vb.net 和c#下的三目运算符的区别

一直都以为这两者之间的三目运算符是没有区别的，只是vb是用iif，c#是？：而已。但是上次用了一次vb的iif之后却发现两者之间简直是天渊之别。先看下面的代码

Sub TestIIF()
        Dim d As DBNull = DBNull.Value
        Dim k As Integer = IIf(TypeOf d Is DBNull, 0, Convert.ToInt32(d))
        Console.WriteLine(k)
    End Sub

IL如下：

.method public static void TestIIF() cil managed
{
// 代码大小       53 (0x35)
.maxstack 3
.locals init ([0] class [mscorlib]System.DBNull d,
           [1] int32 k)
IL_0000: nop
IL_0001: ldsfld     class [mscorlib]System.DBNull [mscorlib]System.DBNull::Value
IL_0006: stloc.0
IL_0007: ldloc.0
IL_0008: isinst     [mscorlib]System.DBNull
IL_000d: ldnull
IL_000e: cgt.un
IL_0010: ldc.i4.0
IL_0011: box        [mscorlib]System.Int32
IL_0016: ldloc.0
IL_0017: call       int32 [mscorlib]System.Convert::ToInt32(object)
IL_001c: box        [mscorlib]System.Int32
IL_0021: call       object [Microsoft.VisualBasic]Microsoft.VisualBasic.Interaction::IIf(bool,
                                                                                            object,
                                                                                            object)
IL_0026: call       int32 [Microsoft.VisualBasic]Microsoft.VisualBasic.CompilerServices.Conversions::ToInteger(object)
IL_002b: stloc.1
IL_002c: ldloc.1
IL_002d: call       void [mscorlib]System.Console::WriteLine(int32)
IL_0032: nop
IL_0033: nop
IL_0034: ret
} // end of method Module1::TestIIF

从il可以看出vb的iif是将前后两个表达是计算出来然后装相成object，然后再调用

object [Microsoft.VisualBasic]Microsoft.VisualBasic.Interaction::IIf(bool,object,object)。

所以如果你要执行以下语句

Dim k As Integer = IIf(TypeOf d Is DBNull, 0, Convert.ToInt32(d))

编译器会直接报错,因为要先将dbnull转化为int装箱。

再来看一下c#的三目运算符

代码如下

static void TestThreeMU()
      {
            DBNull d = DBNull.Value;
            int k = d.GetType() == typeof(DBNull) ? 1 : Convert.ToInt32(DBNull.Value);
            Console.WriteLine(k);

}

IL如下

.method private hidebysig static void TestThreeMU() cil managed
{
// 代码大小       47 (0x2f)
.maxstack 2
.locals init ([0] class [mscorlib]System.DBNull d,
           [1] int32 k)
IL_0000: nop
IL_0001: ldsfld     class [mscorlib]System.DBNull [mscorlib]System.DBNull::Value
IL_0006: stloc.0
IL_0007: ldloc.0
IL_0008: callvirt   instance class [mscorlib]System.Type [mscorlib]System.Object::GetType()
IL_000d: ldtoken    [mscorlib]System.DBNull
IL_0012: call       class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle)
IL_0017: beq.s      IL_0025
IL_0019: ldsfld     class [mscorlib]System.DBNull [mscorlib]System.DBNull::Value
IL_001e: call       int32 [mscorlib]System.Convert::ToInt32(object)
IL_0023: br.s       IL_0026
IL_0025: ldc.i4.1
IL_0026: stloc.1
IL_0027: ldloc.1
IL_0028: call       void [mscorlib]System.Console::WriteLine(int32)
IL_002d: nop
IL_002e: ret
} // end of method Program::TestThreeMU