定义TestClass，在调用的地方
TestClass a;
如果下面有引用
a.Property，VS会报错，而如果
TestClass a = null;
再次调用的话则不会报错。
TestClass a; DebugIL代码:
.method private hidebysig static void Main(string[] args) cil managed
{
.entrypoint
// Code size 2 (0x2)
.maxstack 0
.locals init ([0] class ConsoleApplication1.TestClass a)
IL_0000: nop
IL_0001: ret
} // end of method Program::Main
TestClass a = null; DebugIL代码:
.method private hidebysig static void Main(string[] args) cil managed
{
.entrypoint
// Code size 4 (0x4)
.maxstack 1
.locals init ([0] class ConsoleApplication1.TestClass a)
IL_0000: nop
IL_0001: ldnull
IL_0002: stloc.0
IL_0003: ret
} // end of method Program::Main
TestClass a; Release IL代码,VS会优化掉该语句
.method private hidebysig static void Main(string[] args) cil managed
{
.entrypoint
// Code size 1 (0x1)
.maxstack 8
IL_0000: ret
} // end of method Program::Main
TestClass a = null; Release IL代码，VS同样会优化掉该句
.method private hidebysig static void Main(string[] args) cil managed
{
.entrypoint
// Code size 1 (0x1)
.maxstack 8
IL_0000: ret
} // end of method Program::Main
先Mark，以后在具体研究区别
原文：http://www.cnblogs.com/NailClipper/p/4643064.html