Why are reference types not initialized to null?

Question

Check this code ..

string str; if (str.Contains("something..")) { } 

The compiler throws this error for this code

Using the unassigned local variable 'str'

Why is the reference type not initialized to null?

I just want to know out of curiosity.

I also want to know what happens for the code below. how does this assignment work?

  string str = null; 

Answer 1

Only fields (variables declared at the class level) are automatically initialized:

• Value types are initialized to a default value.
• Reference types are initialized with a null reference.

What you declare is a "local variable" inside the method. Local variables are not automatically initialized, regardless of the value type or reference type.

I also want to know what happens for the code below. how does this assignment work?

This assignment initializes a local variable with a null value with the ldnull , followed by the stloc instruction (in case it is not optimized, of course) and, more importantly, satisfies the rules of compiler data flow analysis for a specific purpose, the C # Specification defines the concept , called a specific assignment, which ensures the assignment of a variable before first use.

Answer 2

C # langauge requires all variables to be assigned before they are read. Local variables are considered initially unassigned, while fields, array elements, etc. They are considered initially assigned to their default values. (Which for the reference type is null.)

There are no technical reasons why we could not treat local variables as originally assigned to their default values ​​and discard all specific assignment checks. This is because using an unassigned local default value is (1) bad coding practice and (2) a very likely source of annoying errors. If you want to explicitly assign local variables before using them, we do not allow the user to use bad practice and exclude a whole class of errors that you then do not have to debug.

Also consider the following:

 while(whatever) { int i; print(i); i = i + 1; } 

Do you expect me to keep its value in all cycles of the cycle or to initialize it to zero every time? Forcing you to explicitly initialize it, the question becomes meaningless, and this difference does not make any difference.

(In addition, in the above case, there is a slight potential performance optimization, since the compiler can reuse the variable without having to generate code to clear its contents, because the compiler knows that you will clear the contents.)

I don’t know how to answer your second question, because I don’t know what you mean by “work”. Can you tell me how to assign "int x = 123;" working? Once I know what you mean by “work,” I can describe how nulling a variable of a reference type works.

Answer 3

It is initialized to null. The compiler just did you a favor, not having to debug the inevitable NullReferenceException that you will get.

Answer 4

Keep in mind that calling Contains in a null string will throw a NullReferenceException.