Difference between variable and string in c#

Question

What is the difference between variable and string in c#

Why the c# is supporting only this

var data = GetData("");  

Why not this?

string data = GetData("");

Or it will support both? Which one is better to use ? How it is implemented?

  private DataTable GetData(string query)
{
    string conString = ConfigurationManager.ConnectionStrings["ProjectConnectionString"].ConnectionString;
    SqlCommand cmd = new SqlCommand(query);
    using (SqlConnection cn = new SqlConnection(conString))
    {
        using (SqlDataAdapter da = new SqlDataAdapter())
        {
            cmd.Connection = cn;
            da.SelectCommand = cmd;
            using (DataTable dt = new DataTable())
            {
                da.Fill(dt);
                return dt;
            }


        }
    }


}

Answer 1

The type of var is not specified in code rather compiler infers it from the code. GetData probably does not return a string.

An implicitly typed local variable is strongly typed just as if you had declared the type yourself, but the compiler determines the type. The following two declarations of i are functionally equivalent, MSDN.

Edit based on updated OP

You can not assign DataTable to string they are two different types. You can use DataTable instead of var if you want to specify the type yourself and you do not want to the compiler to determine it for you using var.

string data = GetData("");

Answer 2

Var is an implicit type.
It aliases any type in the C# programming language.
The aliased type is determined by the C# compiler, where as string is string variable type

Answer 3

if GetData method return type is string below will work

string data = GetData("");

but return type of GetData method is DataTable , you can change the code as

DataTable data = GetData("");

Or use var and compiler will determines the type for you.

Answer 4

var is not a data type. Declaring something as var is synonymous to saying "Compiler, I want you to figure out what type this variable should be by looking at the object I assign to it". Assume you had this:

var myString = "This is a string!";

The C# compiler looks ahead to see that you are assigning a string to myString and it knows that myString is going to be a string type. As far as the compiler is concerned, it is exactly the same as:

string myString = "This is a string!";

As far as what you should use, its entirely your preference and choosing one over the other will not impact your code at all. Personally, I use var because I feel its redundant to declare an object's type twice, like this:

MyClass myClass = new MyClass();

Instead I prefer:

var myClass = new MyClass();

Again, its entirely up to you and your personal coding style. Hope this clears up your questions!