C# 构造函数调用顺序

了解C#的一个类的加载过程，对于语言机制的理解和写出高效的语言很有帮助，这里简单介绍一下类的实例的构造函数调用过程。
C#类的实例的构造过程是，先为实例的数据字段分配内存，并对所有字段按字节置零（0或者null）；然后初始化附加内存（类型对象指针和同步块索引）；调用类型的实例构造器（也就是new关键字调用的普通构造函数）初始化字段。
假如有两个类：基类BaseClass和派生类DerivedClass。DerivedClass继承BaseClass。

namespace GenericSyntax.Constructor
{
    class BaseClass
    {
        public BaseClass()
        {
            Console.WriteLine("Base class Ctor.");
        }
    }

    class DerivedClass:BaseClass
    {
        private string m_Var1 = "Assigned in declaration.";
        private string m_Var2;
        public DerivedClass()
        {
            Console.WriteLine(m_Var2==null);
            Console.WriteLine(m_Var1);
            m_Var1 = "Assigned in constructor of derived Class.";
            Console.WriteLine("Derived class Ctor.");
        }
        public override string ToString()
        {
            return m_Var1;
        }
    }
}
namespace GenericSyntax
{
    class Program
    {
        static void Main(string[] args)
        {
            TestCtor();
            Console.ReadKey();
        }
        private static void TestCtor()
        {
            Constructor.DerivedClass instance = new             
            Constructor.DerivedClass();
            Console.WriteLine(instance.ToString());
        }
    }
}        

生成的BaseClass的IL和DerivedClass的IL如下：

.method public hidebysig specialname rtspecialname 
        instance void  .ctor() cil managed
{
  // 代码大小       20 (0x14)
  .maxstack  8
  IL_0000:  ldarg.0
  IL_0001:  call       instance void [mscorlib]System.Object::.ctor()
  IL_0006:  nop
  IL_0007:  nop
  IL_0008:  ldstr      "Base class Ctor."
  IL_000d:  call       void [mscorlib]System.Console::WriteLine(string)
  IL_0012:  nop
  IL_0013:  ret
} // end of method BaseClass::.ctor

.method public hidebysig specialname rtspecialname 
        instance void  .ctor() cil managed
{
  // 代码大小       69 (0x45)
  .maxstack  2
  IL_0000:  ldarg.0
  IL_0001:  ldstr      "Assigned in declaration."
  IL_0006:  stfld      string GenericSyntax.Constructor.DerivedClass::m_Var1
  IL_000b:  ldarg.0
  IL_000c:  call       instance void GenericSyntax.Constructor.BaseClass::.ctor()
  IL_0011:  nop
  IL_0012:  nop
  IL_0013:  ldarg.0
  IL_0014:  ldfld      string GenericSyntax.Constructor.DerivedClass::m_Var2
  IL_0019:  ldnull
  IL_001a:  ceq
  IL_001c:  call       void [mscorlib]System.Console::WriteLine(bool)
  IL_0021:  nop
  IL_0022:  ldarg.0
  IL_0023:  ldfld      string GenericSyntax.Constructor.DerivedClass::m_Var1
  IL_0028:  call       void [mscorlib]System.Console::WriteLine(string)
  IL_002d:  nop
  IL_002e:  ldarg.0
  IL_002f:  ldstr      "Assigned in constructor of derived Class."
  IL_0034:  stfld      string GenericSyntax.Constructor.DerivedClass::m_Var1
  IL_0039:  ldstr      "Derived class Ctor."
  IL_003e:  call       void [mscorlib]System.Console::WriteLine(string)
  IL_0043:  nop
  IL_0044:  ret
} // end of method DerivedClass::.ctor

可以看出，则初始化DerivedClass时，DerivedClass的构造函数的IL代码顺序执行如下
1. 解析派生类实例字段的直接赋值语句，即：如果字段有直接赋值语句，则用该赋值语句或者字面量对字段进行赋值。
2. 调用基类的构造函数（逐级向上调用，知道Object的构造函数），基类的实例化过程同理。
3. 执行派生类该实例构造函数的函数体。

执行代码如下：