Difference between string str and string str=null

Question

I want to know what exactly happens inside when we declare a variable, like this:

string tr;

string tr = null;

While debugging, I noticed for both values that it was showing null only. But when using ref tr without initializing null it will give error while the second line doesn't.

Please help me to know about it in depth

Answer 1

Your first statement is just declaration and your second statement is Declaration + Initialization.

string tr; // Just Declaration

string tr=null; //Declaration + Initialization. 

You may get compile time errors if you try to use tr with just declaration. (first case) for example:

string tr; // Just Declaration
if (tr == "")   //Use of unassigned local variable
{
}

Same error will be generated for ref keyword, which requires the field to be explicitly assigned something (Declaration + Initialization). With out keyword though, you can use just declaration, but the method would be responsible for making sure that some value is assigned to the out parameter.

Answer 2

While debugging, I noticed for both values that it was showing null only

That's right because string is a reference type and the default value for all reference types is null.

But C# compiler doesn't allow the use of uninitialized variables.

Your first example is just a variable declaration. But your second one is variable initialization.

That's why if you write;

string tr;
Console.WriteLine(tr);

you get compiler error like;

Use of unassigned local variable 'tr'

From ref (C# Reference)

An argument that is passed to a ref parameter must be initialized before it is passed.

Although variables passed as out arguments don't have to be initialized before being passed.