或者对抗其他人

或者和 否则有什么区别？

if temp is dbnull.value or temp = 0

产生错误:

运算符“ =”没有为类型“ DBNull”和类型“ Integer”定义。

而这个却像魔法一样有效？

if temp is dbnull.value OrElse temp = 0
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(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...)

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OrElse is short circuited, this means that only one side of the expression will be tested if the first side is a match.

Just like AndAlso will only test one side of the expression if the first half is a fail.

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最佳答案

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.

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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.

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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("dddd");
}
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      "dddd"
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      "dddd"
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

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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.

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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.
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The reason the compilation fails in the example is the order of operations.

The expression parser is trying to evaluate "dbnull.value or temp" first. 

if temp is (dbnull.value or temp) = 0

The error is here, because you can't do a bitwise OR between an integer (temp) and dbnull.value.

OrElse fixes this, not because it's short-circuited, but because it's lower on the order of operations, and so "temp is dbnull.value" and "3=0" are being evaluated first, rather than the parser trying to compare dbNull and temp.

So the evaluation with OrElse works like you're expecting: (assume temp=3)

if temp is dbnull.value OrElse temp = 0 then
if 3 is dbnull.value OrElse 3 = 0 then
if false OrElse 3=0 then
if false OrElse false then
if false then

This was actually on an entry exam at a software company I used to work for, and it was a common problem I used to encounter in VB6. So it's a good idea to parenthesize your sub-expressions when using boolean operators:

This would have compiled properly:

if (temp is dbnull.value) Or (temp = 0) then

Although, as everyone has already pointed out, OrElse and AndAlso are really the correct operators to use in this context.


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