Are C# uninitialized variables dangerous?

Question

I'm familiar with the C# spec, section 5.3 which says that a variable has to be assigned before use.

In C and unmanaged C++ this makes sense as the stack isn't cleared and the memory location used for a pointer could be anywhere (leading to hard to track down bug).

But I am under the impression that there are not truly "unassigned" values allowed by the runtime. In particular that a reference type that is not initialized will always have a null value, never the value left over from a previous invocation of the method or random value.

Is this correct, or have I been mistakenly assuming that a check for null is sufficient all these years? Can you have trully unintialized variables in C#, or does the CLR take care of this and there's always SOME value set.

Answer 1

All primitive data types have default values so no need to worry for them.

All reference types are initialized to null values so if you leave your reference types uninitialized and then call some method or property on that null ref type you would get a runtime exception which would need to be handled gracefully.

Again, all Nullable types need to be checked for null or default value if they are not initialized as follows:

    int? num = null;
    if (num.HasValue == true)
    {
        System.Console.WriteLine("num = " + num.Value);
    }
    else
    {
        System.Console.WriteLine("num = Null");
    }

    //y is set to zero
    int y = num.GetValueOrDefault();

    // num.Value throws an InvalidOperationException if num.HasValue is false
    try
    {
        y = num.Value;
    }
    catch (System.InvalidOperationException e)
    {
        System.Console.WriteLine(e.Message);
    }

But, you will not get any complile error if you leave all your variables uninitialized as the compiler wont complain, its only the run-time you need to worry about.

Answer 2

I am under the impression that there are not truly "unassigned" values allowed by the runtime. In particular that a reference type that is not initialized will always have a null value, never the value left over from a previous invocation of the method or random value. Is this correct?

I note that no one has actually answered your question yet.

The answer to the question you actually asked is "sorta".

As others have noted, some variables (array elements, fields, and so on) are classified as being automatically "initially assigned" to their default value. (Which is null for reference types, zero for numeric types, false for bools, and the natural recursion for user-defined structs).

Some variables are not classified as initially assigned; local variables in particular are not initially assigned. They must be classified by the compiler as "definitely assigned" at all points where their values are used.

Your question then is actually "is a local variable that is classified as not definitely assigned actually initially assigned the same way that a field would be?" And the answer to that question is yes, in practice, the runtime initially assigns all locals.

This has several nice properties. First, you can observe them in the debugger to be in their default state before their first assignment. Second, there is no chance that the garbage collector will be tricked into dereferencing a bad pointer just because there was garbage left on the stack that is now being treated as a managed reference. And so on.

The runtime is permitted to leave the initial state of locals as whatever garbage happened to be there if it can do so safely. But as an implementation detail, it does not ever choose to do so. It zeros out the memory for a local variable aggressively.

The reason then for the rule that locals must be definitely assigned before they are used is not to prevent you from observing the garbage uninitialized state of the local. That is already unobservable because the CLR aggressively clears locals to their default values, the same as it does for fields and array elements. The reason this is illegal in C# is because using an unassigned local has high likelihood of being a bug. We simply make it illegal, and then the compiler prevents you from ever having such a bug.

Answer 3

In particular that a reference type that is not initialized will always have a null value

I think you are mixing up local variables and member variables. Section 5.3 talks specifically about local variables. Unlike member variables that do get defaulted, local never default to null value or anything else: they simply must be assigned before they are first read. Section 5.3 explains the rules that the compiler uses to determine if a local variable has been assigned or not.

Answer 4

It depends on where the variable is declared. Variables declared within a class are automatically initialized using the default value.

object o;
void Method()
{
    if (o == null)
    {
        // this will execute
    }
}

Variables declared within a method are not initialized, but when the variable is first used the compiler checks to make sure that it was initialized, so the code will not compile.

void Method()
{
    object o;
    if (o == null) // compile error on this line
    {
    }
}

Answer 5

There are 3 ways that a variable can be assigned an initial value:

1. By default -- this happens (for example) if you declare a class variable without assigning an initial value, so the initial value gets default(type) where type is whatever type you declare the variable to be.

2. With an initializer -- this happens when you declare a variable with an initial value, as in int i = 12;

3. Any point before its value is retrieved -- this happens (for example) if you have a local variable with no initial value. The compiler ensures that you have no reachable code paths that will read the value of the variable before it is assigned.

At no point will the compiler allow you to read the value of a variable that hasn't been initialized, so you never have to worry about what would happen if you tried.

Answer 6

As far as I'm aware, every type has a designated default value.

As per this document, fields of classes are assigned the default value.

http://msdn.microsoft.com/en-us/library/aa645756(v=vs.71).aspx

This document says that the following always have default values assigned automatically.

• Static variables.
• Instance variables of class instances.
• Instance variables of initially assigned struct variables.
• Array elements.
• Value parameters.
• Reference parameters.
• Variables declared in a catch clause or a foreach statement.

http://msdn.microsoft.com/en-us/library/aa691173(v=vs.71).aspx

More info on the actual default values here: http://msdn.microsoft.com/en-us/library/83fhsxwc.aspx