand won't even be evaluated (this is useful in cases like:

string a;
//...
if (a is null) or (a = "Hi") //...

to avoid a NullReferenceException throw by the right-hand operand.

I'm sincerely astonished that this (lazy evaluation) isn't the default behaviour of or and and as it is in C/C++ and C# (and many other languages...)

Answer 5

OrElse evaluate first expression then if its true it will proceed to the statement while OR evaluates two expressions before it will proceed to their statement.

Example:

Textbox1.Text= 4

Textbox2.Text= ""

Using OrElse

  If TextBox1.Text > 2 OrElse TextBox2.Text > 3 Then
      MsgBox("True")
  End If

Result is: TRUE

---

Using OR

 If TextBox1.Text > 2 Or TextBox2.Text > 3 Then

            MsgBox("True")
  End If

Result is: Error cannot convert string to double.

Answer 6

This is the same behaviour as with C#, where everyone uses the Coditional Or (||) and the Conditional And (&&), where you also have the normal Or (|) and normal And (&). So comparing C# to VB.Net is:

| => Or

|| => OrElse

& => And

&& => AndAlso

The condifitonal boolean operators are very usefull preventing nested if constructions. But sometimes the normal boolean operators are needed to ensure hitting both code paths.