Why C# local variables must be initialized?

Question

I am reading MCTS Self Paced Training Kit (70-536) Edition 2 and in the 1st chapter we have the following.

How to Declare a Value Type Variable To

Answer 1

You need to assign a value to b

bool b = false;

Until you assign it a value it is "unassigned"

Answer 2

You need to assign something to b first otherwise it doesn't get initialized.

try:

bool b = false; 
Console.WriteLine("The value of b is " + b); 

b is now false.

Answer 3

Local variables have to be assigned before they can be used. Class fields however get their default value.

An example:

public bool MyMethod()
{
    bool a;

    Console.Write(a); // This is NOT OK.

    bool b = false;

    Console.Write(b); // This is OK.
}

class MyClass
{
    private bool _a;

    public void MyMethod()
    {
        Console.Write(_a); // This is OK.
    }
}

Answer 4

In C# local variables are stored in the stack and the compiler does not initialize them to have code optimization.

So the reference, that is in fact a pointer even for value types, points to a undefinded memory space.

Thus if we use a local variable without setting it to someting before, the compiler know that we will get random data.

Here a sample IL code:

static void Test()
{
  int a;
  int b = 2;
}

.method private hidebysig static 
  void Test () cil managed 
{
  // Method begins at RVA 0x3044
  // Code size 4 (0x4)
  .maxstack 1
  .locals init (
    [0] int32 a,
    [1] int32 b
  )

  // (no C# code)
  IL_0000: nop
  // int num = 2;
  IL_0001: ldc.i4.2
  IL_0002: stloc.1
  // }
  IL_0003: ret
} // end of method Program::Test

Int32 is defined in the stack at position 0 for a but it is not assigned to a default value.

So its value is undetermined and can have any value from the memory cells.

For b at position 1, it is assigned later and all is fine.

Answer 5

This statement really should be elaborated to indicate that, although a local variable can be declared without assigning it a value, it cannot be used until it has been assigned an initial value:

The constructor assigns a default value (usually null or 0) to the new instance, but you should always explicitly initialize the variable within the declaration...

Answer 6

A variable in a method (method scope) needs to be initialized explicitly. A variable (or 'field') at class level is initialized automatically with the default value.

class Test
{
   bool b; // =false
   int i; // =0
}